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Date: Mon, 17 Jun 2002 07:00:00 -0000
From: pb@nexus.co.uk
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Subject: target/7056: [ARM] zero extend code worse in 3.1 than 3.0
X-SW-Source: 2002-06/txt/msg00392.txt.bz2
List-Id: <gcc-prs.sourceware.org>


>Number:         7056
>Category:       target
>Synopsis:       [ARM] zero extend code worse in 3.1 than 3.0
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          pessimizes-code
>Submitter-Id:   net
>Arrival-Date:   Mon Jun 17 06:16:01 PDT 2002
>Closed-Date:
>Last-Modified:
>Originator:     pb@nexus.co.uk
>Release:        gcc-3.1
>Organization:
>Environment:
arm-linux
>Description:
See code below.

With gcc-3.0 -O2, this yields

        stmfd   sp!, {r4, lr}
        ldrb    r3, [r0], #1    @ zero_extendqisi2
        ldrb    r4, [r0, #0]    @ zero_extendqisi2
        mov     r0, r3
        bl      g
        mov     r0, r4
        @ Sibcall epilogue
        ldmfd   sp!, {r4, lr}
        b       g

With gcc-3.1, there is an extra, redundant, AND instruction:

        stmfd   sp!, {r4, lr}
        ldrb    r3, [r0], #1
        and     r3, r3, #255
        ldrb    r4, [r0, #0]    @ zero_extendqisi2
        mov     r0, r3
        bl      g
        mov     r0, r4
        @ Sibcall epilogue
        ldmfd   sp!, {r4, lr}
        b       g
>How-To-Repeat:
void f(unsigned char *p)
{
  unsigned long int a,b;
 
  a = *p++;
  b = *p++;

  g(a);
  g(b);
}
>Fix:

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