On Tue, Jul 6, 2021 at 7:31 AM Segher Boessenkool <seg...@kernel.crashing.org> wrote: > > Hi! > > I ran into this in shrink-wrap.c today. > > On Thu, Jun 03, 2021 at 02:54:07PM +0800, liuhongt via Gcc-patches wrote: > > Use "used" flag for CALL_INSN to indicate it's a fake call. If it's a > > fake call, it won't have its own function stack. > > Could you document somewhere what a "fake call" *is*? Including what > that means to RTL, how this is expected to be used, etc.? In rtl.h is fake call is used for TARGET_INSN_CALLEE_ABI, i'll add comments for #define FAKE_CALL_P(RTX) in rtl.h > fine with me, but as it is, no one can know when to use this. What does > "its own function stack" mean in the description here? You can only put > FAKE_CALL on functions that do not have a stack frame? But that is > never true on x86, so that cannot be it, unless there isn't a call > instruction at all? But then, why use an RTL call insn for this? > > Other targets simply do not use an RTL "call" when they want to hide > such an instruction, why can't you do that here, wouldn't that work much > better? There are many more insns that you may want to hide. The > traditional solution is to use unspecs, which very directly hides all > details.
It's explained here, > >> Yeah. Initially clobber_high seemed like the best appraoch for > >> handling the tlsdesc thing, but in practice it was too difficult > >> to shoe-horn the concept in after the fact, when so much rtl > >> infrastructure wasn't prepared to deal with it. The old support > >> didn't handle all cases and passes correctly, and handled others > >> suboptimally. > >> > >> I think it would be worth using the same approach as > >> https://gcc.gnu.org/legacy-ml/gcc-patches/2019-09/msg01466.html for > >> vzeroupper: represent the instructions as call_insns in which the > >> call has a special vzeroupper ABI. I think that's likely to lead > >> to better code than clobber_high would (or at least, it did for tlsdesc). refer to [1] for more details [1] https://gcc.gnu.org/pipermail/gcc-patches/2021-May/570265.html > > > Segher -- BR, Hongtao
