https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88713
--- Comment #19 from Chris Elrod <elrodc at gmail dot com> ---
To add a little more:
I used inline asm for direct access to the rsqrt instruction "vrsqrt14ps" in
Julia. Without adding a Newton step, the answers are wrong beyond just a couple
significant digits.
With the Newton step, the answers are correct.
My point is that LLVM-compiled code (Clang/Flang/ispc) are definitely adding
the Newton step. They get the correct answer.
That leaves my best guess for the performance difference as owing to the masked
"vrsqrt14ps" that gcc is using:
vcmpps $4, %zmm0, %zmm5, %k1
vrsqrt14ps %zmm0, %zmm1{%k1}{z}
Is there any way for me to test that idea?
Edit the asm to remove the vcmppss and mask, compile the asm with gcc, and
benchmark it?
