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From: "tony.poppleton at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug rtl-optimization/47582] New: Combine chains of movl into movq
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47582

           Summary: Combine chains of movl into movq
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: rtl-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: tony.poppleton@gmail.com


The following C code (adapted from
http://stackoverflow.com/questions/4544804/in-what-cases-should-i-use-memcpy-over-standard-operators-in-c)
shows that adjacent sequences of movl could be combined into movq.

extern float a[5];
extern float b[5];

int main()
{
#if defined(M1)
    a[0] = b[0];
    a[1] = b[1];
    a[2] = b[2];
    a[3] = b[3];
    a[4] = b[4];
#elif defined(M2)
    memcpy(a, b, 5*sizeof(float));
 #endif
}

When compiled with "-O2 -fomit-frame-pointer" on GCC 4.3.5, 4.4.5, 4.5.2 and
4.6.0 (20110129), the following asm is produced for the -DM1 branch:
        movl    b(%rip), %eax
        movl    %eax, a(%rip)
        movl    b+4(%rip), %eax
        movl    %eax, a+4(%rip)
        movl    b+8(%rip), %eax
        movl    %eax, a+8(%rip)
        movl    b+12(%rip), %eax
        movl    %eax, a+12(%rip)
        movl    b+16(%rip), %eax
        movl    %eax, a+16(%rip)
        ret

However for the -DM2 branch, the memcpy implementation shows that this can be
done more efficiently:
        movq    b(%rip), %rax
        movq    %rax, a(%rip)
        movq    b+8(%rip), %rax
        movq    %rax, a+8(%rip)
        movl    b+16(%rip), %eax
        movl    %eax, a+16(%rip)
        ret

I presume that the memcpy is being done in hand-written asm?  If so, then once
this enhancment is done, then presumably that portion of memcpy code could be
converted to C code and be just as efficient.