Fw: Possible missed optimization opportunity with const?

[Toshi Morita <tm314159 at yahoo dot com>]

Resending to the list, because I didn't see this show up in the archives.

IWhen this code is compiled with gcc-4.8.2 using -O2:

#include <stdio.h> 
extern const int foo; 
const int * const pfoo = &foo; 
extern void bar(void); 
int main(void) 
{ 
int a, b; 
a = *pfoo; 
bar(); 
b = *pfoo; 
printf("a: %d, b: %d\n", a, b); 

}


The two reads of foo are optimized down to one read, as expected:

test.c.025t.esra:

...
a_2 = foo;
bar ();
b_4 = foo;
...

test.c.026t.fre1

...
a_2 = foo;
bar ();
b_4 = a_2;
...
However, if the definition of pfoo is changed to:

const int * const pfoo = (const int * const 0x1234);


the optimization seems to fail:

test.c.025t.esra:

a_3 = MEM[(const int *)4660B];
bar ();
b_6 = MEM[(const int *)4660B];
...

test.c.026t.fre1

(no change)

The output code seems to have two reads of 0x1234 as well:

test:
    pushq   %rbx
    movl    4660, %ebx
    call    bar
    movl    %ebx, %edx
    movl    4660, %ecx
    movl    $.LC0, %esi
    popq    %rbx
    movl    $1, %edi
    xorl    %eax, %eax
    jmp     __print_chk

I'm assuming "movl 4660, %ebx" is an indirect reference in GNU syntax and not an immediate reference.

This seems odd. Is this correct?

Toshi