ipa-sra: Fix thinko when overriding safe_to_import_accesses (PR 101066)

Hi,

The "new" IPA-SRA has a more difficult job than the previous
not-truly-IPA version when identifying situations in which a parameter
passed by reference can be passed into a third function and only thee
converted to one passed by value (and possibly "split" at the same
time).

In order to allow this, two conditions must be fulfilled.  First the
call to the third function must happen before any modifications of
memory, because it could change the value passed by reference.
Second, in order to make sure we do not introduce new (invalid)
dereferences, the call must postdominate the entry BB.

The second condition is actually not necessary if the caller function
is also certain to dereference the pointer but the first one must
still hold.  Unfortunately, the code making this overriding decision
also happen to trigger when the first condition is not fulfilled.
This is fixed in the following patch.

Bootstrapped, LTO-bootstrapped and tested on x86_64-linux, OK for trunk
and the gcc-11 branch?  On gcc-10, I might just remove the override
altogether, the case might not be important enough to change LTO format.

Thanks,

Martin



gcc/ChangeLog:

2021-06-16  Martin Jambor  <mjambor@suse.cz>

	PR ipa/101066
	* ipa-sra.c (class isra_call_summary): New member
	m_before_any_store, initialize it in the constructor.
	(isra_call_summary::dump): Dump the new field.
	(ipa_sra_call_summaries::duplicate): Copy it.
	(process_scan_results): Set it.
	(isra_write_edge_summary): Stream it.
	(isra_read_edge_summary): Likewise.
	(param_splitting_across_edge): Only override
	safe_to_import_accesses if m_before_any_store is set.

gcc/testsuite/ChangeLog:

2021-06-16  Martin Jambor  <mjambor@suse.cz>

	PR ipa/101066
	* gcc.dg/ipa/pr101066.c: New test.
---
 gcc/ipa-sra.c                       | 15 +++++++++++++--
 gcc/testsuite/gcc.dg/ipa/pr101066.c | 20 ++++++++++++++++++++
 2 files changed, 33 insertions(+), 2 deletions(-)
 create mode 100644 gcc/testsuite/gcc.dg/ipa/pr101066.c

Ping.

On Mon, Jun 21 2021, Martin Jambor wrote:
> Hi,
>
> The "new" IPA-SRA has a more difficult job than the previous
> not-truly-IPA version when identifying situations in which a parameter
> passed by reference can be passed into a third function and only thee
> converted to one passed by value (and possibly "split" at the same
> time).
>
> In order to allow this, two conditions must be fulfilled.  First the
> call to the third function must happen before any modifications of
> memory, because it could change the value passed by reference.
> Second, in order to make sure we do not introduce new (invalid)
> dereferences, the call must postdominate the entry BB.
>
> The second condition is actually not necessary if the caller function
> is also certain to dereference the pointer but the first one must
> still hold.  Unfortunately, the code making this overriding decision
> also happen to trigger when the first condition is not fulfilled.
> This is fixed in the following patch.
>
> Bootstrapped, LTO-bootstrapped and tested on x86_64-linux, OK for trunk
> and the gcc-11 branch?  On gcc-10, I might just remove the override
> altogether, the case might not be important enough to change LTO format.
>
> Thanks,
>
> Martin
>
>
>
> gcc/ChangeLog:
>
> 2021-06-16  Martin Jambor  <mjambor@suse.cz>
>
> 	PR ipa/101066
> 	* ipa-sra.c (class isra_call_summary): New member
> 	m_before_any_store, initialize it in the constructor.
> 	(isra_call_summary::dump): Dump the new field.
> 	(ipa_sra_call_summaries::duplicate): Copy it.
> 	(process_scan_results): Set it.
> 	(isra_write_edge_summary): Stream it.
> 	(isra_read_edge_summary): Likewise.
> 	(param_splitting_across_edge): Only override
> 	safe_to_import_accesses if m_before_any_store is set.
>
> gcc/testsuite/ChangeLog:
>
> 2021-06-16  Martin Jambor  <mjambor@suse.cz>
>
> 	PR ipa/101066
> 	* gcc.dg/ipa/pr101066.c: New test.
> ---
>  gcc/ipa-sra.c                       | 15 +++++++++++++--
>  gcc/testsuite/gcc.dg/ipa/pr101066.c | 20 ++++++++++++++++++++
>  2 files changed, 33 insertions(+), 2 deletions(-)
>  create mode 100644 gcc/testsuite/gcc.dg/ipa/pr101066.c
>
> diff --git a/gcc/ipa-sra.c b/gcc/ipa-sra.c
> index 3272daf56e4..965e246d788 100644
> --- a/gcc/ipa-sra.c
> +++ b/gcc/ipa-sra.c
> @@ -343,7 +343,7 @@ class isra_call_summary
>  public:
>    isra_call_summary ()
>      : m_arg_flow (), m_return_ignored (false), m_return_returned (false),
> -      m_bit_aligned_arg (false)
> +      m_bit_aligned_arg (false), m_before_any_store (false)
>    {}
>  
>    void init_inputs (unsigned arg_count);
> @@ -362,6 +362,10 @@ public:
>  
>    /* Set when any of the call arguments are not byte-aligned.  */
>    unsigned m_bit_aligned_arg : 1;
> +
> +  /* Set to true if the call happend before any (other) store to memory in the
> +     caller.  */
> +  unsigned m_before_any_store : 1;
>  };
>  
>  /* Class to manage function summaries.  */
> @@ -491,6 +495,8 @@ isra_call_summary::dump (FILE *f)
>      fprintf (f, "    return value ignored\n");
>    if (m_return_returned)
>      fprintf (f, "    return value used only to compute caller return value\n");
> +  if (m_before_any_store)
> +    fprintf (f, "    happens before any store to memory\n");
>    for (unsigned i = 0; i < m_arg_flow.length (); i++)
>      {
>        fprintf (f, "    Parameter %u:\n", i);
> @@ -535,6 +541,7 @@ ipa_sra_call_summaries::duplicate (cgraph_edge *, cgraph_edge *,
>    new_sum->m_return_ignored = old_sum->m_return_ignored;
>    new_sum->m_return_returned = old_sum->m_return_returned;
>    new_sum->m_bit_aligned_arg = old_sum->m_bit_aligned_arg;
> +  new_sum->m_before_any_store = old_sum->m_before_any_store;
>  }
>  
>  
> @@ -2374,6 +2381,7 @@ process_scan_results (cgraph_node *node, struct function *fun,
>  	unsigned count = gimple_call_num_args (call_stmt);
>  	isra_call_summary *csum = call_sums->get_create (cs);
>  	csum->init_inputs (count);
> +	csum->m_before_any_store = uses_memory_as_obtained;
>  	for (unsigned argidx = 0; argidx < count; argidx++)
>  	  {
>  	    if (!csum->m_arg_flow[argidx].pointer_pass_through)
> @@ -2546,6 +2554,7 @@ isra_write_edge_summary (output_block *ob, cgraph_edge *e)
>    bp_pack_value (&bp, csum->m_return_ignored, 1);
>    bp_pack_value (&bp, csum->m_return_returned, 1);
>    bp_pack_value (&bp, csum->m_bit_aligned_arg, 1);
> +  bp_pack_value (&bp, csum->m_before_any_store, 1);
>    streamer_write_bitpack (&bp);
>  }
>  
> @@ -2664,6 +2673,7 @@ isra_read_edge_summary (struct lto_input_block *ib, cgraph_edge *cs)
>    csum->m_return_ignored = bp_unpack_value (&bp, 1);
>    csum->m_return_returned = bp_unpack_value (&bp, 1);
>    csum->m_bit_aligned_arg = bp_unpack_value (&bp, 1);
> +  csum->m_before_any_store = bp_unpack_value (&bp, 1);
>  }
>  
>  /* Read intraprocedural analysis information about NODE and all of its outgoing
> @@ -3420,7 +3430,8 @@ param_splitting_across_edge (cgraph_edge *cs)
>  	    }
>  	  else if (!ipf->safe_to_import_accesses)
>  	    {
> -	      if (!all_callee_accesses_present_p (param_desc, arg_desc))
> +	      if (!csum->m_before_any_store
> +		  || !all_callee_accesses_present_p (param_desc, arg_desc))
>  		{
>  		  if (dump_file && (dump_flags & TDF_DETAILS))
>  		    fprintf (dump_file, "  %u->%u: cannot import accesses.\n",
> diff --git a/gcc/testsuite/gcc.dg/ipa/pr101066.c b/gcc/testsuite/gcc.dg/ipa/pr101066.c
> new file mode 100644
> index 00000000000..1ceb6e43136
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/ipa/pr101066.c
> @@ -0,0 +1,20 @@
> +/* { dg-do run } */
> +/* { dg-options "-Os -fno-ipa-cp -fno-inline" } */
> +
> +int a = 1, c, d, e;
> +int *b = &a;
> +static int g(int *h) {
> +  c = *h;
> +  return d;
> +}
> +static void f(int *h) {
> +  e = *h;
> +  *b = 0;
> +  g(h);
> +}
> +int main() {
> +  f(b);
> +  if (c)
> +    __builtin_abort();
> +  return 0;
> +}
> -- 
> 2.31.1

Hi,
> 2021-06-16  Martin Jambor  <mjambor@suse.cz>
> 
> 	PR ipa/101066
> 	* ipa-sra.c (class isra_call_summary): New member
> 	m_before_any_store, initialize it in the constructor.
> 	(isra_call_summary::dump): Dump the new field.
> 	(ipa_sra_call_summaries::duplicate): Copy it.
> 	(process_scan_results): Set it.
> 	(isra_write_edge_summary): Stream it.
> 	(isra_read_edge_summary): Likewise.
> 	(param_splitting_across_edge): Only override
> 	safe_to_import_accesses if m_before_any_store is set.
> 
> gcc/testsuite/ChangeLog:
> 
> 2021-06-16  Martin Jambor  <mjambor@suse.cz>
> 
> 	PR ipa/101066
> 	* gcc.dg/ipa/pr101066.c: New test.
OK, thanks!

The analysis disabling transformation on any memory store is overly
conservative.  We have pointer (which is a parameter and comes from
outer world) and no type infomration, however alias oracle will still be
able to disambiguate when memory access is to non-escaping local memory
or mallocated memory block etc.

Honza