[Bug target/97127] FMA3 code transformation leads to slowdown on Skylake

[amonakov at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97127

--- Comment #16 from Alexander Monakov <amonakov at gcc dot gnu.org> ---
Mostly because prior to register allocation the compiler does not naturally see
that x = *mem + a*b will need an extra mov when both 'a' and 'b' are live (as
in that case registers allocated for them cannot be immediately reused for
'x').

(this is why RTL combine merges the two instructions, but on the isolated
testcase below we already have the combined form thanks to tree-TER)

Isolated testcase, by the way (gcc -O2 -mfma):

double f(double t, double a, double b, double c[], long i)
{
    t = __builtin_fma(a, b, c[i]);
    asm("" :: "x"(t), "x"(a), "x"(b));
    return t;
}