[LLVMdev] Illegal Optimization(?): Combining empty type instances

[Evan Jones]

I think I may have found an illegal optimization in LLVM. The problem 
is that if you have an empty type LLVM optimizes it away so it produces 
weird results. The assembler output from llvm-gcc clearly outputs the 
"alloca" instructions, so some other pass is removing them. I doubt if 
any real program relies on this behaviour, and there is a trivial work 
around. However, my program ran across this problem, so maybe it should 
be corrected.

Example program:

#include <stdio.h>

struct Empty {};

struct Empty globalA;
struct Empty globalB;	

int main( int argc, char* argv[] )
{
	struct Empty a;
	struct Empty b;
	
	printf( "%p %p\n", &a, &b );
	printf( "%p %p\n", &globalA, &globalB );
	return 0;
}


The expected output is that all four pointers will be unique and valid. 
With LLVM the output is:

(nil) (nil)
0x4133aff9 0x4133aff9

Assembling and executing llvm-gcc's output directly is slightly better 
but the globals are still wrong:

0xbfffe0ef 0xbfffe0ee
0x4133aff9 0x4133aff9


Running llvm-gcc's output directly using 'lli -force-interpreter' is 
correct:

0x86b8748 0x86b8758
0x86a4058 0x86b8658


This caused a problem in my program because I was testing if the two 
addresses were equal, and the test was always returning true. The 
workaround is to put something in the structure so that LLVM doesn't 
optimize it away. The question is what does the C standard require? I'm 
assuming that GCC is doing the right thing, but I could be wrong.

Evan Jones

--
Evan Jones
http://evanjones.ca/