Date: Mon, 5 Jun 2017 13:57:01 +0200 From: Dimitry Andric <dim@FreeBSD.org> To: Konstantin Belousov <kostikbel@gmail.com> Cc: Eric van Gyzen <vangyzen@FreeBSD.org>, FreeBSD Hackers <freebsd-hackers@freebsd.org>, toolchain@freebsd.org Subject: Re: Function attribute for optimization level Message-ID: <CE19E497-E069-48BF-924C-30097F346C8F@FreeBSD.org> In-Reply-To: <20170605110911.GZ82323@kib.kiev.ua> References: <85c47390-dd27-aa74-24fe-25a9a5352527@FreeBSD.org> <20170605100324.GV82323@kib.kiev.ua> <20170605110911.GZ82323@kib.kiev.ua>
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On 5 Jun 2017, at 13:09, Konstantin Belousov <kostikbel@gmail.com> wrote:
>
> 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.
Like that, or use -fno-builtin:
$ cat test-memcpy.c
#include <string.h>
void foo(void)
{
long dummy = -1;
memcpy(&dummy, &dummy, sizeof(dummy));
}
$ cc -O2 -fno-builtin -fomit-frame-pointer -S test-memcpy.c -o -
.text
.file "test-memcpy.c"
.globl foo # -- Begin function foo
.p2align 4, 0x90
.type foo,@function
foo: # @foo
.cfi_startproc
# BB#0: # %entry
pushq %rax
.Lcfi0:
.cfi_def_cfa_offset 16
movq $-1, (%rsp)
movq %rsp, %rdi
movl $8, %edx
movq %rdi, %rsi
callq memcpy
popq %rax
retq
.Lfunc_end0:
.size foo, .Lfunc_end0-foo
.cfi_endproc
# -- End function
.ident "FreeBSD clang version 5.0.0 (trunk 304659) (based on LLVM 5.0.0svn)"
.section ".note.GNU-stack","",@progbits
-Dimitry
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