[PATCH] D23940: Delete dead code

[Mehdi Amini via llvm-commits]

> On Aug 26, 2016, at 1:45 PM, Rafael Espíndola <rafael.espindola at gmail.com> wrote:
> 
>> 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
> 
> That already works for ELF. shouldAssumeDSOLocal knows that a symbol
> in a executable cannot be preempted.
> 
> Yes, that is the kind of thing that could be handled uniformly by
> having the IR explicitly say what can be assumed to be dso local, but
> we don't have that feature yet.

Right, my idea was to do something like adding “visibility hidden” on *declarations* when we have this passed-in by the linker, and update shouldAssumeDSOLocal to look for this.

— 
Mehdi