https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96789
--- Comment #22 from Hongtao.liu <crazylht at gmail dot com> --- >One of my workmates found that if we disable vectorization for SPEC2017 >>525.x264_r function sub4x4_dct in source file x264_src/common/dct.c with >?>explicit function attribute __attribute__((optimize("no-tree-vectorize"))), >it >can speed up by 4%. For CLX, if we disable slp vectorization in sub4x4_dct by __attribute__((optimize("no-tree-slp-vectorize"))), it can also speed up by 4%. > Thanks Richi! Should we take care of this case? or neglect this kind of > extension as "no instruction"? I was intent to handle it in target specific > code, but it isn't recorded into cost vector while it seems too heavy to do > the bb_info slp_instances revisits in finish_cost. For i386 backend unsigned char --> unsigned short is no "no instruction", but in this case --- 1033 _134 = MEM[(pixel *)pix1_295 + 2B]; 1034 _135 = (short unsigned int) _134; --- It could be combined and optimized to --- movzbl 19(%rcx), %r8d --- So, if "unsigned char" variable is loaded from memory, then the convertion would also be "no instruction", i'm not sure if backend cost model could handle such situation.