Re: optimization/7632: gcc 3.2 for PowerPC: instruction scheduling ignores dependency in variable asignments

[Daniel Jacobowitz <drow@mvista.com>]

The following reply was made to PR optimization/7632; it has been noted by GNATS.

From: Daniel Jacobowitz <drow@mvista.com>
To: Horst.Lehser@hightec-rt.com
Cc: gcc-gnats@gcc.gnu.org
Subject: Re: optimization/7632: gcc 3.2 for PowerPC: instruction scheduling ignores dependency in variable asignments
Date: Mon, 19 Aug 2002 09:33:59 -0400

 On Mon, Aug 19, 2002 at 11:49:16AM -0000, Horst.Lehser@hightec-rt.com wrote:
 > 
 > >Number:         7632
 > >Category:       optimization
 > >Synopsis:       gcc 3.2 for PowerPC: instruction scheduling ignores dependency in variable asignments
 > >Confidential:   no
 > >Severity:       critical
 > >Priority:       medium
 > >Responsible:    unassigned
 > >State:          open
 > >Class:          sw-bug
 > >Submitter-Id:   net
 > >Arrival-Date:   Mon Aug 19 04:56:01 PDT 2002
 > >Closed-Date:
 > >Last-Modified:
 > >Originator:     Horst Lehser
 > >Release:        unknown-1.0
 > >Organization:
 > >Environment:
 > 
 > >Description:
 > the following C source will generate incorrect code
 > 
 > double a;
 > double foo(void)
 > {
 > 	((int *)&a)[1] += 5;
 > 	return;
 > }
 > Generated assembler
 > foo:
 >         lis %r3,a+4@ha
 >         lis %r5,a@ha
 >         lwz %r4,a+4@l(%r3)
 >         lfd %f1,a@l(%r5)	/* a not modified yet */
 >         addi %r0,%r4,10
 >         stw %r0,a+4@l(%r3)
 >         blr
 
 (I assume you meant "return a" above).
 
 You may want to read the documentation for -fno-strict-aliasing; your
 code is illegal C.  With -fno-strict-aliasing I get the expected
 result.
 
 -- 
 Daniel Jacobowitz
 MontaVista Software                         Debian GNU/Linux Developer