Bug 53977 – gcc produces bad code when using -O2 option

Bug 53977 - gcc produces bad code when using -O2 option

Summary:	gcc produces bad code when using -O2 option

Status:	CLOSED CURRENTRELEASE

Aliases:	None

Product:	Red Hat Linux
Classification:	Retired
Component:	gcc (Show other bugs)
Sub Component:
Version:	7.2
Hardware:	i686 Linux

Priority	medium Severity high
Target Milestone:	---
Target Release:	---
Assigned To:	Jakub Jelinek
QA Contact:	Brian Brock
Docs Contact:

URL:
Whiteboard:
Keywords:

Depends On:
Blocks:
	Show dependency tree / graph

Reported:	2001-09-24 17:09 EDT by Marius Kintel
Modified:	2007-04-18 12:37 EDT (History)
CC List:	1 user (show)

See Also:
Fixed In Version:
Doc Type:	Bug Fix
Doc Text:
Story Points:	---
Clone Of:
Environment:
Last Closed:	2002-12-15 12:47:50 EST
Type:	---
Regression:	---
Mount Type:	---
Documentation:	---
CRM:
Verified Versions:
Category:	---
oVirt Team:	---
RHEL 7.3 requirements from Atomic Host:
Cloudforms Team:	---

Attachments	(Terms of Use)
.cpp file triggering the bug. (374 bytes, text/plain) 2001-09-24 17:11 EDT, Marius Kintel	no flags	Details
Add an attachment (proposed patch, testcase, etc.)

Groups: None (edit)

Description Marius Kintel 2001-09-24 17:09:43 EDT

From Bugzilla Helper:
User-Agent: Mozilla/4.77 [en] (X11; U; Linux 2.4.3-20mdk i686)

Description of problem:
The attached program produces wrong output when compiled with the -O2
option:
$ gcc -o gcctest gcctest.cpp
$ ./gcctest
result: -3.000000
$ gcc -o gcctest gcctest.cpp -O2
$ ./gcctest
result: 3.000000

This is tested under gcc-2.96-69 (RH-7.0), gcc-2.96-85 (RH-7.1) and
gcc-2.96-0.48mdk (Mandrake 8.0).


Version-Release number of selected component (if applicable):


How reproducible:
Always

Steps to Reproduce:
1. compile
2. run
3.
	

Additional info:

Comment 1 Marius Kintel 2001-09-24 17:11:49 EDT

Created attachment 32539 [details]
.cpp file triggering the bug.

Comment 2 William Cohen 2002-04-30 17:33:44 EDT

The compiler is mis-optimizing the code when -O2 set. The following code from
init function implement "f = -(v0+1);":

	flds	12(%ebp)
	fadds	.LC0
	fstps	12(%ebp)
	flds	12(%ebp)       /* load into fp register */
	xorb	$128, 15(%ebp) /* value in fp register, not memory */
	fstps	(%eax)
	movl	-12(%ebp), %eax
	movl	%eax, (%esi)

The "xorb $128, 15(%ebp)" should be a "fchs" instruction.

Comment 3 Alan Cox 2002-12-15 12:47:50 EST

Ok with 3.2

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