On Fri, Sep 11, 2020 at 02:40:06PM -0500, Qing Zhao wrote: > > On Sep 11, 2020, at 12:13 PM, Segher Boessenkool > > <seg...@kernel.crashing.org> wrote: > > On Fri, Sep 11, 2020 at 11:52:29AM -0500, Qing Zhao wrote: > >> I don’t understand why it’s not correct if we clearing call-clobbered > >> registers > >> AFTER restoring call-preserved registers? > > > > Because the compiler backend (or the linker! Or the dynamic linker! > > Etc.) can use volatile registers for their own purposes. > > For the following sequence at the end of a routine: > > *...* > “restore call-preserved registers” > *clear call-clobbered registers"* > *ret* > > “Clear call-clobbered registers” will only clear the call-clobbered registers > that are not live at the end of the routine.
And they can be written again right after the routine, by linker- generated code for example. This is a waste. > In the new version of the patch, the implementation of clearing > call-clobbered registers is done in backend, middle end only > computes a hard register set based on user option, source attribute, data > flow information, and function abi information, and > Then pass this hard register set to the target hook to generate the clearing > sequence. The backend will have all the details > on the special situations you mentioned. > > Let me know any more concerns here. I cannot find that patch? Segher
