[llvm-dev] LTO prevention help

[Mehdi Amini via llvm-dev]

> On Oct 12, 2016, at 5:48 PM, Jon Slenk via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> I'm trying to learn about LTO. I have a static variable that I try to give a non-compile-time-constant value to. With LTO turned off, the sprintf happens i.e. a breakpoint set on that line does get triggered. With LTO turned on (either Monolithic or Incremental), the code appears to be removed/elided/deleted/forgotten/dropped/lost/ignored and the breakpoint is never tripped. 
> 
> What can I do to make my code survive through LTO? The variable makes it through but the sprinf() line doesn't. Can I somehow tell LTO to not apply to this function or line of code or file of code? (In reality this is something I am trying to do via Xcode for iOS targets.)
> 
> thank you.
> 
> static void __attribute__((constructor))_initv1 () {  
>     static char default_[] = "0000000000000000000000000000000000000000000000000000000000000000";  
>     sprintf( default_, "%lu", (unsigned long)5 );  
> }


So:

1) I don’t see what prevents us from deleting this code, even without LTO? It is not doing anything observable.

2) I can’t reproduce: the code is not deleted by LTO on MacOS.

$ clang test.cpp   -O3  -Wl,-save-temps -g -flto
$ lldb -- a.out
(lldb) target create "a.out"
Current executable set to 'a.out' (x86_64).
(lldb) b _initv1
Breakpoint 1: where = a.out`_initv1() + 4 at test.cpp:4, address = 0x0000000100000f64
(lldb) r
Process 15278 launched: '/private/tmp/a.out' (x86_64)
a.out was compiled with optimization - stepping may behave oddly; variables may not be available.
Process 15278 stopped
* thread #1: tid = 0xb82628, 0x0000000100000f64 a.out`_initv1() + 4 at test.cpp:4, queue = 'com.apple.main-thread', stop reason = breakpoint 1.1
    frame #0: 0x0000000100000f64 a.out`_initv1() + 4 at test.cpp:4 [opt]
   1   	#include <stdio.h>
   2   	static void __attribute__((constructor))_initv1 () {  
   3   	    static char default_[] = "0000000000000000000000000000000000000000000000000000000000000000";  
-> 4   	    sprintf( default_, "%lu", (unsigned long)5 );  
   5   	}
   6   	int main() {
   7   		
(lldb) 

— 
Mehdi

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