Date: Mon, 5 Jun 2017 14:09:11 +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: <20170605110911.GZ82323@kib.kiev.ua> In-Reply-To: <20170605100324.GV82323@kib.kiev.ua> References: <85c47390-dd27-aa74-24fe-25a9a5352527@FreeBSD.org> <20170605100324.GV82323@kib.kiev.ua>
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On Mon, Jun 05, 2017 at 01:03:24PM +0300, Konstantin Belousov wrote: > 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. Ok, I realized that I only added -ffreestanding to the rtld Makefile. So clang is optimizing correctly there. Should we compile both libc and libthr in the freestanding environment as well ? I am sure that there are a lot of similar assumptions that libc and libthr code calls into itself and not into the arbitrary re-implementation of the same code as generated by modern compilers. Then hopefully the __no_optimization hack is not needed. > > > > > 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" > _______________________________________________ > freebsd-toolchain@freebsd.org mailing list > https://lists.freebsd.org/mailman/listinfo/freebsd-toolchain > To unsubscribe, send any mail to "freebsd-toolchain-unsubscribe@freebsd.org"
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