Re: optimization/10684: register variable update disappears when using -O

Re: optimization/10684: register variable update disappears when using -O

[Jim Wilson]

The following reply was made to PR optimization/10684; it has been noted by GNATS.

From: Jim Wilson <wilson@tuliptree.org>
To: gcc-gnats@gcc.gnu.org
Cc: Joerg-Cyril.Hoehle@T-Systems.com, gcc-bugs@gcc.gnu.org, bruno@clisp.org,
 sds@gnu.org
Subject: Re: optimization/10684: register variable update disappears when using
 -O
Date: Thu, 08 May 2003 16:39:10 -0400

 http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=10684
 
 This is related to PR c/7871 which I have already done a lot of analysis 
 work on.  There appear to be multiple bugs in the handling of global 
 register variables, and they have been there since the gcc-3.0 release.
 
 Jim
 

Re: optimization/10684: register variable update disappears when using -O

[ehrhardt]

Synopsis: register variable update disappears when using -O

State-Changed-From-To: open->analyzed
State-Changed-By: cae
State-Changed-When: Thu May  8 18:52:25 2003
State-Changed-Why:
    Confirmed, still present in 3.4-cvs. This is a reduced testcase:
    
    register int STACK __asm__("%ebx");
    static void (*p) ();
    void f ()
    {
            STACK += 1;
            p ();
            STACK -= 1;
    }
    
    Looking at the assembly shows that STACK += 1 ist lost if compiled
    with -O. The problem seems to be that life analysis adds a REG_DEAD
    note to the call to (*p) which makes STACK += 1 a dead store.
    
       regards  Christian
    

http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=10684