On May 8, 2015 11:58:18 PM GMT+02:00, Jan Hubicka <hubi...@ucw.cz> wrote: >> On Fri, May 08, 2015 at 01:09:52PM -0600, Jeff Law wrote: >> > On 05/08/2015 01:07 PM, David Edelsohn wrote: >> > >>>>Steve Ellcey wrote: >> > >> >> > >>After your change GCC sees that the code for f1 and f2 are >identical >> > >>so it replaced the body of f2 with a call to f1. This >optimization will >> > >>save space but it is not going to be faster because any call to >f2 will >> > >>now include an extra call/return. Do other platforms have this >same issue >> > >>or is there a way to make f2 an alias for f1 on other targets so >no extra >> > >>call is needed? I looked around to see if there was a target >function or >> > >>macro that is used to make one function an alias of another but I >didn't >> > >>see anything. >> > > >> > >This probably is not good for POWER, and probably not for ARM, in >fact >> > >probably not good for most RISC architectures. >> > But isn't this code going to be creating aliases when the output >format >> > supports them -- which avoids the call/return overheads? >> > >> > Which might argue that if we don't have aliases, then ICF might >need to be a >> > -Os thing. >> >> ICF uses aliases only when possible (when it can prove that the >callers >> don't care about function address equality/non-equality. In other >cases >> ICF uses caller redirection (if it is possible to redirect all >callers and >> remove unused other call), or thunks (otherwise). >> >> Thunks indeed can slow things down somewhat, especially if they can't >tail >> call the function. > >Yep, the idea here is that inliner should undo the splitting in most >cases >where the hot path performance matters. >For GCC 6 we plan to work bit on this and try to chose thunks to point >in right >direction. It also may be possible to arrange backend to output those >thunks >as simple nops just ahead of the target functions (and also to output >thunks >there without the redundant jump instructions)
... which will hopefully address https://gcc.gnu.org/PR65534 thanks,
