The "split one function to two” was handle by gcc "-freorder-blocks-and-partition” I don’t know this phase really handle __builtin_expect and [[unlikely]] ? We can ensure from the gcc source code :) > 2021年5月7日 上午4:49，Xi Ruoyao <xry111@xxxxxxxxxxxxxxxx> 写道： > > 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 >
