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Subject: [Bug target/55026] New: [Multiple targets] Inefficient code with structs passed by value
Date: Mon, 22 Oct 2012 16:39:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55026

             Bug #: 55026
           Summary: [Multiple targets] Inefficient code with structs
                    passed by value
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: mans@mansr.com


Multiple targets (including ARM, HPPA, and MIPS) generate inefficient code in
functions taking struct arguments by value.  Consider this code:

struct foo {
    int a;
    int b;
};

int f(struct foo x)
{
    return x.a + x.b;
}

On ARM, this compiles to the following at -O3 optimisation:

f:
        @ args = 0, pretend = 0, frame = 8
        @ frame_needed = 0, uses_anonymous_args = 0
        @ link register save eliminated.
        sub     sp, sp, #8
        add     r3, sp, #8
        stmdb   r3, {r0, r1}
        ldmia   sp, {r0, r3}
        add     r0, r0, r3
        add     sp, sp, #8
        @ sp needed
        bx      lr

Note the entirely unnecessary (and inefficiently done to boot) storing and
loading of the argument registers to/from the stack.

The x86_64 and SH4 targets do not show this behaviour.