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Bug#484784: gcc-4.3: -O2 -O3 - wrong arguments are passing to inlined body of function

On Fri, Jun 06, 2008 at 02:15:38PM +0200, Wojciech Muła wrote:
> Consider this simple program:

This is not simple. Inline assembler could never be simple.

> 		"	xorl %%ebx, %%ebx	\n"

Don't use explicit register names but placeholder.

> 		"	movzbl (%%eax), %%edx	\n"

eax is unclobbered, therefor the compiler is allowed to assume it

> 		"	decl %%ecx		\n"

Same for ecx.

> Function inline_asm just add all values from array; all modified/input
> registers are listed in __asm__ statement.

Nope, they are not.

> When program is compiled without any optimization flag, it works ok.
> However if compiled with -O3 or -O3 --- segfaults.  I quickly analyzed
> assembly output, and it is clear, that gcc fully inlined procedure,
> however in a while loop only address is restored (%eax), but inner loop
> counter (%ecx) isn't.  Thus in a second iteration %ecx has value 0, and
> loop would execute 0xffffffff times, but segfault appear faster.

No bug. You cheated so the compiler is free to do whatever it wants.


Beam me up, Scotty!  It ate my phaser!

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