[LLVMdev] Multiple Definition error with LTO

[Daniel Stewart]

I'm trying to understand why using a local memcpy with LTO results in a
"multiple definition" error. 

 

I have an local (optimized) mempy.c (clearly simplified!):

 

void* memcpy(void* dest, const void* src, unsigned int count) {

  return 0;

}

 

void* __aeabi_memcpy(void *dest, const void *src, unsigned int size) {

  return memcpy(dest,src,size);

}

---

I also have a simple main.c  to test it out (also simplified):

#include <stdio.h>

#include <string.h>

 

struct {

  char name[40];

} person;

 

char myname[] = "abcd";

 

int main ()

{

  memcpy ( person.name, myname, strlen(myname)+1 );

 

  printf ("name :%s\n", person.name );

  return 0;

}

---

 

During compilation with:

clang -O0 memcpy.c -o memcpy.o  -c

ar cr memcpy.a memcpy.o

clang -O0 -o memcpyTest.exe  -flto -static main.c  memcpy.a 

 

I get a multiple definition of memcpy error. Now building it with
-fno-builtin works fine. I'm trying to understand what is happening behind
the scenes that requires my use of -fno-builtin (or
-Wl,--allow-multiple-definitions). Did the backend put in references to libc
that are trying to get resolved before loading the optimized archive? I also
notice that replacing memcpy.a with memcpy.o works just fine as well. Can
someone give me some insight?

 

I've found this discussion
<https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55994>  regarding GCC that
seems to center around the same issue. 

 

Daniel 

 

--

Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted by
The Linux Foundation

 

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