On 5 Jun 2017, at 13:09, Konstantin Belousov <kostik...@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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