Re: optimization/9739: With -Os while and for loops can be arranged with one test instead of two

Re: optimization/9739: With -Os while and for loops can be arranged with one test instead of two

[bangerth]

Synopsis: With -Os while and for loops can be arranged with one test instead of two

State-Changed-From-To: open->closed
State-Changed-By: bangerth
State-Changed-When: Tue Mar  4 15:53:35 2003
State-Changed-Why:
    By request of submitter

http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=9739

optimization/9739: With -Os while and for loops can be arranged with one test instead of two

[lac]

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>Number:         9739
>Category:       optimization
>Synopsis:       With -Os while and for loops can be arranged with one test instead of two
>Confidential:   no
>Severity:       non-critical
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          pessimizes-code
>Submitter-Id:   net
>Arrival-Date:   Tue Feb 18 10:06:00 UTC 2003
>Closed-Date:
>Last-Modified:
>Originator:     László Vidács
>Release:        gcc version 3.3 20030210 (prerelease)
>Organization:
>Environment:
BUILD & HOST: Linux 2.4.20 i686 unknown
TARGET: arm-unknown-elf
>Description:
In the case of a while or for loop, gcc first tests whether to enter the loop or not; and at the end of the loop, it tests again whether to jump back to the beginning of the loop or not. One of these two test-and-conditional-jumps should be replaced with an unconditional jump to the other test when optimizing for size. When optimizing for speed this is not a problem.

Example (01.c): The compare and conditional return at lines 24-25 (in 01.s) are not needed, they are replaced by unconditional jump to the test (see 01_modified.s).
>How-To-Repeat:
arm-elf-gcc -S -g0 -Os

// 01.c:

# 1 "/home/alec/gcc/test/issues/_src/lac_5/01.c"
# 1 "<built-in>"
# 1 "<command line>"
# 1 "/home/alec/gcc/test/issues/_src/lac_5/01.c"
int F()
{
  return 2;
}

int main()
{
  int i=F();
  while (i>0)
  {
        i--;
  }
  return i;
}
>Fix:

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