[PATCH] D23940: Delete dead code

[Mehdi Amini via llvm-commits]

> On Aug 26, 2016, at 12:51 PM, Rafael Espíndola <rafael.espindola at gmail.com> wrote:
> 
> On 26 August 2016 at 15:37, Mehdi AMINI <mehdi.amini at apple.com> wrote:
>> mehdi_amini added a comment.
>> 
>> We should be able to skip the GOT/PLT for such symbols in the codegen right now, wouldn't we?
> 
> They are skipped right now by a combination of visibility and codegen model.



Let me take an example of what I mean:

$ cat foo.cint foo =42;
int bar();int bar2();
int main() {
	return bar() + bar2();
}
$ cat bar.c
extern int foo;
int locally_defined = 0;
int bar(){
	return foo;
}
int bar2() {
	return locally_defined;
}

clang foo.c -c
clang bar.c -c -flto
clang foo.o bar.o 

You can see the difference in the codegen for bar and bar2 when I disassemble the binary:

_bar:
100000fa0:	55 	pushq	%rbp
100000fa1:	48 89 e5 	movq	%rsp, %rbp
100000fa4:	48 8d 05 55 00 00 00 	leaq	85(%rip), %rax
100000fab:	8b 00 	movl	(%rax), %eax
100000fad:	5d 	popq	%rbp
100000fae:	c3 	retq
100000faf:	90 	nop

_bar2:
100000fb0:	55 	pushq	%rbp
100000fb1:	48 89 e5 	movq	%rsp, %rbp
100000fb4:	31 c0 	xorl	%eax, %eax
100000fb6:	5d 	popq	%rbp
100000fb7:	c3 	retq



— 
Mehdi