On Fri, 2021-05-07 at 00:12 +0800, 陈云星 via Gcc-help wrote: > In my test : https://godbolt.org/z/87dWMejPP ;< > https://godbolt.org/z/87dWMejPP> > > It seems [[likely]] and [[unlikely]] just have same effective like > __builtin_expect; > > Will not split function into cold function and hot function ( which > __attribute__((cold)) on callee function can do so). In your test case, if the compiler split out a function for unlikely branch, there will be no performance gain at all. If the compiler just transform it into two separate functions blindly, the behavior will be a bug. A famous quote: > All optimizations are transformations, but not all transformations are > optimizations. Segher replied that GCC is already optimizing on this. You can run objdump -t on some "real-life" binaries (they are large enough so the optimization will really make some difference, for example /lib/libc.so.6) and find some functions with name "xxxx.cold", which is the "cold" function seperated from the normal one. -- Xi Ruoyao <xry111@xxxxxxxxxxxxxxxx> School of Aerospace Science and Technology, Xidian University
