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math-opts: Only build FMA when use is in addends. [PR123940]

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When looking for an FMA opportunity we can find a multiplication use in an
else operand:

  vect_pretmp_50.23_121 = MEM <vector(2) charD.2> [(charD.2 *)&dD.2916 + 14B];
  vect__28.25_123 = vect_pretmp_50.23_121 * { 2, 2 };
  vect_patt_99.26_124 = .COND_ADD ({ -1, -1 }, vect_pretmp_50.23_121, { 14, 15 }, vect__28.25_123);

and build it:
  vect_pretmp_50.23_121 = MEM <vector(2) charD.2> [(charD.2 *)&dD.2916 + 14B];
  vect_patt_99.26_124 = .FMA (vect_pretmp_50.23_121, { 2, 2 }, vect_pretmp_50.23_121);

This patch checks if the use is in one of the addends.

	PR tree-optimization/123940

gcc/ChangeLog:

	* tree-ssa-math-opts.cc (convert_mult_to_fma): Check
	multiplication result is an addend.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/pr123940.c: New test.

Signed-off-by: Robin Dapp <rdapp@oss.qualcomm.com>
This commit is contained in:
Robin Dapp
2026-02-03 21:24:33 +01:00
parent bcb3b1eef1
commit bd67b5a8c9
2 changed files with 30 additions and 0 deletions
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25
gcc/testsuite/gcc.target/riscv/rvv/autovec/pr123940.c Normal file
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@@ -0,0 +1,25 @@
/* { dg-do run } */
/* { dg-options "-O3 -march=rv64gcv -mabi=lp64d -std=gnu99 -fdump-tree-widening_mul" } */
long a;
long long b;
_Bool c[16];
char d[16];
char e = 0;
int f = 1;
int main ()
{
for (long i = 0; i < 16; ++i)
c[i] = 40;
for (int j = 0; j < 16; j++)
{
e = (c[j] ? j : d[j]) + d[j];
a = f * c[j] ?: ~0;
}
b = (int) e;
if (b != 15)
__builtin_abort ();
}
/* { dg-final { scan-tree-dump-not "FMA" "widening_mul" } } */

5
gcc/tree-ssa-math-opts.cc
View File

@@ -3481,6 +3481,7 @@ convert_mult_to_fma (gimple *mul_stmt, tree op1, tree op2,
use_operand_p tmp_use_p;
if (single_imm_use (cast_lhs, &tmp_use_p, &tmp_use))
use_stmt = tmp_use;
result = cast_lhs;
}
/* For now restrict this operations to single basic blocks. In theory
@@ -3535,6 +3536,10 @@ convert_mult_to_fma (gimple *mul_stmt, tree op1, tree op2,
&else_value, &len, &bias))
return false;
/* The multiplication result must be one of the addition operands. */
if (ops[0] != result && ops[1] != result)
return false;
switch (code)
{
case MINUS_EXPR: