https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101705
Bug ID: 101705 Summary: Missed optimization opportunity when copying lots of bitfields Product: gcc Version: 11.1.1 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c Assignee: unassigned at gcc dot gnu.org Reporter: jengelh at inai dot de Target Milestone: --- Using gcc-11.1.1 [revision 62bbb113ae68a7e724255e17143520735bcb9ec9], I observe that gcc is able to recognize and combine lots of "structb->member = structa->member" assignments into SIMD instructions. However, this only works as long as "member" has exactly 8*n bits. It would appear gcc is not smart enough to consider smaller entities (if and when they add up to multiples of 8 or 64). Observed ======== [On x86_64] »gcc -O3 -c 1.c; gcc -O3 -c 2.c »objdump -d 1.o 0: f3 0f 6f 3e movdqu (%rsi),%xmm7 ... »objdump -d 2.o 0: 0f b6 16 movzbl (%rsi),%edx 3: 0f b6 07 movzbl (%rdi),%eax 6: 83 e2 01 and $0x1,%edx 9: 83 e0 fe and $0xfffffffe,%eax c: 09 d0 or %edx,%eax e: 88 07 mov %al,(%rdi) 10: 0f b6 16 movzbl (%rsi),%edx 13: 83 e0 fd and $0xfffffffd,%eax 16: 83 e2 02 and $0x2,%edx 19: 09 d0 or %edx,%eax 1b: 88 07 mov %al,(%rdi) ... Expected ======== Emit some movdqu/movups even for 2.c. Other info ========== gcc version 11.1.1 20210625 [revision 62bbb113ae68a7e724255e17143520735bcb9ec9] (SUSE Linux) & gcc version 7.5.0 (SUSE Linux)