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Date: Fri, 23 May 2008 21:37:00 -0000
Subject: [Bug tree-optimization/36318]  New: SRA pessimizes struct copies without -Os
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From: "astrange at ithinksw dot com" <gcc-bugzilla@gcc.gnu.org>
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> /usr/local/gcc44/bin/gcc -v
Using built-in specs.
Target: i386-apple-darwin9.2.2
Configured with: ../gcc/configure --prefix=/usr/local/gcc44
--enable-threads=posix --with-arch=core2 --with-tune=core2 --with-gmp=/sw
--with-mpfr=/sw --disable-nls --disable-bootstrap --enable-checking=yes,rtl
--enable-languages=c,c++,objc
Thread model: posix
gcc version 4.4.0 20080523 (experimental) (GCC) 

and these options:
> gcc -fno-pic -fomit-frame-pointer -O3 -S wc.c

For the attached source, gcc generates good code for global variable
assignment:
_foo0:
        subl    $12, %esp
        movl    _b, %eax
        movl    %eax, _a
        addl    $12, %esp
        ret

but uses byte copies for pointer assignment:
_foo1:
        subl    $12, %esp
        movl    %ebx, 4(%esp)
        movl    %esi, 8(%esp)
        movl    20(%esp), %eax
        movl    16(%esp), %edx
        movzbl  (%eax), %esi
        movzbl  1(%eax), %ebx
        movzbl  2(%eax), %ecx
        movzbl  3(%eax), %eax
        movb    %cl, 2(%edx)
        movb    %al, 3(%edx)
        movb    %bl, 1(%edx)
        movl    %esi, %eax
        movb    %al, (%edx)
        movl    4(%esp), %ebx
        movl    8(%esp), %esi
        addl    $12, %esp
        ret

Using either -Os or -fno-tree-sra fixes it.


-- 
           Summary: SRA pessimizes struct copies without -Os
           Product: gcc
           Version: 4.4.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: astrange at ithinksw dot com
GCC target triplet: i?86-*-*


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36318