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Date: Fri, 26 Sep 2003 14:55:00 -0000
From: "aph at gcc dot gnu dot org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Message-ID: <20030926105501.12419.aph@gcc.gnu.org>
Reply-To: gcc-bugzilla@gcc.gnu.org
Subject: [Bug optimization/12419] New: Performace regression: poor optimization of const memory
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X-SW-Source: 2003-09/txt/msg02006.txt.bz2
List-Id: <gcc-bugs.sourceware.org>

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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12419

           Summary: Performace regression: poor optimization of const memory
           Product: gcc
           Version: 3.4
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: aph at gcc dot gnu dot org
                CC: gcc-bugs at gcc dot gnu dot org

This program:

extern const int a[];
extern void f();

int foo ()
{
  int n = a[0]; 
  f();
  n += a[0];
  return n;
}

generates this code:

foo:
        pushl   %ebx
        subl    $8, %esp
        movl    a, %ebx
        call    f
        movl    a, %eax
        addl    $8, %esp
        addl    %eax, %ebx
        movl    %ebx, %eax
        popl    %ebx
        ret

Note that the memory 'a' is read twice.  3.2 did this:

foo:
        pushl   %ebx
        subl    $8, %esp
        movl    a, %ebx
        sall    $1, %ebx
        call    f
        addl    $8, %esp
        movl    %ebx, %eax
        popl    %ebx
        ret