Hey.
Am 25.03.2019 um 13:36 schrieb Moritz Strübe:
> I have the following code:
> ----------
> #include <stdint.h>
> #ifdef UN
> #define UNLIKELY(x) __builtin_expect((x),0)
> #else
> #define UNLIKELY(x) (x)
> #endif
>
> float a = 66;
>
> int test (float b, int test) {
> if(UNLIKELY(test)) {
> return b / a;
> } else {
> return b * a;
> }
> }
> ----------
> "gcc -O2" reorders the code depending on a passed -DUN, while -Os always
> produces the same output (seehttps://godbolt.org/z/cL-Pbg)
Actually that is wrong. I was diverted by looking for reorderings. When
compiling with -Os -DUN there is an additional, and unnecessary, "movaps
%xmm0, %xmm1". (I seen the same thing on arm, as it is more visible
there: https://bugs.launchpad.net/gcc-arm-embedded/+bug/1821703)
I also had a quick look at the output of -fdump-rtl-bbro, as suggested
by Martin, but I'm afraid I'll need a week of digging into the specifics
before I can interpret it.
Cheers
Morty
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