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From:	"Alexei A. Frounze" <alexfru AT chat DOT ru>
Newsgroups:	comp.os.msdos.djgpp
Subject:	gcc bug?
Date:	Mon, 16 Apr 2007 20:27:07 -0700
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I was checking my understanding of volatile's effect and encountered an odd 
thing.

Consider the following code:
----8<----
typedef struct {char x;} T, *PT;

void f1 ()
{
  volatile T* pt = 0;
  while (pt->x != 0);
}

void f2 ()
{
  volatile PT pt = 0;
  while (pt->x != 0);
}

int main()
{
  return 0;
}

----8<----

Now, let's compile it to asm using: gcc -Wall -c -S -O ptr.c -o ptr.s:
----8<----
        .file   "ptr.c"
        .section .text
.globl _f1
_f1:
        pushl   %ebp
        movl    %esp, %ebp
        movl    $0, %edx
L2:
        movb    (%edx), %al
        testb   %al, %al
        jne     L2
        popl    %ebp
        ret
.globl _f2
_f2:
        pushl   %ebp
        movl    %esp, %ebp
        subl    $4, %esp
        movl    $0, -4(%ebp)
L6:
        movl    -4(%ebp), %eax
        cmpb    $0, (%eax)
        jne     L6
        movl    %ebp, %esp
        popl    %ebp
        ret
.globl _main
_main:
        pushl   %ebp
        movl    %esp, %ebp
        subl    $8, %esp
        andl    $-16, %esp
        movl    $0, %eax
        movl    %ebp, %esp
        popl    %ebp
        ret
        .ident  "GCC: (GNU) 3.3.4"
----8<----

Let's now compile it to asm using higher optimization level: 
gcc -Wall -c -S -O ptr.c -o ptr.s:
----8<----
        .file   "ptr.c"
        .section .text
        .p2align 4,,15
.globl _f1
_f1:
        pushl   %ebp
        movl    %esp, %ebp
        .p2align 4,,7
L2:
        movb    0, %al
        testb   %al, %al
        jne     L2
        popl    %ebp
        ret
        .p2align 4,,15
.globl _f2
_f2:
        pushl   %ebp
        movl    %esp, %ebp
        pushl   %eax
        movl    $0, -4(%ebp)
        .p2align 4,,7
L6:
        movl    -4(%ebp), %eax
        cmpb    $0, (%eax)
        jne     L6
        movl    %ebp, %esp
        popl    %ebp
        ret
        .p2align 4,,15
.globl _main
_main:
        pushl   %ebp
        movl    %esp, %ebp
        xorl    %eax, %eax
        pushl   %edx
        pushl   %edx
        andl    $-16, %esp
        movl    %ebp, %esp
        popl    %ebp
        ret
        .ident  "GCC: (GNU) 3.3.4"
----8<----

Note the difference in f1:
- if the optimization is -O, then there's a pointer in the routine and it 
gets dereferenced
- if the optimization is -O2 (or -O3), then there's no pointer in the 
routine at all, instead there's a non-pointer variable that is initialized 
with the value that should've initialized the missing pointer

Am I missing something, or is this really a bug?

Alex

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