Inline assembly problem (GCC bug?)

[Simon Kagstrom <simon dot kagstrom at bth dot se>]

Hello!

I have a problem with some inline assembly, which to me looks like a
GCC bug but I first wanted to check that I haven't done any obvious
mistake in my implementation. I see the problem both on MIPS and IA-32.
The code I have is

  /* Assign 1 to v0 (the MIPS register used for passing function
results) */
  static inline int inline_asm(void) {
   register unsigned long __v0 asm("$2");

   __asm__ volatile (
   "li %0, 1\n"
   : "=r" (__v0)
   :
   : "memory"
   );
  
   return (int) __v0;
  }

  void problem (void) {
    int a = inline_asm();
    int b = overwriter();
  
    tst(a, b);
  }

When disassembling the generated code, this generates the following
MIPS assembly code, with my comments

00000000 <problem>:
   0:	27bdffe8 	addiu	sp,sp,-24
   4:	afbf0010 	sw	ra,16(sp)
   8:	24020001 	li	v0,1          # Result of the inline assembly piece
   c:	0c000000 	jal	overwriter    # Overwrites v0
  10:	00000000 	nop                   # jal delay slot - v0 should have been saved here!
  14:	00402021 	move	a0,v0         # GCC thinks v0 still contains the result of the inline assembly
  18:	0c000000 	jal	tst
  1c:	00402821 	move	a1,v0         # ... *and* the result of overwriter()!
  20:	8fbf0010 	lw	ra,16(sp)
  24:	00000000 	nop
  28:	03e00008 	jr	ra
  2c:	27bd0018 	addiu	sp,sp,24

This bug seems to be portable - I rewrote the inline assembly part for
IA-32 - just a move to %eax - and %eax was indeed overwritten as well.
The versions where I have this problem is

   mips-linux-gnu-gcc (GCC) 4.1.2 20061115 (prerelease) (Debian 4.1.1-21)
   gcc (GCC) 4.1.2 20061115 (prerelease) (Debian 4.1.1-21)

but gcc-3.3 (IA-32) handles this just fine.


So the usual question: Did I miss something in the inline assembly
piece which can cause this problem or is this a proper GCC bug?

(The problem was first manifested in some inline assembly for the Cibyl
project, http://spel.bth.se/index.php/Cibyl, and the code is based on
the Linux system calls for MIPS)

-- 
// Simon

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