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tree-optimization/110979 - fold-left reduction and partial vectors

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When we vectorize fold-left reductions with partial vectors but
no target operation available we use a vector conditional to force
excess elements to zero.  But that doesn't correctly preserve
the sign of zero.  The following patch disables partial vector
support when we have to do that and also need to honor rounding
modes other than round-to-nearest.  When round-to-nearest is in
effect and we have to preserve the sign of zero instead use
negative zero for the excess elements.

	PR tree-optimization/110979
	* tree-vect-loop.cc (vectorizable_reduction): For
	FOLD_LEFT_REDUCTION without target support make sure
	we don't need to honor signed zeros and sign dependent rounding.

	* gcc.dg/torture/pr110979.c: New testcase.
This commit is contained in:
Richard Biener 2023-08-11 13:00:17 +02:00
parent 3a13884b23
commit 798a880a0b
2 changed files with 48 additions and 1 deletions
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25
gcc/testsuite/gcc.dg/torture/pr110979.c Normal file
View File

@ -0,0 +1,25 @@
/* { dg-do run } */
/* { dg-additional-options "--param vect-partial-vector-usage=2" } */
#define FLT double
#define N 20
__attribute__((noipa))
FLT
foo3 (FLT *a)
{
FLT sum = -0.0;
for (int i = 0; i != N; i++)
sum += a[i];
return sum;
}
int main()
{
FLT a[N];
for (int i = 0; i != N; i++)
a[i] = -0.0;
if (!__builtin_signbit(foo3(a)))
__builtin_abort();
return 0;
}

24
gcc/tree-vect-loop.cc
View File

@ -6905,7 +6905,17 @@ vectorize_fold_left_reduction (loop_vec_info loop_vinfo,
tree vector_identity = NULL_TREE;
if (LOOP_VINFO_FULLY_MASKED_P (loop_vinfo))
vector_identity = build_zero_cst (vectype_out);
{
vector_identity = build_zero_cst (vectype_out);
if (!HONOR_SIGNED_ZEROS (vectype_out))
;
else
{
gcc_assert (!HONOR_SIGN_DEPENDENT_ROUNDING (vectype_out));
vector_identity = const_unop (NEGATE_EXPR, vectype_out,
vector_identity);
}
}
tree scalar_dest_var = vect_create_destination_var (scalar_dest, NULL);
int i;
@ -8037,6 +8047,18 @@ vectorizable_reduction (loop_vec_info loop_vinfo,
" no conditional operation is available.\n");
LOOP_VINFO_CAN_USE_PARTIAL_VECTORS_P (loop_vinfo) = false;
}
else if (reduction_type == FOLD_LEFT_REDUCTION
&& reduc_fn == IFN_LAST
&& FLOAT_TYPE_P (vectype_in)
&& HONOR_SIGNED_ZEROS (vectype_in)
&& HONOR_SIGN_DEPENDENT_ROUNDING (vectype_in))
{
if (dump_enabled_p ())
dump_printf_loc (MSG_MISSED_OPTIMIZATION, vect_location,
"can't operate on partial vectors because"
" signed zeros cannot be preserved.\n");
LOOP_VINFO_CAN_USE_PARTIAL_VECTORS_P (loop_vinfo) = false;
}
else
{
internal_fn mask_reduc_fn