Nathan Chancellor <natechancellor@xxxxxxxxx> writes: > On Wed, Sep 04, 2019 at 08:01:35AM -0500, Segher Boessenkool wrote: >> On Wed, Sep 04, 2019 at 08:16:45AM +0000, David Laight wrote: >> > From: Nathan Chancellor [mailto:natechancellor@xxxxxxxxx] >> > > Fair enough so I guess we are back to just outright disabling the >> > > warning. >> > >> > Just disabling the warning won't stop the compiler generating code >> > that breaks a 'user' implementation of setjmp(). >> >> Yeah. I have a patch (will send in an hour or so) that enables the >> "returns_twice" attribute for setjmp (in <asm/setjmp.h>). In testing >> (with GCC trunk) it showed no difference in code generation, but >> better save than sorry. >> >> It also sets "noreturn" on longjmp, and that *does* help, it saves a >> hundred insns or so (all in xmon, no surprise there). >> >> I don't think this will make LLVM shut up about this though. And >> technically it is right: the C standard does say that in hosted mode >> setjmp is a reserved name and you need to include <setjmp.h> to access >> it (not <asm/setjmp.h>). > > It does not fix the warning, I tested your patch. > >> So why is the kernel compiled as hosted? Does adding -ffreestanding >> hurt anything? Is that actually supported on LLVM, on all relevant >> versions of it? Does it shut up the warning there (if not, that would >> be an LLVM bug)? > > It does fix this warning because -ffreestanding implies -fno-builtin, > which also solves the warning. LLVM has supported -ffreestanding since > at least 3.0.0. There are some parts of the kernel that are compiled > with this and it probably should be used in more places but it sounds > like there might be some good codegen improvements that are disabled > with it: > > https://lore.kernel.org/lkml/CAHk-=wi-epJZfBHDbKKDZ64us7WkF=LpUfhvYBmZSteO8Q0RAg@xxxxxxxxxxxxxx/ For xmon.c and crash.c I think using -ffreestanding would be fine. They're both crash/debug code, so we don't care about minor optimisation differences. If anything we don't want the compiler being too clever when generating that code. cheers
