Re: inline-asm/9910: gcc dies on inline assembly

[Zack Weinberg <zack@codesourcery.com>]

The following reply was made to PR inline-asm/9910; it has been noted by GNATS.

From: Zack Weinberg <zack@codesourcery.com>
To: jay@remotepoint.com
Cc: gcc-gnats@gcc.gnu.org,  gcc@sources.redhat.com,  dies@sources.redhat.com
Subject: Re: inline-asm/9910: gcc dies on inline assembly
Date: Sun, 02 Mar 2003 17:19:31 -0800

 jay@remotepoint.com writes:
 
 > will not compile very simple piece of inline assembly code.  gcc-3.2
 > gives an internal compiler error where gcc-2.95 gave invalid asm
 > statement (fixed or forbidden register 0 (ax) was spilled for class
 > AREG).  From what I can find, the code should actually be legal.
 ...
 
 > int main() {
 >         int foo=10, bar=15;
 >
 >         __asm__ __volatile__ ("addl     %%ebx,%%eax"
 >                         : "=eax"(foo)
 >                         : "eax"(foo),"ebx"(bar)
 >                         : "eax"
 >                         );
 
 gcc should not ICE, but this is emphatically not correct code.  A
 correct asm statement would read
 
    __asm__ ("addl     %1, %0"
             : "+a" (foo)
             : "b" (bar));
 
 or better (since there is no requirement for eax and ebx to be the
 registers used):
 
    __asm__ ("addl     %1, %0"
             : "+r" (foo)
             : "r" (bar));
 
 The difference between constraint strings and clobber lists is
 unfortunate and confusing, but no one has had a better idea.
 
 zw