target/9832: ARM: Arithmetic and move could be combined using liveness information

target/9832: ARM: Arithmetic and move could be combined using liveness information

[lac]

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>Number:         9832
>Category:       target
>Synopsis:       ARM: Arithmetic and move could be combined using liveness information
>Confidential:   no
>Severity:       non-critical
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          pessimizes-code
>Submitter-Id:   net
>Arrival-Date:   Mon Feb 24 15:26:00 UTC 2003
>Closed-Date:
>Last-Modified:
>Originator:     László Vidács
>Release:        gcc version 3.3 20030217 (prerelease)
>Organization:
>Environment:
BUILD & HOST: Linux 2.4.20 i686 unknown
TARGET: arm-unknown-elf
>Description:
Arithmetic and move instructions are replaced by a combined arithmetic in ARM target. The combine cannot be done if the destination of the arithmetic is also used. However, if based on liveness information this destination is never used the combine could be safely applied.

For example, in file ... if r4 is not used after the and instruction, the combine can be applied as in the modified file.
>How-To-Repeat:
gcc -S -Os 01.i

// 01.i

# 1 "01.c"
# 1 "<built-in>"
# 1 "<command line>"
# 1 "01.c"
void func(char c, int t)
{
  ;
}
void foo(int u)
{
  func ( 8, (u >> 24) & 0xffL );
  func ( 8, (u >> 16) & 0xffL );
}
>Fix:

>Release-Note:
>Audit-Trail:
>Unformatted:
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Re: target/9832: ARM: Arithmetic and move could be combined using liveness information

[rearnsha]

Synopsis: ARM: Arithmetic and move could be combined using liveness information

Responsible-Changed-From-To: unassigned->rearnsha
Responsible-Changed-By: rearnsha
Responsible-Changed-When: Mon Mar  3 14:14:27 2003
Responsible-Changed-Why:
    .
State-Changed-From-To: open->closed
State-Changed-By: rearnsha
State-Changed-When: Mon Mar  3 14:14:27 2003
State-Changed-Why:
    The analysis for the reasons behind this are mistaken.  GCC does get the liveness correct, but combine prevented from doing the combine into a hard register.  The current register allocator fails to spot that the move can be eliminated and thus generates sub-optimal code.  Compiling the same code with -fnew-ra shows that the new register allocator does a much better job in this area and eliminates the redundant move.

http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=9832