On Fri, Aug 21, 2026 at 4:30 AM Kyrylo Tkachov <[email protected]> wrote:
>
>
>
> > On 20 Aug 2026, at 21:32, Andrea Pinski <[email protected]>
> > wrote:
> >
> > On Thu, Aug 20, 2026 at 4:57 AM <[email protected]> wrote:
> >>
> >> From: Kyrylo Tkachov <[email protected]>
> >>
> >> A VECTOR_CST can encode a stepped series without storing every lane. The
> >> fold-const negate_expr_p checks only the encoded elements. It can
> >> therefore
> >> approve a signed vector whose implicit final lane is INT_MIN.
> >>
> >> typedef int v4si __attribute__ ((vector_size (16)));
> >>
> >> const v4si c = { 2147483645, 2147483646, 2147483647,
> >> (-2147483647 - 1) };
> >>
> >> v4si f (v4si x) { return (-x) - c; }
> >> v4si g (v4si x) { return -(x + c); }
> >> v4si h () { return -c; }
> >>
> >> The old predicate lets f and g negate c. It represents -INT_MIN as
> >> INT_MIN,
> >> which adds a false signed overflow for defined inputs. fold_negate_expr_1
> >> can also negate the encoded elements of c directly. This removes the
> >> required overflow diagnostic from h when overflow is instrumented.
> >>
> >> aarch64 -O2 -fsanitize=signed-integer-overflow
> >> -fsanitize-trap=signed-integer-overflow before:
> >>
> >> h:
> >> adrp x0, .LANCHOR0
> >> ldr q0, [x0, #:lo12:.LANCHOR0]
> >> ret
> >>
> >> After:
> >>
> >> h:
> >> brk #1000
> >>
> >> Reject a non-wrapping integral stepped vector in negate_expr_p. Also keep
> >> a
> >> direct stepped integral negation when sanitizer instrumentation makes
> >> overflow
> >> observable. Use vector-aware predicates directly. Keep the integral type
> >> check before TYPE_OVERFLOW_WRAPS because fixed-point vectors can reach the
> >> predicate, but TYPE_OVERFLOW_WRAPS does not accept them. The preceding
> >> patch
> >> makes TYPE_OVERFLOW_SANITIZED accept integral vectors.
> >>
> >> The two stepped checks have different conditions. negate_expr_p is a proof
> >> for moving a negation. Moving an implicit INT_MIN negation can introduce
> >> undefined overflow into f or g even when no overflow option is enabled, so
> >> that predicate rejects every non-wrapping integral stepped constant.
> >> fold_negate_expr_1 handles a direct source negation whose INT_MIN lane
> >> already
> >> overflows. GCC normally folds that constant overflow, and -ftrapv does not
> >> trap vector constant negation. Only sanitizer instrumentation makes the
> >> missing operation observable, so the direct-fold check is conditional on
> >> TYPE_OVERFLOW_SANITIZED.
> >>
> >> The test checks that f and g do not report overflow for defined inputs. It
> >> also checks that h reports its real INT_MIN negation.
> >>
> >> Bootstrapped and tested on aarch64-none-linux-gnu and x86_64-pc-linux-gnu.
> >> Ok for trunk?
> >> Thanks,
> >> Kyrill
> >>
> >> gcc/ChangeLog:
> >>
> >> * fold-const.cc (negate_expr_p): Reject non-wrapping stepped vector
> >> constants.
> >> (fold_negate_expr_1): Preserve sanitized stepped integral negations.
> >>
> >> gcc/testsuite/ChangeLog:
> >>
> >> * g++.dg/ubsan/fold-negate-vector-1.C: New test.
> >>
> >> Signed-off-by: Kyrylo Tkachov <[email protected]>
> >> ---
> >> gcc/fold-const.cc | 12 ++++-
> >> .../g++.dg/ubsan/fold-negate-vector-1.C | 46 +++++++++++++++++++
> >> 2 files changed, 56 insertions(+), 2 deletions(-)
> >> create mode 100644 gcc/testsuite/g++.dg/ubsan/fold-negate-vector-1.C
> >>
> >> diff --git a/gcc/fold-const.cc b/gcc/fold-const.cc
> >> index 420e3185a2a..b2fc63ce5aa 100644
> >> --- a/gcc/fold-const.cc
> >> +++ b/gcc/fold-const.cc
> >> @@ -406,10 +406,14 @@ negate_expr_p (tree t)
> >>
> >> case VECTOR_CST:
> >> {
> >> - if (FLOAT_TYPE_P (TREE_TYPE (type)) || TYPE_OVERFLOW_WRAPS (type))
> >> + if (FLOAT_TYPE_P (type)
> >> + || (ANY_INTEGRAL_TYPE_P (type) && TYPE_OVERFLOW_WRAPS (type)))
> >> return true;
> >
> > I think you don't need to change this part any more. Otherwise ok.
>
> I think we still need it. A VECTOR_CST can also contain fixed-point lanes.
> Such a vector is not ANY_INTEGRAL_TYPE_P.
> e.g. typedef _Fract v2hq __attribute__ ((vector_size (4)))
Hmm, I always forget about fixed point since they are conditional
types in the first place (only arm, mips and avr enables them; I
wonder if we can enable them in a more major target).
It would be a good idea to add many more fixed point testcases because
I noticed they are lacking and folks forget about them too.
> Thanks,
> Kyrill
>
> >
> >>
> >> - /* Steps don't prevent negation. */
> >> + /* An implicit element of a stepped vector can be the minimum
> >> + value. */
> >> + if (VECTOR_CST_STEPPED_P (t))
> >> + return false;
> >> unsigned int count = vector_cst_encoded_nelts (t);
> >> for (unsigned int i = 0; i < count; ++i)
> >> if (!negate_expr_p (VECTOR_CST_ENCODED_ELT (t, i)))
> >> @@ -566,6 +570,10 @@ fold_negate_expr_1 (location_t loc, tree t)
> >>
> >> case VECTOR_CST:
> >> {
> >> + if (VECTOR_CST_STEPPED_P (t)
> >> + && TYPE_OVERFLOW_SANITIZED (type))
> >> + return NULL_TREE;
> >> +
> >> tree_vector_builder elts;
> >> elts.new_unary_operation (type, t, true);
> >> unsigned int count = elts.encoded_nelts ();
> >> diff --git a/gcc/testsuite/g++.dg/ubsan/fold-negate-vector-1.C
> >> b/gcc/testsuite/g++.dg/ubsan/fold-negate-vector-1.C
> >> new file mode 100644
> >> index 00000000000..cda0fd9817f
> >> --- /dev/null
> >> +++ b/gcc/testsuite/g++.dg/ubsan/fold-negate-vector-1.C
> >> @@ -0,0 +1,46 @@
> >> +// { dg-do run { target int32 } }
> >> +// { dg-options "-O2 -Wno-psabi -fsanitize=signed-integer-overflow" }
> >> +
> >> +#define INT_MAX __INT_MAX__
> >> +#define INT_MIN (-INT_MAX - 1)
> >> +
> >> +typedef int v4si __attribute__ ((vector_size (16)));
> >> +
> >> +const v4si c = { INT_MAX - 2, INT_MAX - 1, INT_MAX, INT_MIN };
> >> +
> >> +v4si __attribute__ ((noipa))
> >> +f (v4si x)
> >> +{
> >> + return (-x) - c;
> >> +}
> >> +
> >> +v4si __attribute__ ((noipa))
> >> +g (v4si x)
> >> +{
> >> + return -(x + c);
> >> +}
> >> +
> >> +v4si __attribute__ ((noipa))
> >> +h ()
> >> +{
> >> + return -c;
> >> +}
> >> +
> >> +int
> >> +main ()
> >> +{
> >> + v4si x = { 0, 0, 0, 1 };
> >> + v4si y = f (x);
> >> + if (y[3] != INT_MAX)
> >> + __builtin_abort ();
> >> +
> >> + y = g (x);
> >> + if (y[3] != INT_MAX)
> >> + __builtin_abort ();
> >> +
> >> + volatile v4si z = h ();
> >> + if (z[3] != INT_MIN)
> >> + __builtin_abort ();
> >> +}
> >> +
> >> +// { dg-output "negation of -2147483648 cannot be represented in type
> >> 'int'; cast to an unsigned type to negate this value to itself" }
> >> --
> >> 2.50.1 (Apple Git-155)
> >>
>
