| Message ID | 001001d99d36$a38111c0$ea833540$@nextmovesoftware.com |
|---|---|
| State | New |
| Headers | show |
| Series | New finish_compare_by_pieces target hook (for x86). | expand |
On Mon, Jun 12, 2023 at 4:03 PM Roger Sayle <roger@nextmovesoftware.com> wrote: > > > The following simple test case, from PR 104610, shows that memcmp () == 0 > can result in some bizarre code sequences on x86. > > int foo(char *a) > { > static const char t[] = "0123456789012345678901234567890"; > return __builtin_memcmp(a, &t[0], sizeof(t)) == 0; > } > > with -O2 currently contains both: > xorl %eax, %eax > xorl $1, %eax > and also > movl $1, %eax > xorl $1, %eax > > Changing the return type of foo to _Bool results in the equally > bizarre: > xorl %eax, %eax > testl %eax, %eax > sete %al > and also > movl $1, %eax > testl %eax, %eax > sete %al > > All these sequences set the result to a constant, but this optimization > opportunity only occurs very late during compilation, by basic block > duplication in the 322r.bbro pass, too late for CSE or peephole2 to > do anything about it. The problem is that the idiom expanded by > compare_by_pieces for __builtin_memcmp_eq contains basic blocks that > can't easily be optimized by if-conversion due to the multiple > incoming edges on the fail block. > > In summary, compare_by_pieces generates code that looks like: > > if (x[0] != y[0]) goto fail_label; > if (x[1] != y[1]) goto fail_label; > ... > if (x[n] != y[n]) goto fail_label; > result = 1; > goto end_label; > fail_label: > result = 0; > end_label: > > In theory, the RTL if-conversion pass could be enhanced to tackle > arbitrarily complex if-then-else graphs, but the solution proposed > here is to allow suitable targets to perform if-conversion during > compare_by_pieces. The x86, for example, can take advantage that > all of the above comparisons set and test the zero flag (ZF), which > can then be used in combination with sete. Hence compare_by_pieces > could instead generate: > > if (x[0] != y[0]) goto fail_label; > if (x[1] != y[1]) goto fail_label; > ... > if (x[n] != y[n]) goto fail_label; > fail_label: > sete result > > which requires one less basic block, and the redundant conditional > branch to a label immediately after is cleaned up by GCC's existing > RTL optimizations. > > For the test case above, where -O2 -msse4 previously generated: > > foo: movdqu (%rdi), %xmm0 > pxor .LC0(%rip), %xmm0 > ptest %xmm0, %xmm0 > je .L5 > .L2: movl $1, %eax > xorl $1, %eax > ret > .L5: movdqu 16(%rdi), %xmm0 > pxor .LC1(%rip), %xmm0 > ptest %xmm0, %xmm0 > jne .L2 > xorl %eax, %eax > xorl $1, %eax > ret > > we now generate: > > foo: movdqu (%rdi), %xmm0 > pxor .LC0(%rip), %xmm0 > ptest %xmm0, %xmm0 > jne .L2 > movdqu 16(%rdi), %xmm0 > pxor .LC1(%rip), %xmm0 > ptest %xmm0, %xmm0 > .L2: sete %al > movzbl %al, %eax > ret > > Using a target hook allows the large amount of intelligence already in > compare_by_pieces to be re-used by the i386 backend, but this can also > help other backends with condition flags where the equality result can > be materialized. > > This patch has been tested on x86_64-pc-linux-gnu with make bootstrap > and make -k check, both with and without --target_board=unix{-m32} > with no new failures. Ok for mainline? > > > 2023-06-12 Roger Sayle <roger@nextmovesoftware.com> > > gcc/ChangeLog > * config/i386/i386.cc (ix86_finish_compare_by_pieces): New > function to provide a backend specific implementation. > (TARGET_FINISH_COMPARE_BY_PIECES): Use the above function. > > * doc/tm.texi.in (TARGET_FINISH_COMPARE_BY_PIECES): New @hook. > * doc/tm.texi: Regenerate. > > * expr.cc (compare_by_pieces): Call finish_compare_by_pieces in > targetm to finalize the RTL expansion. Move the current > implementation to a default target hook. > * target.def (finish_compare_by_pieces): New target hook to allow > compare_by_pieces to be customized by the target. > * targhooks.cc (default_finish_compare_by_pieces): Default > implementation moved here from expr.cc's compare_by_pieces. > * targhooks.h (default_finish_compare_by_pieces): Prototype. > > gcc/testsuite/ChangeLog > * gcc.target/i386/pieces-memcmp-1.c: New test case. This patch needs middle-end approval first. Uros. > > > Thanks in advance, > Roger > -- >
On Mon, Jun 12, 2023 at 4:04 PM Roger Sayle <roger@nextmovesoftware.com> wrote: > > > The following simple test case, from PR 104610, shows that memcmp () == 0 > can result in some bizarre code sequences on x86. > > int foo(char *a) > { > static const char t[] = "0123456789012345678901234567890"; > return __builtin_memcmp(a, &t[0], sizeof(t)) == 0; > } > > with -O2 currently contains both: > xorl %eax, %eax > xorl $1, %eax > and also > movl $1, %eax > xorl $1, %eax > > Changing the return type of foo to _Bool results in the equally > bizarre: > xorl %eax, %eax > testl %eax, %eax > sete %al > and also > movl $1, %eax > testl %eax, %eax > sete %al > > All these sequences set the result to a constant, but this optimization > opportunity only occurs very late during compilation, by basic block > duplication in the 322r.bbro pass, too late for CSE or peephole2 to > do anything about it. The problem is that the idiom expanded by > compare_by_pieces for __builtin_memcmp_eq contains basic blocks that > can't easily be optimized by if-conversion due to the multiple > incoming edges on the fail block. > > In summary, compare_by_pieces generates code that looks like: > > if (x[0] != y[0]) goto fail_label; > if (x[1] != y[1]) goto fail_label; > ... > if (x[n] != y[n]) goto fail_label; > result = 1; > goto end_label; > fail_label: > result = 0; > end_label: > > In theory, the RTL if-conversion pass could be enhanced to tackle > arbitrarily complex if-then-else graphs, but the solution proposed > here is to allow suitable targets to perform if-conversion during > compare_by_pieces. The x86, for example, can take advantage that > all of the above comparisons set and test the zero flag (ZF), which > can then be used in combination with sete. Hence compare_by_pieces > could instead generate: > > if (x[0] != y[0]) goto fail_label; > if (x[1] != y[1]) goto fail_label; > ... > if (x[n] != y[n]) goto fail_label; > fail_label: > sete result > > which requires one less basic block, and the redundant conditional > branch to a label immediately after is cleaned up by GCC's existing > RTL optimizations. > > For the test case above, where -O2 -msse4 previously generated: > > foo: movdqu (%rdi), %xmm0 > pxor .LC0(%rip), %xmm0 > ptest %xmm0, %xmm0 > je .L5 > .L2: movl $1, %eax > xorl $1, %eax > ret > .L5: movdqu 16(%rdi), %xmm0 > pxor .LC1(%rip), %xmm0 > ptest %xmm0, %xmm0 > jne .L2 > xorl %eax, %eax > xorl $1, %eax > ret > > we now generate: > > foo: movdqu (%rdi), %xmm0 > pxor .LC0(%rip), %xmm0 > ptest %xmm0, %xmm0 > jne .L2 > movdqu 16(%rdi), %xmm0 > pxor .LC1(%rip), %xmm0 > ptest %xmm0, %xmm0 > .L2: sete %al > movzbl %al, %eax > ret > > Using a target hook allows the large amount of intelligence already in > compare_by_pieces to be re-used by the i386 backend, but this can also > help other backends with condition flags where the equality result can > be materialized. > > This patch has been tested on x86_64-pc-linux-gnu with make bootstrap > and make -k check, both with and without --target_board=unix{-m32} > with no new failures. Ok for mainline? What's the guarantee that the zero flag is appropriately set on all edges incoming now and forever? Does this require target specific knowledge on how do_compare_rtx_and_jump is emitting RTL? Do you see matching this in ifcvt to be unreasonable? I'm thinking of "reducing" the incoming edges pairwise without actually looking at the ifcvt code. Thanks, Richard. > > 2023-06-12 Roger Sayle <roger@nextmovesoftware.com> > > gcc/ChangeLog > * config/i386/i386.cc (ix86_finish_compare_by_pieces): New > function to provide a backend specific implementation. > (TARGET_FINISH_COMPARE_BY_PIECES): Use the above function. > > * doc/tm.texi.in (TARGET_FINISH_COMPARE_BY_PIECES): New @hook. > * doc/tm.texi: Regenerate. > > * expr.cc (compare_by_pieces): Call finish_compare_by_pieces in > targetm to finalize the RTL expansion. Move the current > implementation to a default target hook. > * target.def (finish_compare_by_pieces): New target hook to allow > compare_by_pieces to be customized by the target. > * targhooks.cc (default_finish_compare_by_pieces): Default > implementation moved here from expr.cc's compare_by_pieces. > * targhooks.h (default_finish_compare_by_pieces): Prototype. > > gcc/testsuite/ChangeLog > * gcc.target/i386/pieces-memcmp-1.c: New test case. > > > Thanks in advance, > Roger > -- >
On Tue, 13 June 2023 12:02, Richard Biener wrote: > On Mon, Jun 12, 2023 at 4:04 PM Roger Sayle <roger@nextmovesoftware.com> > wrote: > > The following simple test case, from PR 104610, shows that memcmp () > > == 0 can result in some bizarre code sequences on x86. > > > > int foo(char *a) > > { > > static const char t[] = "0123456789012345678901234567890"; > > return __builtin_memcmp(a, &t[0], sizeof(t)) == 0; } > > > > with -O2 currently contains both: > > xorl %eax, %eax > > xorl $1, %eax > > and also > > movl $1, %eax > > xorl $1, %eax > > > > Changing the return type of foo to _Bool results in the equally > > bizarre: > > xorl %eax, %eax > > testl %eax, %eax > > sete %al > > and also > > movl $1, %eax > > testl %eax, %eax > > sete %al > > > > All these sequences set the result to a constant, but this > > optimization opportunity only occurs very late during compilation, by > > basic block duplication in the 322r.bbro pass, too late for CSE or > > peephole2 to do anything about it. The problem is that the idiom > > expanded by compare_by_pieces for __builtin_memcmp_eq contains basic > > blocks that can't easily be optimized by if-conversion due to the > > multiple incoming edges on the fail block. > > > > In summary, compare_by_pieces generates code that looks like: > > > > if (x[0] != y[0]) goto fail_label; > > if (x[1] != y[1]) goto fail_label; > > ... > > if (x[n] != y[n]) goto fail_label; > > result = 1; > > goto end_label; > > fail_label: > > result = 0; > > end_label: > > > > In theory, the RTL if-conversion pass could be enhanced to tackle > > arbitrarily complex if-then-else graphs, but the solution proposed > > here is to allow suitable targets to perform if-conversion during > > compare_by_pieces. The x86, for example, can take advantage that all > > of the above comparisons set and test the zero flag (ZF), which can > > then be used in combination with sete. Hence compare_by_pieces could > > instead generate: > > > > if (x[0] != y[0]) goto fail_label; > > if (x[1] != y[1]) goto fail_label; > > ... > > if (x[n] != y[n]) goto fail_label; > > fail_label: > > sete result > > > > which requires one less basic block, and the redundant conditional > > branch to a label immediately after is cleaned up by GCC's existing > > RTL optimizations. > > > > For the test case above, where -O2 -msse4 previously generated: > > > > foo: movdqu (%rdi), %xmm0 > > pxor .LC0(%rip), %xmm0 > > ptest %xmm0, %xmm0 > > je .L5 > > .L2: movl $1, %eax > > xorl $1, %eax > > ret > > .L5: movdqu 16(%rdi), %xmm0 > > pxor .LC1(%rip), %xmm0 > > ptest %xmm0, %xmm0 > > jne .L2 > > xorl %eax, %eax > > xorl $1, %eax > > ret > > > > we now generate: > > > > foo: movdqu (%rdi), %xmm0 > > pxor .LC0(%rip), %xmm0 > > ptest %xmm0, %xmm0 > > jne .L2 > > movdqu 16(%rdi), %xmm0 > > pxor .LC1(%rip), %xmm0 > > ptest %xmm0, %xmm0 > > .L2: sete %al > > movzbl %al, %eax > > ret > > > > Using a target hook allows the large amount of intelligence already in > > compare_by_pieces to be re-used by the i386 backend, but this can also > > help other backends with condition flags where the equality result can > > be materialized. > > > > This patch has been tested on x86_64-pc-linux-gnu with make bootstrap > > and make -k check, both with and without --target_board=unix{-m32} > > with no new failures. Ok for mainline? > > What's the guarantee that the zero flag is appropriately set on all edges incoming > now and forever? Is there any reason why this target hook can't be removed (in future) should it stop being useful? It's completely optional and not required for the correct functioning of the compiler. > Does this require target specific knowledge on how do_compare_rtx_and_jump > is emitting RTL? Yes. Each backend can decide how best to implement finish_compare_by_pieces given its internal knowledge of how do_compare_rtx_and_jump works. It's not important to the middle-end how the underlying invariants are guaranteed, just that they are and the backend produces correct code. A backend may store flags on the target label, or maintain state in cfun. Future changes to the i386 backend might cause it to revert to the default finish_compare_by_pieces, or provide an alternate implementation, but at the moment this patch improves the code that GCC generates. Very little (in software like GCC) is forever. > Do you see matching this in ifcvt to be unreasonable? I'm thinking of "reducing" > the incoming edges pairwise without actually looking at the ifcvt code. There's nothing about the proposed patch that prevents or blocks improvements to ifcvt, which I agree completely could be improved. But even (in future) if later RTL passes could clean things up, that's no reason for RTL expansion to initially generate poor/inefficient code. I'm not sure that a (hypothetical) ifcvt improvement would be sufficient reason to revert/remove enhancements to compare_by_pieces. Is it that there's not enough (bootstrap and) testsuite coverage of compare_by_pieces to make you feel comfortable with this change? The proposed implementation should be "obvious enough" to future generations what the intended behaviour should be. And the x86 target hook implementation (i.e. the change) is only four lines long, a fraction of the size of the new documentation and comments. Thanks in advance. Roger > > 2023-06-12 Roger Sayle <roger@nextmovesoftware.com> > > > > gcc/ChangeLog > > * config/i386/i386.cc (ix86_finish_compare_by_pieces): New > > function to provide a backend specific implementation. > > (TARGET_FINISH_COMPARE_BY_PIECES): Use the above function. > > > > * doc/tm.texi.in (TARGET_FINISH_COMPARE_BY_PIECES): New @hook. > > * doc/tm.texi: Regenerate. > > > > * expr.cc (compare_by_pieces): Call finish_compare_by_pieces in > > targetm to finalize the RTL expansion. Move the current > > implementation to a default target hook. > > * target.def (finish_compare_by_pieces): New target hook to allow > > compare_by_pieces to be customized by the target. > > * targhooks.cc (default_finish_compare_by_pieces): Default > > implementation moved here from expr.cc's compare_by_pieces. > > * targhooks.h (default_finish_compare_by_pieces): Prototype. > > > > gcc/testsuite/ChangeLog > > * gcc.target/i386/pieces-memcmp-1.c: New test case. > >
On Sun, Jun 25, 2023 at 7:39 AM Roger Sayle <roger@nextmovesoftware.com> wrote: > > > On Tue, 13 June 2023 12:02, Richard Biener wrote: > > On Mon, Jun 12, 2023 at 4:04 PM Roger Sayle <roger@nextmovesoftware.com> > > wrote: > > > The following simple test case, from PR 104610, shows that memcmp () > > > == 0 can result in some bizarre code sequences on x86. > > > > > > int foo(char *a) > > > { > > > static const char t[] = "0123456789012345678901234567890"; > > > return __builtin_memcmp(a, &t[0], sizeof(t)) == 0; } > > > > > > with -O2 currently contains both: > > > xorl %eax, %eax > > > xorl $1, %eax > > > and also > > > movl $1, %eax > > > xorl $1, %eax > > > > > > Changing the return type of foo to _Bool results in the equally > > > bizarre: > > > xorl %eax, %eax > > > testl %eax, %eax > > > sete %al > > > and also > > > movl $1, %eax > > > testl %eax, %eax > > > sete %al > > > > > > All these sequences set the result to a constant, but this > > > optimization opportunity only occurs very late during compilation, by > > > basic block duplication in the 322r.bbro pass, too late for CSE or > > > peephole2 to do anything about it. The problem is that the idiom > > > expanded by compare_by_pieces for __builtin_memcmp_eq contains basic > > > blocks that can't easily be optimized by if-conversion due to the > > > multiple incoming edges on the fail block. > > > > > > In summary, compare_by_pieces generates code that looks like: > > > > > > if (x[0] != y[0]) goto fail_label; > > > if (x[1] != y[1]) goto fail_label; > > > ... > > > if (x[n] != y[n]) goto fail_label; > > > result = 1; > > > goto end_label; > > > fail_label: > > > result = 0; > > > end_label: > > > > > > In theory, the RTL if-conversion pass could be enhanced to tackle > > > arbitrarily complex if-then-else graphs, but the solution proposed > > > here is to allow suitable targets to perform if-conversion during > > > compare_by_pieces. The x86, for example, can take advantage that all > > > of the above comparisons set and test the zero flag (ZF), which can > > > then be used in combination with sete. Hence compare_by_pieces could > > > instead generate: > > > > > > if (x[0] != y[0]) goto fail_label; > > > if (x[1] != y[1]) goto fail_label; > > > ... > > > if (x[n] != y[n]) goto fail_label; > > > fail_label: > > > sete result > > > > > > which requires one less basic block, and the redundant conditional > > > branch to a label immediately after is cleaned up by GCC's existing > > > RTL optimizations. > > > > > > For the test case above, where -O2 -msse4 previously generated: > > > > > > foo: movdqu (%rdi), %xmm0 > > > pxor .LC0(%rip), %xmm0 > > > ptest %xmm0, %xmm0 > > > je .L5 > > > .L2: movl $1, %eax > > > xorl $1, %eax > > > ret > > > .L5: movdqu 16(%rdi), %xmm0 > > > pxor .LC1(%rip), %xmm0 > > > ptest %xmm0, %xmm0 > > > jne .L2 > > > xorl %eax, %eax > > > xorl $1, %eax > > > ret > > > > > > we now generate: > > > > > > foo: movdqu (%rdi), %xmm0 > > > pxor .LC0(%rip), %xmm0 > > > ptest %xmm0, %xmm0 > > > jne .L2 > > > movdqu 16(%rdi), %xmm0 > > > pxor .LC1(%rip), %xmm0 > > > ptest %xmm0, %xmm0 > > > .L2: sete %al > > > movzbl %al, %eax > > > ret > > > > > > Using a target hook allows the large amount of intelligence already in > > > compare_by_pieces to be re-used by the i386 backend, but this can also > > > help other backends with condition flags where the equality result can > > > be materialized. > > > > > > This patch has been tested on x86_64-pc-linux-gnu with make bootstrap > > > and make -k check, both with and without --target_board=unix{-m32} > > > with no new failures. Ok for mainline? > > > > What's the guarantee that the zero flag is appropriately set on all edges incoming > > now and forever? > > Is there any reason why this target hook can't be removed (in future) should it stop > being useful? It's completely optional and not required for the correct functioning > of the compiler. > > > Does this require target specific knowledge on how do_compare_rtx_and_jump > > is emitting RTL? > > Yes. Each backend can decide how best to implement finish_compare_by_pieces > given its internal knowledge of how do_compare_rtx_and_jump works. It's not > important to the middle-end how the underlying invariants are guaranteed, just > that they are and the backend produces correct code. A backend may store flags > on the target label, or maintain state in cfun. Future changes to the i386 backend > might cause it to revert to the default finish_compare_by_pieces, or provide an > alternate implementation, but at the moment this patch improves the code that > GCC generates. Very little (in software like GCC) is forever. > > > Do you see matching this in ifcvt to be unreasonable? I'm thinking of "reducing" > > the incoming edges pairwise without actually looking at the ifcvt code. > > There's nothing about the proposed patch that prevents or blocks improvements > to ifcvt, which I agree completely could be improved. But even (in future) if later > RTL passes could clean things up, that's no reason for RTL expansion to initially > generate poor/inefficient code. I'm not sure that a (hypothetical) ifcvt improvement > would be sufficient reason to revert/remove enhancements to compare_by_pieces. > > Is it that there's not enough (bootstrap and) testsuite coverage of compare_by_pieces > to make you feel comfortable with this change? The proposed implementation should > be "obvious enough" to future generations what the intended behaviour should be. > And the x86 target hook implementation (i.e. the change) is only four lines long, a > fraction of the size of the new documentation and comments. My main concern was that we are communicating "implicit" dependences between the target hook and expand RTL code generation - we don't seem to pass down enough info for example to have the target verify constraints and excuse itself if they do not hold. They also seem to be poorly documented and the compare_by_pieces (and all _by_pieces) stuff has been extended to be quite "configurable" and thus is probably prone to emit vastly different RTL in some special cases? That said, I'm not familiar enough with the compare_by_pices logic to say that this looks obviously safe, but maybe it is. Yes - all target hooks have this kind of "implicit" dependence on how the calling code works, so I guess the main thing is that it's not obvious how the calling code works in this case? Anyway, I'm not standing in the way of somebody approving this change - the code generation improvements are definitely worth it. Maybe it's the abstraction boundary that makes me worry - can we make the target simply indicate whether a do_compare_rtx_and_jump sets a flag (maybe query the CC mode used?) and have the generic code then emit the setcc? The calling code would then know it emitted suitable CC producing compare and branch and it would emit the corresponding setcc with the proper CCmode? Richard. > Thanks in advance. > Roger > > > > > 2023-06-12 Roger Sayle <roger@nextmovesoftware.com> > > > > > > gcc/ChangeLog > > > * config/i386/i386.cc (ix86_finish_compare_by_pieces): New > > > function to provide a backend specific implementation. > > > (TARGET_FINISH_COMPARE_BY_PIECES): Use the above function. > > > > > > * doc/tm.texi.in (TARGET_FINISH_COMPARE_BY_PIECES): New @hook. > > > * doc/tm.texi: Regenerate. > > > > > > * expr.cc (compare_by_pieces): Call finish_compare_by_pieces in > > > targetm to finalize the RTL expansion. Move the current > > > implementation to a default target hook. > > > * target.def (finish_compare_by_pieces): New target hook to allow > > > compare_by_pieces to be customized by the target. > > > * targhooks.cc (default_finish_compare_by_pieces): Default > > > implementation moved here from expr.cc's compare_by_pieces. > > > * targhooks.h (default_finish_compare_by_pieces): Prototype. > > > > > > gcc/testsuite/ChangeLog > > > * gcc.target/i386/pieces-memcmp-1.c: New test case. > > > > >
Richard Biener via Gcc-patches <gcc-patches@gcc.gnu.org> writes: > On Sun, Jun 25, 2023 at 7:39 AM Roger Sayle <roger@nextmovesoftware.com> wrote: >> >> >> On Tue, 13 June 2023 12:02, Richard Biener wrote: >> > On Mon, Jun 12, 2023 at 4:04 PM Roger Sayle <roger@nextmovesoftware.com> >> > wrote: >> > > The following simple test case, from PR 104610, shows that memcmp () >> > > == 0 can result in some bizarre code sequences on x86. >> > > >> > > int foo(char *a) >> > > { >> > > static const char t[] = "0123456789012345678901234567890"; >> > > return __builtin_memcmp(a, &t[0], sizeof(t)) == 0; } >> > > >> > > with -O2 currently contains both: >> > > xorl %eax, %eax >> > > xorl $1, %eax >> > > and also >> > > movl $1, %eax >> > > xorl $1, %eax >> > > >> > > Changing the return type of foo to _Bool results in the equally >> > > bizarre: >> > > xorl %eax, %eax >> > > testl %eax, %eax >> > > sete %al >> > > and also >> > > movl $1, %eax >> > > testl %eax, %eax >> > > sete %al >> > > >> > > All these sequences set the result to a constant, but this >> > > optimization opportunity only occurs very late during compilation, by >> > > basic block duplication in the 322r.bbro pass, too late for CSE or >> > > peephole2 to do anything about it. The problem is that the idiom >> > > expanded by compare_by_pieces for __builtin_memcmp_eq contains basic >> > > blocks that can't easily be optimized by if-conversion due to the >> > > multiple incoming edges on the fail block. >> > > >> > > In summary, compare_by_pieces generates code that looks like: >> > > >> > > if (x[0] != y[0]) goto fail_label; >> > > if (x[1] != y[1]) goto fail_label; >> > > ... >> > > if (x[n] != y[n]) goto fail_label; >> > > result = 1; >> > > goto end_label; >> > > fail_label: >> > > result = 0; >> > > end_label: >> > > >> > > In theory, the RTL if-conversion pass could be enhanced to tackle >> > > arbitrarily complex if-then-else graphs, but the solution proposed >> > > here is to allow suitable targets to perform if-conversion during >> > > compare_by_pieces. The x86, for example, can take advantage that all >> > > of the above comparisons set and test the zero flag (ZF), which can >> > > then be used in combination with sete. Hence compare_by_pieces could >> > > instead generate: >> > > >> > > if (x[0] != y[0]) goto fail_label; >> > > if (x[1] != y[1]) goto fail_label; >> > > ... >> > > if (x[n] != y[n]) goto fail_label; >> > > fail_label: >> > > sete result >> > > >> > > which requires one less basic block, and the redundant conditional >> > > branch to a label immediately after is cleaned up by GCC's existing >> > > RTL optimizations. >> > > >> > > For the test case above, where -O2 -msse4 previously generated: >> > > >> > > foo: movdqu (%rdi), %xmm0 >> > > pxor .LC0(%rip), %xmm0 >> > > ptest %xmm0, %xmm0 >> > > je .L5 >> > > .L2: movl $1, %eax >> > > xorl $1, %eax >> > > ret >> > > .L5: movdqu 16(%rdi), %xmm0 >> > > pxor .LC1(%rip), %xmm0 >> > > ptest %xmm0, %xmm0 >> > > jne .L2 >> > > xorl %eax, %eax >> > > xorl $1, %eax >> > > ret >> > > >> > > we now generate: >> > > >> > > foo: movdqu (%rdi), %xmm0 >> > > pxor .LC0(%rip), %xmm0 >> > > ptest %xmm0, %xmm0 >> > > jne .L2 >> > > movdqu 16(%rdi), %xmm0 >> > > pxor .LC1(%rip), %xmm0 >> > > ptest %xmm0, %xmm0 >> > > .L2: sete %al >> > > movzbl %al, %eax >> > > ret >> > > >> > > Using a target hook allows the large amount of intelligence already in >> > > compare_by_pieces to be re-used by the i386 backend, but this can also >> > > help other backends with condition flags where the equality result can >> > > be materialized. >> > > >> > > This patch has been tested on x86_64-pc-linux-gnu with make bootstrap >> > > and make -k check, both with and without --target_board=unix{-m32} >> > > with no new failures. Ok for mainline? >> > >> > What's the guarantee that the zero flag is appropriately set on all edges incoming >> > now and forever? >> >> Is there any reason why this target hook can't be removed (in future) should it stop >> being useful? It's completely optional and not required for the correct functioning >> of the compiler. >> >> > Does this require target specific knowledge on how do_compare_rtx_and_jump >> > is emitting RTL? >> >> Yes. Each backend can decide how best to implement finish_compare_by_pieces >> given its internal knowledge of how do_compare_rtx_and_jump works. It's not >> important to the middle-end how the underlying invariants are guaranteed, just >> that they are and the backend produces correct code. A backend may store flags >> on the target label, or maintain state in cfun. Future changes to the i386 backend >> might cause it to revert to the default finish_compare_by_pieces, or provide an >> alternate implementation, but at the moment this patch improves the code that >> GCC generates. Very little (in software like GCC) is forever. >> >> > Do you see matching this in ifcvt to be unreasonable? I'm thinking of "reducing" >> > the incoming edges pairwise without actually looking at the ifcvt code. >> >> There's nothing about the proposed patch that prevents or blocks improvements >> to ifcvt, which I agree completely could be improved. But even (in future) if later >> RTL passes could clean things up, that's no reason for RTL expansion to initially >> generate poor/inefficient code. I'm not sure that a (hypothetical) ifcvt improvement >> would be sufficient reason to revert/remove enhancements to compare_by_pieces. >> >> Is it that there's not enough (bootstrap and) testsuite coverage of compare_by_pieces >> to make you feel comfortable with this change? The proposed implementation should >> be "obvious enough" to future generations what the intended behaviour should be. >> And the x86 target hook implementation (i.e. the change) is only four lines long, a >> fraction of the size of the new documentation and comments. > > My main concern was that we are communicating "implicit" dependences between > the target hook and expand RTL code generation - we don't seem to pass down > enough info for example to have the target verify constraints and > excuse itself if > they do not hold. They also seem to be poorly documented and the > compare_by_pieces (and all _by_pieces) stuff has been extended to be quite > "configurable" and thus is probably prone to emit vastly different RTL in > some special cases? FWIW, I share these concerns, especially about the target hook not being able to verify its assumptions. Maybe it wouldn't matter too much if the worst outcome was a missed optimisation, but I don't think the mechanism is reliable enough for us to rely on it for correctness. Richard
diff --git a/gcc/config/i386/i386.cc b/gcc/config/i386/i386.cc index 3a1444d..509c0ee 100644 --- a/gcc/config/i386/i386.cc +++ b/gcc/config/i386/i386.cc @@ -16146,6 +16146,20 @@ ix86_fp_compare_code_to_integer (enum rtx_code code) } } +/* Override compare_by_pieces' default implementation using the state + of the CCZmode FLAGS_REG and sete instruction. TARGET is the integral + mode result, and FAIL_LABEL is the branch target of mismatched + comparisons. */ + +void +ix86_finish_compare_by_pieces (rtx target, rtx_code_label *fail_label) +{ + rtx tmp = gen_reg_rtx (QImode); + emit_label (fail_label); + ix86_expand_setcc (tmp, NE, gen_rtx_REG (CCZmode, FLAGS_REG), const0_rtx); + convert_move (target, tmp, 1); +} + /* Zero extend possibly SImode EXP to Pmode register. */ rtx ix86_zero_extend_to_Pmode (rtx exp) @@ -25127,6 +25141,8 @@ ix86_run_selftests (void) #undef TARGET_OVERLAP_OP_BY_PIECES_P #define TARGET_OVERLAP_OP_BY_PIECES_P hook_bool_void_true +#undef TARGET_FINISH_COMPARE_BY_PIECES +#define TARGET_FINISH_COMPARE_BY_PIECES ix86_finish_compare_by_pieces #undef TARGET_FLAGS_REGNUM #define TARGET_FLAGS_REGNUM FLAGS_REG diff --git a/gcc/doc/tm.texi b/gcc/doc/tm.texi index 95ba56e..28d8361 100644 --- a/gcc/doc/tm.texi +++ b/gcc/doc/tm.texi @@ -6922,6 +6922,18 @@ particular mode from being used for block comparisons by returning a negative number from this hook. @end deftypefn +@deftypefn {Target Hook} void TARGET_FINISH_COMPARE_BY_PIECES (rtx @var{target}, rtx_code_label *@var{fail_label}) +Allow targets with a zero flag and suitable setcc instruction to provide +an alternate implementation for @code{compare_by_pieces}. The function +@code{compare_by_pieces} generates a sequence of equality tests that +branch to @var{failure_label} on mismatches, and fall through on success. +By default, this hook assigns @code{const1_rtx} to @var{target} in the +current basic block and @code{const0_rtx} to @var{target} in a new +@var{fail_label} basic block. Targets like x86 can take advantage +of the property that the condition codes/zero flag are appropriately +set to avoid introducing a new basic block. +@end deftypefn + @defmac MOVE_MAX_PIECES A C expression used by @code{move_by_pieces} to determine the largest unit a load or store used to copy memory is. Defaults to @code{MOVE_MAX}. diff --git a/gcc/doc/tm.texi.in b/gcc/doc/tm.texi.in index 4ac96dc..45711db 100644 --- a/gcc/doc/tm.texi.in +++ b/gcc/doc/tm.texi.in @@ -4529,6 +4529,8 @@ If you don't define this, a reasonable default is used. @hook TARGET_COMPARE_BY_PIECES_BRANCH_RATIO +@hook TARGET_FINISH_COMPARE_BY_PIECES + @defmac MOVE_MAX_PIECES A C expression used by @code{move_by_pieces} to determine the largest unit a load or store used to copy memory is. Defaults to @code{MOVE_MAX}. diff --git a/gcc/expr.cc b/gcc/expr.cc index 868fa6e..25fca17 100644 --- a/gcc/expr.cc +++ b/gcc/expr.cc @@ -1923,7 +1923,6 @@ compare_by_pieces (rtx arg0, rtx arg1, unsigned HOST_WIDE_INT len, by_pieces_constfn a1_cfn, void *a1_cfn_data) { rtx_code_label *fail_label = gen_label_rtx (); - rtx_code_label *end_label = gen_label_rtx (); if (target == NULL_RTX || !REG_P (target) || REGNO (target) < FIRST_PSEUDO_REGISTER) @@ -1934,12 +1933,8 @@ compare_by_pieces (rtx arg0, rtx arg1, unsigned HOST_WIDE_INT len, data.run (); - emit_move_insn (target, const0_rtx); - emit_jump (end_label); - emit_barrier (); - emit_label (fail_label); - emit_move_insn (target, const1_rtx); - emit_label (end_label); + /* Allow the backend to override the default implementation. */ + targetm.finish_compare_by_pieces (target, fail_label); return target; } diff --git a/gcc/target.def b/gcc/target.def index 7d68429..0593917 100644 --- a/gcc/target.def +++ b/gcc/target.def @@ -3749,6 +3749,20 @@ negative number from this hook.", default_compare_by_pieces_branch_ratio) DEFHOOK +(finish_compare_by_pieces, + "Allow targets with a zero flag and suitable setcc instruction to provide\n\ +an alternate implementation for @code{compare_by_pieces}. The function\n\ +@code{compare_by_pieces} generates a sequence of equality tests that\n\ +branch to @var{failure_label} on mismatches, and fall through on success.\n\ +By default, this hook assigns @code{const1_rtx} to @var{target} in the\n\ +current basic block and @code{const0_rtx} to @var{target} in a new\n\ +@var{fail_label} basic block. Targets like x86 can take advantage\n\ +of the property that the condition codes/zero flag are appropriately\n\ +set to avoid introducing a new basic block.", + void, (rtx target, rtx_code_label *fail_label), + default_finish_compare_by_pieces) + +DEFHOOK (slow_unaligned_access, "This hook returns true if memory accesses described by the\n\ @var{mode} and @var{alignment} parameters have a cost many times greater\n\ diff --git a/gcc/targhooks.cc b/gcc/targhooks.cc index e190369..767fe38 100644 --- a/gcc/targhooks.cc +++ b/gcc/targhooks.cc @@ -2094,6 +2094,24 @@ default_compare_by_pieces_branch_ratio (machine_mode) return 1; } +/* This hook allows the backend to modify/override code generation for + compare_by_pieces. Targets with a zero flag and a suitable setcc + function can use them instead of the default "compare ? 0 : 1" + implementation below. TARGET is the integral mode result and + FAIL_LABEL is the destination for comparison mismatches. */ + +void +default_finish_compare_by_pieces (rtx target, rtx_code_label *fail_label) +{ + rtx_code_label *end_label = gen_label_rtx (); + emit_move_insn (target, const0_rtx); + emit_jump (end_label); + emit_barrier (); + emit_label (fail_label); + emit_move_insn (target, const1_rtx); + emit_label (end_label); +} + /* Write PATCH_AREA_SIZE NOPs into the asm outfile FILE around a function entry. If RECORD_P is true and the target supports named sections, the location of the NOPs will be recorded in a special object section diff --git a/gcc/targhooks.h b/gcc/targhooks.h index 1a0db8d..b99fff6 100644 --- a/gcc/targhooks.h +++ b/gcc/targhooks.h @@ -237,6 +237,7 @@ extern bool default_use_by_pieces_infrastructure_p (unsigned HOST_WIDE_INT, enum by_pieces_operation, bool); extern int default_compare_by_pieces_branch_ratio (machine_mode); +extern void default_finish_compare_by_pieces (rtx, rtx_code_label *); extern void default_print_patchable_function_entry (FILE *, unsigned HOST_WIDE_INT, diff --git a/gcc/testsuite/gcc.target/i386/pieces-memcmp-1.c b/gcc/testsuite/gcc.target/i386/pieces-memcmp-1.c new file mode 100644 index 0000000..de1d82f --- /dev/null +++ b/gcc/testsuite/gcc.target/i386/pieces-memcmp-1.c @@ -0,0 +1,10 @@ +/* { dg-do compile } */ +/* { dg-options "-O2" } */ + +int foo(char *a) +{ + static const char t[] = "0123456789012345678901234567890"; + return __builtin_memcmp(a, &t[0], sizeof(t)) == 0; +} + +/* { dg-final { scan-assembler-not "xorl\[ \\t]*\\\$1," } } */
The following simple test case, from PR 104610, shows that memcmp () == 0 can result in some bizarre code sequences on x86. int foo(char *a) { static const char t[] = "0123456789012345678901234567890"; return __builtin_memcmp(a, &t[0], sizeof(t)) == 0; } with -O2 currently contains both: xorl %eax, %eax xorl $1, %eax and also movl $1, %eax xorl $1, %eax Changing the return type of foo to _Bool results in the equally bizarre: xorl %eax, %eax testl %eax, %eax sete %al and also movl $1, %eax testl %eax, %eax sete %al All these sequences set the result to a constant, but this optimization opportunity only occurs very late during compilation, by basic block duplication in the 322r.bbro pass, too late for CSE or peephole2 to do anything about it. The problem is that the idiom expanded by compare_by_pieces for __builtin_memcmp_eq contains basic blocks that can't easily be optimized by if-conversion due to the multiple incoming edges on the fail block. In summary, compare_by_pieces generates code that looks like: if (x[0] != y[0]) goto fail_label; if (x[1] != y[1]) goto fail_label; ... if (x[n] != y[n]) goto fail_label; result = 1; goto end_label; fail_label: result = 0; end_label: In theory, the RTL if-conversion pass could be enhanced to tackle arbitrarily complex if-then-else graphs, but the solution proposed here is to allow suitable targets to perform if-conversion during compare_by_pieces. The x86, for example, can take advantage that all of the above comparisons set and test the zero flag (ZF), which can then be used in combination with sete. Hence compare_by_pieces could instead generate: if (x[0] != y[0]) goto fail_label; if (x[1] != y[1]) goto fail_label; ... if (x[n] != y[n]) goto fail_label; fail_label: sete result which requires one less basic block, and the redundant conditional branch to a label immediately after is cleaned up by GCC's existing RTL optimizations. For the test case above, where -O2 -msse4 previously generated: foo: movdqu (%rdi), %xmm0 pxor .LC0(%rip), %xmm0 ptest %xmm0, %xmm0 je .L5 .L2: movl $1, %eax xorl $1, %eax ret .L5: movdqu 16(%rdi), %xmm0 pxor .LC1(%rip), %xmm0 ptest %xmm0, %xmm0 jne .L2 xorl %eax, %eax xorl $1, %eax ret we now generate: foo: movdqu (%rdi), %xmm0 pxor .LC0(%rip), %xmm0 ptest %xmm0, %xmm0 jne .L2 movdqu 16(%rdi), %xmm0 pxor .LC1(%rip), %xmm0 ptest %xmm0, %xmm0 .L2: sete %al movzbl %al, %eax ret Using a target hook allows the large amount of intelligence already in compare_by_pieces to be re-used by the i386 backend, but this can also help other backends with condition flags where the equality result can be materialized. This patch has been tested on x86_64-pc-linux-gnu with make bootstrap and make -k check, both with and without --target_board=unix{-m32} with no new failures. Ok for mainline? 2023-06-12 Roger Sayle <roger@nextmovesoftware.com> gcc/ChangeLog * config/i386/i386.cc (ix86_finish_compare_by_pieces): New function to provide a backend specific implementation. (TARGET_FINISH_COMPARE_BY_PIECES): Use the above function. * doc/tm.texi.in (TARGET_FINISH_COMPARE_BY_PIECES): New @hook. * doc/tm.texi: Regenerate. * expr.cc (compare_by_pieces): Call finish_compare_by_pieces in targetm to finalize the RTL expansion. Move the current implementation to a default target hook. * target.def (finish_compare_by_pieces): New target hook to allow compare_by_pieces to be customized by the target. * targhooks.cc (default_finish_compare_by_pieces): Default implementation moved here from expr.cc's compare_by_pieces. * targhooks.h (default_finish_compare_by_pieces): Prototype. gcc/testsuite/ChangeLog * gcc.target/i386/pieces-memcmp-1.c: New test case. Thanks in advance, Roger --