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Date: Tue, 09 Mar 2010 19:06:00 -0000
Subject: [Bug target/43311]  New: missed 'movw' optimization.
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From: "pluto at agmk dot net" <gcc-bugzilla@gcc.gnu.org>
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reduced testcase:

typedef struct { unsigned char b1, b2; } __attribute__((aligned(8))) S;
void f( S const* s, unsigned char* b1, unsigned char* b2 )
{
        *b1 = s->b1;
        *b2 = s->b2;
}

generates at -Os and -O3:

f:
        movb    (%rdi), %al     # <variable>.b1, <variable>.b1
        movb    %al, (%rsi)     # <variable>.b1,* b1
        movb    1(%rdi), %al    # <variable>.b2, <variable>.b2
        movb    %al, (%rdx)     # <variable>.b2,* b2
        ret

gcc could (at least at -Os) reduce code size and memory accesses to:

        movw    (%rdi), %ax
        movb    %al, (%rsi)     # <variable>.b1,* b1
        movb    %ah, (%rdx)     # <variable>.b2,* b2
        ret


-- 
           Summary: missed 'movw' optimization.
           Product: gcc
           Version: 4.4.3
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: target
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: pluto at agmk dot net
 GCC build triplet: x86_64-gnu-linux
  GCC host triplet: x86_64-gnu-linux
GCC target triplet: x86_64-gnu-linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43311