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Date: Wed, 20 Feb 2002 14:16:00 -0000
From: dann@godzilla.ics.uci.edu
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Subject: optimization/5738: GCSE missed optimization
X-SW-Source: 2002-02/txt/msg00524.txt.bz2
List-Id: <gcc-prs.sourceware.org>


>Number:         5738
>Category:       optimization
>Synopsis:       GCSE missed optimization
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          pessimizes-code
>Submitter-Id:   net
>Arrival-Date:   Wed Feb 20 13:56:01 PST 2002
>Closed-Date:
>Last-Modified:
>Originator:     dann@godzilla.ics.uci.edu
>Release:        CVS
>Organization:
>Environment:
sparc-sun-solaris2.8
>Description:
GCSE not working

The following code: 


struct foo 
{
  unsigned short *p;
};


void
func (struct foo *s, unsigned int *coord, _Bool delta)
{
  unsigned short change;

  if (delta) {
    change = *((s)->p++);
    *coord += change;
  } else {
    change = *((s)->p++);
    *coord += change;
    *coord += *((s)->p++) << 8;
  }
}

generates when compiled with gcc from CVS on SPARC (with -O2)

func:
        !#PROLOGUE# 0
        !#PROLOGUE# 1
        andcc   %o2, 0xff, %g0
        mov     %o0, %o5
        be      .LL2
        mov     %o1, %o4
        ld      [%o0], %o0
        ld      [%o1], %o2
        lduh    [%o0], %o3
        sll     %o3, 16, %o1
        srl     %o1, 16, %o1
        add     %o2, %o1, %o2
        add     %o0, 2, %o0
        st      %o0, [%o5]
        b       .LL1
        st      %o2, [%o4]
.LL2:
        ld      [%o0], %o0
        lduh    [%o0], %o3
        ld      [%o1], %o2
        add     %o0, 2, %o0
        lduh    [%o0], %o1
        add     %o2, %o3, %o2
        sll     %o1, 8, %o1
        add     %o2, %o1, %o2
        add     %o0, 2, %o0
        st      %o2, [%o4]
        st      %o0, [%o5]
.LL1:
        retl
        nop

GCSE should be able to realize that
The sequence:
    change = *((s)->p++);
    *coord += change;

occurs on both branches of the conditional and move it before.
>How-To-Repeat:
Compile the code above with -O2 and look at the assembly,
one branch of the conditional should be empty. 

 
>Fix:
n/a
>Release-Note:
>Audit-Trail:
>Unformatted: