https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102294
Bug ID: 102294 Summary: structure assignment slower than memberwise initialization Product: gcc Version: 11.2.1 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c Assignee: unassigned at gcc dot gnu.org Reporter: bart.vanassche at gmail dot com Target Milestone: --- Created attachment 51444 --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=51444&action=edit Test program that illustrates the issue The output of the attached test program is as follows for an Intel Core i7-4790 CPU (3.6 GHz) when compiled with -O2: $ ~/test/bio_init Elapsed time: 0.874763 s Elapsed time: 0.480335 s Elapsed time: 0.733273 s The above output shows that bio_init2() runs faster than bio_init3() and that bio_init3() runs faster than bio_init1(). bio_init3() uses structure assignment to initialize struct bio while bio_init2() uses memberwise initialization. bio_init1() uses memset(). To me it was a big surprise to see that bio_init3() is slower than bio_init2(). Apparently clang generates better code: $ clang -O2 -o bio_init-clang bio_init.c $ ./bio_init-clang Elapsed time: 0.446804 s Elapsed time: 0.455009 s Elapsed time: 0.407392 s Can gcc be modified such that bio_init3() runs at least as fast as bio_init2()? The bio_init[123]() source code comes from the Linux kernel. Optimization level -O2 has been chosen because that is what the Linux kernel uses.