> 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)))
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)
>>
