https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113560
Roger Sayle <roger at nextmovesoftware dot com> changed:
What |Removed |Added
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CC| |roger at nextmovesoftware dot
com
--- Comment #2 from Roger Sayle <roger at nextmovesoftware dot com> ---
The costs look sane, and I'd expect the synth_mult generated sequence to be
faster, though it would be good to get some microbenchmarking.
A reduced test case is:
__int128 foo(__int128 x) { return x*100; }
The x86 backend thinks that a 128-bit (TImode) multiplication would take 14
cycles, so instead generates:
x2 = x+x 2 cycles
x3 = x2+x 2 cycles
x24 = x<<3 2 cycles
x25 = x24+x 2 cycles
x100 = x<<2 2 cycles
which is a total of 10 cycles, and predicted to be faster than the generic
implementation (requiring 2 IMULQ, 1 MULQ and 2 ADDQ) for
__int128 bar(__int128 x, int y) { return x*y; }
