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optimization/3507: appalling optimisation with sub/cmp on i386
- To: gcc-gnats at gcc dot gnu dot org, debian-gcc at lists dot debian dot org
- Subject: optimization/3507: appalling optimisation with sub/cmp on i386
- From: Matthias Klose <doko at klose dot in-berlin dot de>
- Date: Sun, 01 Jul 2001 18:07:42 +0200
- Reply-To: 75773-quiet at bugs dot debian dot org
>Number: 3507
>Category: optimization
>Synopsis: appalling optimisation with sub/cmp on i386
>Confidential: no
>Severity: non-critical
>Priority: low
>Responsible: unassigned
>State: open
>Class: pessimizes-code
>Submitter-Id: net
>Arrival-Date: Sun Jul 01 09:16:00 PDT 2001
>Closed-Date:
>Last-Modified:
>Originator: herbert@gondor.apana.org.au
>Release: 3.0 (Debian GNU/Linux) and HEAD 20010701
>Organization:
The Debian project
>Environment:
System: Debian GNU/Linux (testing/unstable)
Architecture: i686
host: i386-linux
build: i386-linux
target: i386-linux
configured with: ../src/configure -v --enable-languages=c,c++,java,f77,proto,objc --prefix=/usr --infodir=/share/info --mandir=/share/man --enable-shared --with-gnu-as --with-gnu-ld --with-system-zlib --enable-long-long --enable-nls --without-included-gettext --disable-checking --enable-threads=posix --enable-java-gc=boehm --with-cpp-install-dir=bin --enable-objc-gc i386-linux
>Description:
[ Reported to the Debian BTS as report #75773.
Please CC 75773-quiet@bugs.debian.org on replies.
Log of report can be found at http://bugs.debian.org/75773 ]
For the file
unsigned long foo(unsigned long a, unsigned long b) {
unsigned long c = a - b;
if (a < b) {
c += 100;
}
return c;
}
gcc -O2 -S generates (The cmpl after the subl is unnecessary):
.file "bug-75773.c"
.text
.align 2
.p2align 2,,3
.globl foo
.type foo,@function
foo:
pushl %ebp
movl %esp, %ebp
movl 8(%ebp), %edx
movl 12(%ebp), %eax
movl %edx, %ecx
subl %eax, %ecx
cmpl %eax, %edx
jae .L2
addl $100, %ecx
.L2:
movl %ecx, %eax
popl %ebp
ret
.Lfe1:
.size foo,.Lfe1-foo
.ident "GCC: (GNU) 3.1 20010701 (experimental)"
>How-To-Repeat:
>Fix:
>Release-Note:
>Audit-Trail:
>Unformatted: