[cfe-dev] How to prevent insertion of memcpy()

[Chris Lattner]

On May 29, 2012, at 9:51 AM, Dmitry Vyukov wrote:

> Hi,
> 
> I have the following program:
> 
> // test.c
> #include <stdlib.h>
> struct foo_t {
>   int x[1024];
> };
> __thread struct foo_t g_foo;
> void bar(struct foo_t* foo) {
>   g_foo = *foo;
> }
> int main() {
>   struct foo_t* f = (struct foo_t*)malloc(sizeof(struct foo_t));
>   bar(f);
>   return 0;
> }
> 
> When I compile it with clang I see that it inserts memcpy() in function bar():

Hi Dmitry,

Short answer: you can't.  Freestanding implementations are required to provide memcpy.

-Chris

> 
> $ clang -v
> clang version 3.2 (trunk 157390)
> Target: x86_64-unknown-linux-gnu
> Thread model: posix
> $ clang test.c -g && objdump -dCS a.out
> 
> void bar(struct foo_t* foo) {
>   4005b0:       55                      push   %rbp
>   4005b1:       48 89 e5                mov    %rsp,%rbp
>   4005b4:       48 83 ec 10             sub    $0x10,%rsp
>   4005b8:       48 89 7d f8             mov    %rdi,-0x8(%rbp)
>   g_foo = *foo;
>   4005bc:       48 8b 7d f8             mov    -0x8(%rbp),%rdi
>   4005c0:       64 48 8b 04 25 00 00    mov    %fs:0x0,%rax
>   4005c7:       00 00 
>   4005c9:       48 8d 80 00 f0 ff ff    lea    -0x1000(%rax),%rax
>   4005d0:       ba 00 10 00 00          mov    $0x1000,%edx
>   4005d5:       48 89 7d f0             mov    %rdi,-0x10(%rbp)
>   4005d9:       48 89 c7                mov    %rax,%rdi
>   4005dc:       48 8b 75 f0             mov    -0x10(%rbp),%rsi
>   4005e0:       e8 c3 fe ff ff          callq  4004a8 <memcpy at plt>
> }
>   4005e5:       48 83 c4 10             add    $0x10,%rsp
>   4005e9:       5d                      pop    %rbp
>   4005ea:       c3                      retq   
>   4005eb:       0f 1f 44 00 00          nopl   0x0(%rax,%rax,1)
> 
> How do I disable that feature? I've tried -fno-builtin and/or -ffreestanding with no success.
> TIA
> 
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> cfe-dev at cs.uiuc.edu
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