| Message ID | ZpfNPBSLF+QDqr85@tucnak |
|---|---|
| State | New |
| Headers | show |
| Series | varasm: Shorten assembly of strings with larger zero regions | expand |
> Am 17.07.2024 um 15:55 schrieb Jakub Jelinek <jakub@redhat.com>: > > Hi! > > When not using .base64 directive, we emit for long sequences of zeros > .string "foobarbaz" > .string "" > .string "" > .string "" > .string "" > .string "" > .string "" > .string "" > .string "" > .string "" > .string "" > .string "" > .string "" > The following patch changes that to > .string "foobarbaz" > .zero 12 > It keeps emitting .string "" if there is just one zero or two zeros where > the first one is preceded by non-zeros, so we can have > .string "foobarbaz" > .string "" > or > .base64 "VG8gYmUgb3Igbm90IHRvIGJlLCB0aGF0IGlzIHRoZSBxdWVzdGlvbg==" > .string "" > but not 2 .string "" in a row. > > On a testcase I have with around 310440 0-255 unsigned char character > constants mostly derived from cc1plus start but with too long sequences of > 0s which broke transformation to STRING_CST adjusted to have at most 126 > consecutive 0s, I see: > 1504498 bytes long assembly without this patch on i686-linux (without > .base64 support in binutils) > 1155071 bytes long assembly with this patch on i686-linux (without .base64 > support in binutils) > 431390 bytes long assembly without this patch on x86_64-linux (with > .base64 support in binutils) > 427593 bytes long assembly with this patch on x86_64-linux (with .base64 > support in binutils) > All 4 assemble to identical *.o file when using x86_64-linux .base64 > supporting gas, and the former 2 when using older x86_64-linux gas assemble > to identical content as well. > > Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk? Ok. Is there a more general repeat byte op available? Richard > 2024-07-17 Jakub Jelinek <jakub@redhat.com> > > * varasm.cc (default_elf_asm_output_ascii): Use ASM_OUTPUT_SKIP instead > of 2 or more default_elf_asm_output_limited_string (f, "") calls and > adjust base64 heuristics correspondingly. > > --- gcc/varasm.cc.jj 2024-07-16 13:36:54.259748720 +0200 > +++ gcc/varasm.cc 2024-07-16 14:08:19.211753867 +0200 > @@ -8538,6 +8538,7 @@ default_elf_asm_output_ascii (FILE *f, c > if (s >= last_base64) > { > unsigned cnt = 0; > + unsigned char prev_c = ' '; > const char *t; > for (t = s; t < limit && (t - s) < (long) ELF_STRING_LIMIT - 1; t++) > { > @@ -8560,7 +8561,13 @@ default_elf_asm_output_ascii (FILE *f, c > break; > case 1: > if (c == 0) > - cnt += 2 + strlen (STRING_ASM_OP) + 1; > + { > + if (prev_c == 0 > + && t + 1 < limit > + && (t + 1 - s) < (long) ELF_STRING_LIMIT - 1) > + break; > + cnt += 2 + strlen (STRING_ASM_OP) + 1; > + } > else > cnt += 4; > break; > @@ -8568,6 +8575,7 @@ default_elf_asm_output_ascii (FILE *f, c > cnt += 2; > break; > } > + prev_c = c; > } > if (cnt > ((unsigned) (t - s) + 2) / 3 * 4 && (t - s) >= 3) > { > @@ -8633,8 +8641,18 @@ default_elf_asm_output_ascii (FILE *f, c > bytes_in_chunk = 0; > } > > - default_elf_asm_output_limited_string (f, s); > - s = p; > + if (p == s && p + 1 < limit && p[1] == '\0') > + { > + for (p = s + 2; p < limit && *p == '\0'; p++) > + continue; > + ASM_OUTPUT_SKIP (f, (unsigned HOST_WIDE_INT) (p - s)); > + s = p - 1; > + } > + else > + { > + default_elf_asm_output_limited_string (f, s); > + s = p; > + } > } > else > { > > Jakub >
On Wed, Jul 17, 2024 at 04:15:16PM +0200, Richard Biener wrote:
> Ok. Is there a more general repeat byte op available?
I think
.skip bytes, fill
but not sure what assemblers do support that, not sure it is that
common to have long sequences of non-zero identical bytes and
for '\0's we were already splitting the string into separate directives,
while say for
.string "fooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo"
we aren't.
The intent of this patch was just to get some common savings without
sacrifying readability too much.
Jakub
> Am 17.07.2024 um 16:45 schrieb Jakub Jelinek <jakub@redhat.com>: > > On Wed, Jul 17, 2024 at 04:15:16PM +0200, Richard Biener wrote: >> Ok. Is there a more general repeat byte op available? > > I think > .skip bytes, fill > but not sure what assemblers do support that, not sure it is that > common to have long sequences of non-zero identical bytes and > for '\0's we were already splitting the string into separate directives, > while say for > .string "fooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo" > we aren't. > The intent of this patch was just to get some common savings without > sacrifying readability too much. Fair enough, I was just wondering. Richard > Jakub >
--- gcc/varasm.cc.jj 2024-07-16 13:36:54.259748720 +0200 +++ gcc/varasm.cc 2024-07-16 14:08:19.211753867 +0200 @@ -8538,6 +8538,7 @@ default_elf_asm_output_ascii (FILE *f, c if (s >= last_base64) { unsigned cnt = 0; + unsigned char prev_c = ' '; const char *t; for (t = s; t < limit && (t - s) < (long) ELF_STRING_LIMIT - 1; t++) { @@ -8560,7 +8561,13 @@ default_elf_asm_output_ascii (FILE *f, c break; case 1: if (c == 0) - cnt += 2 + strlen (STRING_ASM_OP) + 1; + { + if (prev_c == 0 + && t + 1 < limit + && (t + 1 - s) < (long) ELF_STRING_LIMIT - 1) + break; + cnt += 2 + strlen (STRING_ASM_OP) + 1; + } else cnt += 4; break; @@ -8568,6 +8575,7 @@ default_elf_asm_output_ascii (FILE *f, c cnt += 2; break; } + prev_c = c; } if (cnt > ((unsigned) (t - s) + 2) / 3 * 4 && (t - s) >= 3) { @@ -8633,8 +8641,18 @@ default_elf_asm_output_ascii (FILE *f, c bytes_in_chunk = 0; } - default_elf_asm_output_limited_string (f, s); - s = p; + if (p == s && p + 1 < limit && p[1] == '\0') + { + for (p = s + 2; p < limit && *p == '\0'; p++) + continue; + ASM_OUTPUT_SKIP (f, (unsigned HOST_WIDE_INT) (p - s)); + s = p - 1; + } + else + { + default_elf_asm_output_limited_string (f, s); + s = p; + } } else {