[LLVMdev] clang optimizer does not remove unused/uneeded variables(and accesses) from global scope

[Dennis Luehring]

hi llvmdev list

im currently investigating a missing optimizer feature in VS2010 - and 
comparing the VS2010 results with the result of clang 3.1

i've downloaded clang from http://llvm.org/releases/download.html -> 
Experimental Clang Binaries for Mingw32/x86

clang --version
clang version 3.1 (branches/release_31)
Target: i686-pc-mingw32
Thread model: posix

----- test.c

typedef unsigned char byte_t;
typedef unsigned int dword_t;

byte_t byte;
dword_t dword;

int main(int argc, char** argv)
{
     dword_t random = (dword_t)argv;

     byte = (byte_t)random;
     dword = (dword_t)random;

     dword_t result = 3*(byte+dword);

     return result;
}

-------

compiled with "clang -O3 test.c"

produces this code for the main

.text:004012E0 sub_4012E0      proc near               ; CODE XREF: 
sub_401020+91
.text:004012E0
.text:004012E0 arg_4           = dword ptr  0Ch
.text:004012E0
.text:004012E0                 push    ebp
.text:004012E1                 mov     ebp, esp
.text:004012E3                 call    sub_4014C0
.text:004012E8                 mov     eax, [ebp+arg_4] ; dword_t random 
= (dword_t)argv;
.text:004012EB                 mov     byte_402000, al
.text:004012F0                 mov     dword_402004, eax
.text:004012F5                 movzx   ecx, al
.text:004012F8                 add     ecx, eax
.text:004012FA                 lea     eax, [ecx+ecx*2]
.text:004012FD                 pop     ebp
.text:004012FE                 retn
.text:004012FE sub_4012E0      endp

but why are these two lines not removed by the optimizer?

.text:004012EB                 mov     byte_402000, al
.text:004012F0                 mov     dword_402004, eax

no access before, no access after - even the main-code itself isn't 
using the variables for reading

VS2010 behaves the same - but clang is still much better in other cases :)