https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88713
--- Comment #36 from H.J. Lu <hjl.tools at gmail dot com> ---
(In reply to Richard Biener from comment #34)
> GCC definitely fails to see the FMA use as opportunity in
> ix86_emit_swsqrtsf, the a == 0 checking is because of the missing
> expander w/o avx512er where we could still use the NR sequence
> with the other instruction. HJ?
Like this?
diff --git a/gcc/config/i386/i386.c b/gcc/config/i386/i386.c
index e0d7c74fcec..0bbe3772ab7 100644
--- a/gcc/config/i386/i386.c
+++ b/gcc/config/i386/i386.c
@@ -44855,14 +44855,22 @@ void ix86_emit_swsqrtsf (rtx res, rtx a, machine_mode
mode, bool recip)
}
}
+ mthree = force_reg (mode, mthree);
+
/* e0 = x0 * a */
emit_insn (gen_rtx_SET (e0, gen_rtx_MULT (mode, x0, a)));
- /* e1 = e0 * x0 */
- emit_insn (gen_rtx_SET (e1, gen_rtx_MULT (mode, e0, x0)));
- /* e2 = e1 - 3. */
- mthree = force_reg (mode, mthree);
- emit_insn (gen_rtx_SET (e2, gen_rtx_PLUS (mode, e1, mthree)));
+ if (TARGET_FMA || TARGET_AVX512F)
+ emit_insn (gen_rtx_SET (e2,
+ gen_rtx_FMA (mode, e0, x0, mthree)));
+ else
+ {
+ /* e1 = e0 * x0 */
+ emit_insn (gen_rtx_SET (e1, gen_rtx_MULT (mode, e0, x0)));
+
+ /* e2 = e1 - 3. */
+ emit_insn (gen_rtx_SET (e2, gen_rtx_PLUS (mode, e1, mthree)));
+ }
mhalf = force_reg (mode, mhalf);
if (recip)
