https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102294
--- Comment #7 from Bart Van Assche <bart.vanassche at gmail dot com> --- Initializing small data structures via structure assignment is a common approach in the Linux kernel. This is the code gcc generates with the no-sse option applied: (gdb) disas bio_init3 Dump of assembler code for function bio_init3: 0x00000000004011b0 <+0>: mov %rdi,%r8 0x00000000004011b3 <+3>: mov $0xf,%ecx 0x00000000004011b8 <+8>: xor %eax,%eax 0x00000000004011ba <+10>: rep stos %rax,%es:(%rdi) 0x00000000004011bd <+13>: movl $0x1,0x20(%r8) 0x00000000004011c5 <+21>: mov %dx,0x62(%r8) 0x00000000004011ca <+26>: movl $0x1,0x64(%r8) 0x00000000004011d2 <+34>: mov %rsi,0x68(%r8) 0x00000000004011d6 <+38>: ret This is the code clang generates with the no-sse option applied: (gdb) disas bio_init3 Dump of assembler code for function bio_init3: 0x00000000004012c0 <+0>: movq $0x0,0x18(%rdi) 0x00000000004012c8 <+8>: movq $0x0,0x10(%rdi) 0x00000000004012d0 <+16>: movq $0x0,0x8(%rdi) 0x00000000004012d8 <+24>: movq $0x0,(%rdi) 0x00000000004012df <+31>: movl $0x1,0x20(%rdi) 0x00000000004012e6 <+38>: movq $0x0,0x24(%rdi) 0x00000000004012ee <+46>: movq $0x0,0x2c(%rdi) 0x00000000004012f6 <+54>: movq $0x0,0x34(%rdi) 0x00000000004012fe <+62>: movq $0x0,0x3c(%rdi) 0x0000000000401306 <+70>: movq $0x0,0x44(%rdi) 0x000000000040130e <+78>: movq $0x0,0x4c(%rdi) 0x0000000000401316 <+86>: movq $0x0,0x54(%rdi) 0x000000000040131e <+94>: movq $0x0,0x5a(%rdi) 0x0000000000401326 <+102>: mov %dx,0x62(%rdi) 0x000000000040132a <+106>: movl $0x1,0x64(%rdi) 0x0000000000401331 <+113>: mov %rsi,0x68(%rdi) 0x0000000000401335 <+117>: movq $0x0,0x70(%rdi) 0x000000000040133d <+125>: ret Is there any x86_64 CPU on which the latter code runs slower than the former?
