On Tue, Aug 18, 2020 at 1:24 PM Arvind Sankar <nivedita@xxxxxxxxxxxx> wrote: > > On Tue, Aug 18, 2020 at 12:13:22PM -0700, Linus Torvalds wrote: > > On Tue, Aug 18, 2020 at 12:03 PM H. Peter Anvin <hpa@xxxxxxxxx> wrote: > > > > > > I'm not saying "change the semantics", nor am I saying that playing > > > whack-a-mole *for a limited time* is unreasonable. But I would like to go back > > > to the compiler authors and get them to implement such a #pragma: "this > > > freestanding implementation *does* support *this specific library function*, > > > and you are free to call it." > > > > I'd much rather just see the library functions as builtins that always > > do the right thing (with the fallback being "just call the standard > > function"). > > > > IOW, there's nothing wrong with -ffreestanding if you then also have > > __builtin_memcpy() etc, and they do the sane compiler optimizations > > for memcpy(). > > > > What we want to avoid is the compiler making *assumptions* based on > > standard names, because we may implement some of those things > > differently. > > > > -ffreestanding as it stands today does have __builtin_memcpy and > friends. But you need to then use #define memcpy __builtin_memcpy etc, > which is messy and also doesn't fully express what you want. #pragma, or > even just allowing -fbuiltin-foo options would be useful. > > The two compilers have some peculiarities, which means you really can't > have functions with the same name that do something else if you want to > use builtins at all, and can also lead to missed optimizations. > > For eg, __builtin_strchr(s,'\0') can be optimized to strlen. gcc will > optimize it that way even if -ffreestanding is used (so strlen has to > mean strlen), while clang won't, so it misses a potential optimization. > This is admittedly a silly example, but you could imagine something like > strncpy being optimized to memcpy+memset if the source length was > previously computed. > > PS: clang optimizes sprintf, but doesn't provide __builtin_sprintf? https://bugs.llvm.org/show_bug.cgi?id=47224 -- Thanks, ~Nick Desaulniers
