[Bug c++/96535] [10/11 Regression] GCC 10 ignoring function __attribute__ optimize for all x86 since r11-1019

[danielhanchen at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96535

--- Comment #3 from Daniel Han-Chen <danielhanchen at gmail dot com> ---
Oh lolll I was just about to add a comment about further experimentation

Seems like Jakub and Hongtao have found the root cause of the issues?

Anyways what I was gonna write [probs not necessary anymore so no need to read]

"""
Anyways from more experimentation, it seems like O1, O2, O3 are not ignored,
but the unrolling only gets turned on via O3. So if one passes O1, O2 in
__attribute__, but the command line is O3, the function still unrolls.

For eg, when commandline is O3, in GCC 9, __attribute__((optimize("O1 / 2"))
causes code to use VMULPS and VADDPS with an unroll factor of 1.

However in GCC 10.x, when the commandline is O3, VMULPS and VADDPS is used
(optimize("O1/2")), however, unrolling is still done??? Passing
"no-unroll-loops" in attribute also does not work.

It seems like the commandline O3 overrides unrolling or something? The
resulting assembly does use VMULPS/VADDPS and not VFMADDPS for O1/O2, but O3
causes an unrolling factor of 6 or so [it should be 1]

https://gcc.godbolt.org/z/qb3d5M for new example.
"""