On 2020-08-18 13:58, Nick Desaulniers wrote: > On Tue, Aug 18, 2020 at 1:27 PM Nick Desaulniers > <ndesaulniers@xxxxxxxxxx> wrote: >> >> 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. > > I do really like the idea of -fbuiltin-foo. For example, you'd specify: > > -ffreestanding -fbuiltin-bcmp > > as an example. `-ffreestanding` would opt you out of ALL libcall > optimizations, `-fbuiltin-bcmp` would then opt you back in to > transforms that produce bcmp. That way you're informing the compiler > more precisely about the environment you'd be targeting. It feels > symmetric to existing `-fno-` flags (clang makes -f vs -fno- pretty > easy when there is such symmetry). And it's already convention that > if you specify multiple conflicting compiler flags, then the latter > one specified "wins." In that sense, turning back on specific > libcalls after disabling the rest looks more ergonomic to me. > > Maybe Eli or David have thoughts on why that may or may not be as > ergonomic or possible to implement as I imagine? > I would prefer this to be a #pragma for a header file, rather than having a very long command line for everything... -hpa
