Date: Sun, 3 Apr 2016 12:50:28 -0500 From: Bobby Bingham <koorogi@...rogi.info> To: musl@...ts.openwall.com Subject: Re: [PATCH 2/2] add powerpc64 port On Sun, Apr 03, 2016 at 01:26:31PM -0400, Rich Felker wrote: > On Sun, Apr 03, 2016 at 12:10:44PM -0500, Bobby Bingham wrote: > > > > r2 is call-saved when calling to the local entry point, so setjmp needs > > > > to save it. > > > > > > OK, I see how this works for local calls to setjmp. But how does the > > > linker PLT magic work for setjmp? > > > > > > After the first return, the caller's stack slot where r2 was saved > > > belongs to the caller, and the compiler can clobber it. Upon the > > > > The ABI is very prescriptive about the layout of a stack frame. Each > > stack frame has several slots where callees are allowed to use part of > > their caller's frame. For example, the link register is saved to the > > caller's frame, not the callee's. > > > > For several of these slots, the ABI explicitly documents that they may > > be used as temporary storage which should be considered call-clobbered. > > For the slot used for saving the toc pointer (r2), the ABI makes no > > mention of it being available for temporary storage. It would be nice > > if it were more explicit here, but I believe the intent is that the > > compiler may not use this slot for any other purpose. > > My concern was not that the function itself could clobber it (although > I think it would be entitled to if it's no longer live, i.e. if no > code paths remain that reference its value) but that future function > calls might clobber it. However I think they all necessarily either > don't write to this slot at all, or write the same value that was > already there, so it's probably safe without longjmp having to restore > it. Yes, that's right. All function calls from a single caller will either not clobber that stack slot, or will write the same value to it. > > Rich
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