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optimization/8013: writing all bitfields in a word
- From: jreiser at BitWagon dot com
- To: gcc-gnats at gcc dot gnu dot org
- Date: 23 Sep 2002 22:26:02 -0000
- Subject: optimization/8013: writing all bitfields in a word
- Reply-to: jreiser at BitWagon dot com
>Number: 8013
>Category: optimization
>Synopsis: writing all bitfields in a word
>Confidential: no
>Severity: serious
>Priority: medium
>Responsible: unassigned
>State: open
>Class: sw-bug
>Submitter-Id: net
>Arrival-Date: Mon Sep 23 15:36:00 PDT 2002
>Closed-Date:
>Last-Modified:
>Originator: John Reiser
>Release: gcc version 3.2
>Organization:
>Environment:
Reading specs from /usr/local/gcc-3.2/lib/gcc-lib/i686-pc-linux-gnu/3.2/specs
Configured with: ../gcc-3.2/configure --prefix=/usr/local/gcc-3.2
Thread model: posix
gcc version 3.2
>Description:
There is an opportunity for improved optimization when writing all the bitfields in a word ('int', 'unsigned', or 'long'): construct the entire word result in a register, then perform only one store to memory, instead of writing the fields to memory separately in turn.
The attached bitfield.i generates (with "-O" optimization)
movl (%ebx), %ecx
andl $16777215, %ecx
movl (%edx), %eax
andl $-16777216, %eax
orl %ecx, %eax
movl %eax, (%edx)
movzbl 3(%ebx), %ecx
andb $127, %cl
movzbl 3(%edx), %eax
andb $-128, %al
orb %cl, %al
orb $-128, %al
movb %al, 3(%edx)
which could be improved to
movl (%ebx), %eax
orl $1<<31, %eax
movl %eax, (%edx)
which is much shorter and faster.
>How-To-Repeat:
gcc -v -save-temps -O -S bitfield.c
>Fix:
>Release-Note:
>Audit-Trail:
>Unformatted:
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