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From: "jay.foad at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/52415] New: memcpy to local variable generates unnecessary stack frame for armv7
Date: Tue, 28 Feb 2012 10:40:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52415

             Bug #: 52415
           Summary: memcpy to local variable generates unnecessary stack
                    frame for armv7
    Classification: Unclassified
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: target
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: jay.foad@gmail.com


This test case uses memcpy to load the bytes of a float into an int:

$ cat m.c
int f(float *p) {
    int i;
    __builtin_memcpy(&i, p, sizeof i);
    return i;
}

When compiled for armv7 I get:

$ cc1 -o - -O3 m.c -quiet -march=armv7 -mthumb
...
f:
    @ args = 0, pretend = 0, frame = 4
    @ frame_needed = 0, uses_anonymous_args = 0
    @ link register save eliminated.
    ldr    r0, [r0, #0]    @ unaligned
    sub    sp, sp, #4
    str    r0, [sp, #0]    @ unaligned
    ldr    r0, [sp, #0]
    add    sp, sp, #4
    bx    lr
...

The stack frame is unnecessary; it could compile to just:

    ldr    r0, [r0, #0]
    bx    lr

I'm using trunk rev 184597, configured with --target=arm-elf.