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Date: Wed, 05 Mar 2003 14:26:00 -0000
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Subject: middle-end/9967: Some standard C function calls should not be replaced when optimizing for size
X-SW-Source: 2003-03/txt/msg00216.txt.bz2
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>Number:         9967
>Category:       middle-end
>Synopsis:       Some standard C function calls should not be replaced when optimizing for size
>Confidential:   no
>Severity:       non-critical
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          pessimizes-code
>Submitter-Id:   net
>Arrival-Date:   Wed Mar 05 14:26:00 UTC 2003
>Closed-Date:
>Last-Modified:
>Originator:     Árpád Beszédes
>Release:        gcc version 3.3 20030224 (prerelease)
>Organization:
>Environment:
BUILD & HOST: Linux 2.4.20 i686 unknown
TARGET: arm-unknown-elf
>Description:
GCC replaces calls with special parameters to some standard C functions to equivalent ones in order to increase performance. E.g. fputs with string parameter -> fwrite.
However, some of these calls take more instructions, which is not good if we optimize for size. These replacements should be avoided depending on the optimization switches. (See example below.)
>How-To-Repeat:
This simple program demonstrates that fprintf is replaced by calling fwrite in the assembly output (arm-elf target with newlib library):

#include <stdio.h>
void foo ()
{
  fprintf(stderr, "Comment"); // fprintf->fwrite
}

ldr r3, .L2
ldr r3, [r3, #0]
ldr r0, .L2+4
ldr r3, [r3, #12]
mov r1, #1
mov r2, #7
b fwrite

However, calling fprintf would require two instructions less:

ldr r3, .L2
ldr r3, [r3, #0]
ldr r1, .L2+4
ldr r0, [r3, #12]
b fprintf
>Fix:

>Release-Note:
>Audit-Trail:
>Unformatted: