On Wed, Aug 28, 2024 at 07:12:55PM +0200, Ard Biesheuvel wrote: > On Wed, 28 Aug 2024 at 18:24, Segher Boessenkool > <segher@xxxxxxxxxxxxxxxxxxx> wrote: > > > In my experience, this is likely to do the opposite: it causes the > > > compiler to 'forget' the semantics of memcpy() and memset(), so that > > > explicit trivial calls will no longer be elided and replaced with > > > plain loads and stores (as it can no longer guarantee the equivalence) > > > > No, the compiler will never forget those semantics. But if you tell it > > your function named memset() is not the actual standard memset -- via > > -fno-builtin-memset for example -- the compiler won't optimise things > > involving it quite as much. You told it so eh? > > That is exactly the point I am making. So how would this help in this case? I think we agree? :-) > > > This needs to be fixed for Clang as well, so throwing GCC specific > > > flags at it will at best be a partial solution. > > > > clang says it is a 100% plug-in replacement for GCC, so they will have > > to accept all GCC flags. And in many cases they do. Cases where they > > don't are bugs. > > Even if this were true, we support Clang versions today that do not > support -fno-tree-loop-distribute-patterns so my point stands. It is true. Yes, this cause problems for their users. > > > It is not a complete solution, unfortunately, and I guess there may be > > > other situations (compiler/arch combinations) where this might pop up > > > again. > > > > Why do mem* not work in VDSOs? Fix that, and all these problems > > disappear, and you do not need workrarounds :-) > > Good point. We should just import memcpy and memset in the VDSO ELF metadata. Yeah. In many cases GCC will replace such calls by (faster and/or smaller) inline code anyway, but when it does leave a call, there needs to be an external function implementing it :-) > Not sure about static binaries, though: do those even use the VDSO? With "static binary" people usually mean "a binary not using any DSOs", I think the VDSO is a DSO, also in this respect? As always, -static builds are *way* less problematic (and faster and smaller :-) ) Segher
