Missed Optimization: Missed Unlikely Optimization #167117
Description
look at this emample:
https://godbolt.org/z/jrrK8q3Ex
#include <stdbool.h> # define likely(x) __builtin_expect(!!(x), 1) # define unlikely(x) __builtin_expect(!!(x), 0) extern int xxx; void temp(void) { if (({ const bool __ret_do_once = unlikely(xxx != 2); // define unlikely here static bool __attribute__((__section__(".data..once"))) __already_done; if (__ret_do_once && !__already_done) __already_done = true; // -> unlikely path __ret_do_once; })) __asm__ volatile ("nop":::); // -> unlikely path }The assemble of clang is:
temp: # @temp .cfi_startproc movq xxx@GOTPCREL(%rip), %rax movl (%rax), %eax cmpl $2, %eax je .LBB0_3 // eax ==2 is likely, so here jmp likely testb $1, temp.__already_done(%rip) jne .LBB0_3 movb $1, temp.__already_done(%rip) jmp .LBB0_4 .LBB0_3: cmpl $2, %eax // eax is not changed, so jmp again je .LBB0_5 .LBB0_4: #APP nop #NO_APP .LBB0_5: retq The gcc assemble is :
temp: cmpl $2, xxx(%rip) jne .L5 // jump unlikely right ret // likely run here, return fast .L5: cmpb $0, __already_done.0(%rip) jne .L3 movb $1, __already_done.0(%rip) .L3: nop ret This results in all the WARN_ONCE branches in the Linux kernel source code being judged twice, causing significant performance loss:
https://elixir.bootlin.com/linux/v6.18-rc4/source/include/linux/once_lite.h#L19