Richard Biener <rguent...@suse.de> writes:
> 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 <rguent...@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).
That's also the intention behind maybe_ne though. The point is that
-1 isn't really a number on a scale [M, N] (M < -1, N > -1). It's a
just a C way of representing "nothing" in an "X or nothing" (std::optional).
So whether an offset happens to be numerically equal to -1 at runtime
isn't relevant. A runtime off is an X in the "X or nothing" and so
needs to be treated in the same was as other Xs.
To put it another way: known_ne is harder to prove than maybe_ne.
known_ne must return false if it can't prove the arguments are
different for all possible combinations of indeterminates.
maybe_ne is instead the direct opposite of known_eq.
Another analogy might be: suppose that we used a -1 INTEGER_CST
as a special marker. (Not a good choice, but bear with me.)
If we wanted to test whether a tree was this special marker,
we'd use integer_minus_onep. To test that it isn't the marker
we'd use !integer_minus_onep. known_ne would instead be the
equivalent of:
integer_zerop (fold_build2 (boolean_type_node, NE_EXPR,
x, ...-1 node...))
and so a general x would be treated in the same way as -1.
> Yes, -1 is a special
> marker but still. Practically of course
> known_ne (poly, integer constant) == maybe_ne (poly, integer_constant)?
Not in general. E.g. if VL is the SVE vector length in bytes:
maybe_ne (VL, 16)
is true (VL can be [1,16]*16) but:
known_ne (VL, 16)
is false (VL might be 16 but might not).
> That is, if we'd have a poly-int that might be -1 we should still
> not use that as 'off'?
known_ne might be safe in this particular instance if no runtime offset
would ever evaluate to -1. But IMO it's harder to reason about and
less obviously safe.
Thanks,
Richard
> 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. */
>>
