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> * Inefficient instructions are replaced with more efficient instructions. For example gcc will for a simple x % 19 generate no less than 16 instructions inste

> * Inefficient instructions are replaced with more efficient instructions. For example gcc will for a simple x % 19 generate no less than 16 instructions instead of a single div/idiv. This is probably still faster, but it may still be detrimental if it's not in a hot path. It should be noted that gcc emits this even at -O0. Does it emit it at -Os ?

poizan42 10y ago
Curiously not. -O0: main: .LFB0: .cfi_startproc pushq %rbp .cfi_def_cfa_offset 16 .cfi_offset 6, -16 movq %rsp, %rbp .cfi_def_cfa_register 6 subq $16, %rsp movl %edi, -4(%rbp) movq %rsi, -16(%rbp) movl -4(%rbp), %ecx movl $1808407283, %edx movl %ecx, %eax imull %edx sarl $3, %edx movl %ecx, %eax sarl $31, %eax subl %eax, %edx movl %edx, %eax sall $3, %eax addl %edx, %eax addl %eax, %eax addl %edx, %eax subl %eax, %ecx movl %ecx, %edx movl %edx, %esi movl $.LC0, %edi movl $0, %eax call printf movl $0, %eax leave .cfi_def_cfa 7, 8 ret .cfi_endproc -Os: main: .LFB13: .cfi_startproc pushq %rax .cfi_def_cfa_offset 16 movl %edi, %eax movl $19, %ecx cltd movl $.LC0, %esi movl $1, %edi idivl %ecx xorl %eax, %eax call __printf_chk xorl %eax, %eax popq %rdx .cfi_def_cfa_offset 8 ret .cfi_endproc So it kinda performs an optimization when disabling all optimizations that it doesn't do when optimizing for size. Or well, the default codegen is the optimized version. Interesting.

poizan42 10y ago

Curiously not. -O0: main: .LFB0: .cfi_startproc pushq %rbp .cfi_def_cfa_offset 16 .cfi_offset 6, -16 movq %rsp, %rbp .cfi_def_cfa_register 6 subq $16, %rsp movl %edi, -4(%rbp) movq %rsi, -16(%rbp) movl -4(%rbp), %ecx movl $1808407283, %edx movl %ecx, %eax imull %edx sarl $3, %edx movl %ecx, %eax sarl $31, %eax subl %eax, %edx movl %edx, %eax sall $3, %eax addl %edx, %eax addl %eax, %eax addl %edx, %eax subl %eax, %ecx movl %ecx, %edx movl %edx, %esi movl $.LC0, %edi movl $0, %eax call printf movl $0, %eax leave .cfi_def_cfa 7, 8 ret .cfi_endproc -Os: main: .LFB13: .cfi_startproc pushq %rax .cfi_def_cfa_offset 16 movl %edi, %eax movl $19, %ecx cltd movl $.LC0, %esi movl $1, %edi idivl %ecx xorl %eax, %eax call __printf_chk xorl %eax, %eax popq %rdx .cfi_def_cfa_offset 8 ret .cfi_endproc So it kinda performs an optimization when disabling all optimizations that it doesn't do when optimizing for size. Or well, the default codegen is the optimized version. Interesting.