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

[Richard Biener <rguenther@suse.de>]

On Fri, 1 Jul 2022, Richard Sandiford wrote:

> 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.

Hmm, to me 'known_ne' was visually more correct (only if 'off' is
an actual offset we may treat it as such).  Yes, -1 is a special
marker but still.  Practically of course
known_ne (poly, integer constant) == maybe_ne (poly, integer_constant)?
That is, if we'd have a poly-int that might be -1 we should still
not use that as 'off'?

I can of course revert that hunk.

Thanks,
Richard.

> 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.  */
> 

-- 
Richard Biener <rguenther@suse.de>
SUSE Software Solutions Germany GmbH, Frankenstra