Re: [PATCH] tree-optimization/106131 - wrong code with FRE rewriting

[Richard Sandiford <richard.sandiford@arm.com>]

Richard Biener via Gcc-patches <gcc-patches@gcc.gnu.org> writes:
> The following makes sure to not use the original TBAA type for
> looking up a value across an aggregate copy when we had to offset
> the read.
>
> Bootstrapped and tested on x86_64-unknown-linux-gnu, pushed to trunk.
>
> 2022-06-30  Richard Biener  <rguenther@suse.de>
>
> 	PR tree-optimization/106131
> 	* tree-ssa-sccvn.cc (vn_reference_lookup_3): Force alias-set
> 	zero when offsetting the read looking through an aggregate
> 	copy.
>
> 	* g++.dg/torture/pr106131.C: New testcase.
> ---
>  gcc/testsuite/g++.dg/torture/pr106131.C | 34 +++++++++++++++++++++++++
>  gcc/tree-ssa-sccvn.cc                   | 16 +++++++++---
>  2 files changed, 46 insertions(+), 4 deletions(-)
>  create mode 100644 gcc/testsuite/g++.dg/torture/pr106131.C
>
> diff --git a/gcc/testsuite/g++.dg/torture/pr106131.C b/gcc/testsuite/g++.dg/torture/pr106131.C
> new file mode 100644
> index 00000000000..e110f4a8fe6
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/torture/pr106131.C
> @@ -0,0 +1,34 @@
> +// { dg-do run { target c++11 } }
> +
> +struct Pair {
> +    int a, b;
> +    Pair(const Pair &) = default;
> +    Pair(int _a, int _b) : a(_a), b(_b) {}
> +    Pair &operator=(const Pair &z) {
> +	a = z.a;
> +	b = z.b;
> +	return *this;
> +    }
> +};
> +
> +const int &max(const int &a, const int &b)
> +{
> +  return a < b ? b : a;
> +}
> +
> +int foo(Pair x, Pair y)
> +{
> +  return max(x.b, y.b);
> +}
> +
> +int main()
> +{
> +  auto f = new Pair[3] {{0, -11}, {0, -8}, {0, 2}};
> +  for (int i = 0; i < 1; i++) {
> +      f[i] = f[0];
> +      if(i == 0)
> +	f[i] = f[2];
> +      if (foo(f[i], f[1]) != 2)
> +	__builtin_abort();
> +  }
> +}
> diff --git a/gcc/tree-ssa-sccvn.cc b/gcc/tree-ssa-sccvn.cc
> index 9deedeac378..76d92895a3a 100644
> --- a/gcc/tree-ssa-sccvn.cc
> +++ b/gcc/tree-ssa-sccvn.cc
> @@ -3243,12 +3243,12 @@ vn_reference_lookup_3 (ao_ref *ref, tree vuse, void *data_,
>        poly_int64 extra_off = 0;
>        if (j == 0 && i >= 0
>  	  && lhs_ops[0].opcode == MEM_REF
> -	  && maybe_ne (lhs_ops[0].off, -1))
> +	  && known_ne (lhs_ops[0].off, -1))
>  	{
>  	  if (known_eq (lhs_ops[0].off, vr->operands[i].off))
>  	    i--, j--;
>  	  else if (vr->operands[i].opcode == MEM_REF
> -		   && maybe_ne (vr->operands[i].off, -1))
> +		   && known_ne (vr->operands[i].off, -1))

These changes don't look right.  If -1 is a special marker,
it should be tested with known_eq (positive) or maybe_ne (negative).

In other words, we should enter the if body if off is not the
compile-time constant -1.

Thanks,
Richard

>  	    {
>  	      extra_off = vr->operands[i].off - lhs_ops[0].off;
>  	      i--, j--;
> @@ -3275,6 +3275,7 @@ vn_reference_lookup_3 (ao_ref *ref, tree vuse, void *data_,
>        copy_reference_ops_from_ref (rhs1, &rhs);
>  
>        /* Apply an extra offset to the inner MEM_REF of the RHS.  */
> +      bool force_no_tbaa = false;
>        if (maybe_ne (extra_off, 0))
>  	{
>  	  if (rhs.length () < 2)
> @@ -3287,6 +3288,10 @@ vn_reference_lookup_3 (ao_ref *ref, tree vuse, void *data_,
>  	  rhs[ix].op0 = int_const_binop (PLUS_EXPR, rhs[ix].op0,
>  					 build_int_cst (TREE_TYPE (rhs[ix].op0),
>  							extra_off));
> +	  /* When we have offsetted the RHS, reading only parts of it,
> +	     we can no longer use the original TBAA type, force alias-set
> +	     zero.  */
> +	  force_no_tbaa = true;
>  	}
>  
>        /* Save the operands since we need to use the original ones for
> @@ -3339,8 +3344,11 @@ vn_reference_lookup_3 (ao_ref *ref, tree vuse, void *data_,
>        /* Adjust *ref from the new operands.  */
>        ao_ref rhs1_ref;
>        ao_ref_init (&rhs1_ref, rhs1);
> -      if (!ao_ref_init_from_vn_reference (&r, ao_ref_alias_set (&rhs1_ref),
> -					  ao_ref_base_alias_set (&rhs1_ref),
> +      if (!ao_ref_init_from_vn_reference (&r,
> +					  force_no_tbaa ? 0
> +					  : ao_ref_alias_set (&rhs1_ref),
> +					  force_no_tbaa ? 0
> +					  : ao_ref_base_alias_set (&rhs1_ref),
>  					  vr->type, vr->operands))
>  	return (void *)-1;
>        /* This can happen with bitfields.  */