Date: Mon, 5 Jun 2017 13:03:24 +0300 From: Konstantin Belousov <kostikbel@gmail.com> To: Eric van Gyzen <vangyzen@FreeBSD.org> Cc: FreeBSD Hackers <freebsd-hackers@freebsd.org>, toolchain@freebsd.org Subject: Re: Function attribute for optimization level Message-ID: <20170605100324.GV82323@kib.kiev.ua> In-Reply-To: <85c47390-dd27-aa74-24fe-25a9a5352527@FreeBSD.org> References: <85c47390-dd27-aa74-24fe-25a9a5352527@FreeBSD.org>
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I think that toolchain@ is more suitable list for the discussion. On Sun, Jun 04, 2017 at 05:44:31PM -0500, Eric van Gyzen wrote: > _thr_rtld_init() calls memcpy() for the sole purpose of resolving its > PLT entry. With clang 4.0 and the current code, compiler optimization > defeats this attempt by completely eliding the call. Other compilers > or code might emit inline instructions instead of the library call, > also defeating the purpose. After looking more closely at the whole situation, I have a question that we probably must answer first. Is clang -ffreestanding mode broken ? memcpy(3) is not included into the set of the environment features required for a C11 freestanding implementation, and clang pretending that it knows the semantic of the call sounds broken. > > I propose adding "__no_optimization" to sys/cdefs.h. The patch is > below. Empirical testing shows that clang 3.7 and later support > "optnone", and gcc 4.6 and later support "optimize()". Clang 3.4 does > not support either, so it takes the define-to-empty case. I did not > test clang 3.5 or 3.6. Where this attribute should be applied ? To the _thr_rtld_init() function ? > > Side note: GCC 4.6 with optimize(0) on amd64 emits two movq > instructions for memset(x,0,16), but GCC 5 emits a call to memset(). > > I have done no research to see if other popular codebases have such a > definition. If you know of one, please tell me; I would gladly adopt > an already common name for this proposal, for the sake of portability. > > Thanks in advance for your feedback. > > Eric > > > diff --git a/sys/sys/cdefs.h b/sys/sys/cdefs.h index > 9cdc03c861cb..e370f6d6459e 100644 --- a/sys/sys/cdefs.h +++ > b/sys/sys/cdefs.h @@ -396,6 +396,14 @@ #define __unreachable() > ((void)0) #endif > > +#if __has_attribute(optnone) +#define __no_optimization > __attribute__((optnone)) +#elif __has_attribute(optimize) +#define > __no_optimization __attribute__((optimize(0))) +#else +#define > __no_optimization +#endif + /* XXX: should use `#if __STDC_VERSION__ < > 199901'. */ #if !__GNUC_PREREQ__(2, 7) && !defined(__INTEL_COMPILER) > #define __func__ NULL _______________________________________________ > freebsd-hackers@freebsd.org mailing list > https://lists.freebsd.org/mailman/listinfo/freebsd-hackers > To unsubscribe, send any mail to > "freebsd-hackers-unsubscribe@freebsd.org"
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