Hi, I observe a slowdown of some code compiled with gcc when I use -std=gnu18 instead of -std=c99. My computer is a i5-4590, and I use gcc version 11.3.0 (Debian 11.3.0-1). To reproduce: $ git clone https://gitlab.inria.fr/core-math/core-math.git $ cd core-math $ CORE_MATH_PERF_MODE=rdtsc CFLAGS="-O3 -march=native -ffinite-math-only -std=gnu18" ./perf.sh exp10f GNU libc version: 2.33 GNU libc release: release 31.746 11.780 $ CORE_MATH_PERF_MODE=rdtsc CFLAGS="-O3 -march=native -ffinite-math-only -std=c99" ./perf.sh exp10f GNU libc version: 2.33 GNU libc release: release 21.514 11.751 The difference is seen between the first figures in each run (31.746 and 21.514), which indicate the average number of cycles of the exp10f function from the core-math library. The code is very simple (a few dozen lines): https://gitlab.inria.fr/core-math/core-math/-/blob/master/src/binary32/exp10/exp10f.c Some more remarks: * this slowdown does not happen on all machines, for example it does not appear on an AMD EPYC 7282 with gcc gcc version 10.2.1 (Debian 10.2.1-6). * this slowdown disappears when I replace __builtin_expect(ex>(127+6), 0) by ex>(127+6) at line 45 of the code, however that branch is never taken in the above experiment. Does anyone have a clue? Best regards, Paul Zimmermann
