[Bug target/90204] [8/9 Regression] C code is optimized worse than C++

crazylht at gmail dot com gcc-bugzilla@gcc.gnu.org
Wed Apr 24 03:12:00 GMT 2019


https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90204

--- Comment #2 from Hongtao.liu <crazylht at gmail dot com> ---
It seems such code generation is r254855's intention.

                /* Use 256-bit AVX instructions instead of 512-bit AVX
instructions
4695                  in the auto-vectorizer.  */
4696               if (ix86_tune_features[X86_TUNE_AVX256_OPTIMAL]
4697                   && !(opts_set->x_ix86_target_flags &
OPTION_MASK_PREFER_AVX256))
4698                 opts->x_ix86_target_flags |= OPTION_MASK_PREFER_AVX256;

I know there is a frequency reduction issue when many zmm registers are used,
but i don't know what exact situation did r254855 deal with?


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