target/10242: ARM: peephole for effective use of plus and minus operators

[alga@rgai.hu]

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>Number:         10242
>Category:       target
>Synopsis:       ARM: peephole for effective use of plus and minus operators
>Confidential:   no
>Severity:       non-critical
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          pessimizes-code
>Submitter-Id:   net
>Arrival-Date:   Thu Mar 27 13:56:00 UTC 2003
>Closed-Date:
>Last-Modified:
>Originator:     Gábor Lóki
>Release:        GCC: (GNU) 3.3 20030324 (prerelease)
>Organization:
>Environment:
BUILD & HOST: Linux 2.4.20 i686 unknown
TARGET: arm-unknown-elf
>Description:
In some cases the generated code contains (for arm target) a plus operator with positive constant which is followed by another plus operator with a negative constant and both operate on the same register (eg: "add rD, rB, #c0" and "sub rD, rD, #c1" ).
This makes an opportunity for effective use of these operators if c0+c1 can be stored in the Immediate Operand filed. Specifically, in this case "add rD, rB, #c2" where c2 = c0 + c1 and c2 >= 0, or "sub rD, rB, #c2" where c2 = -(c0 + c1) and c2 < 0.
This problem generally occurs when storing local variables on the stack.
The same can be applied for all four combinations of plus and minus operators.
In the example attached, in function foo there is a "sub r0,r0,#12" and "sub r0,r0,#4" which can be replaced by "sub r0,r0,#16".
>How-To-Repeat:
arm-elf-gcc -S -g0 -Os plus-minus.c

void func(char*);
void foo()
{
  char buf[1028];
  func(buf);
}
>Fix:

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