https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58073
--- Comment #5 from dhowells at redhat dot com <dhowells at redhat dot com> --- There's a further potential optimisation. If shift is large enough that the bits under test are outside of the LSB, the TESTB changes to a TESTL at the same address: Shift 2: 0: f6 07 1c testb $0x1c,(%rdi) 3: 0f 94 c0 sete %al 6: c3 retq Shift 10: 0: f7 07 00 1c 00 00 testl $0x1c00,(%rdi) 6: 0f 94 c0 sete %al 9: c3 retq Shift 18: 0: f7 07 00 00 1c 00 testl $0x1c0000,(%rdi) 6: 0f 94 c0 sete %al 9: c3 retq However, one could do a TESTW or a TESTB instead with a smaller immediate value and a displaced address.
