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From: "b.grayson at samsung dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/58623] New: lack of ldp/stp optimization
Date: Fri, 04 Oct 2013 19:19:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58623

            Bug ID: 58623
           Summary: lack of ldp/stp optimization
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: b.grayson at samsung dot com
            Target: AArch64
             Build: 20130602

The following C code:

long long a, b;
int c, d;

int foo() { return a+b; }
int bar() { return c+d; }

generates this assembly code under -O3 -fsection-anchors -fno-common:

foo:
        adrp    x1, .LANCHOR0
        add     x1, x1, :lo12:.LANCHOR0
        ldr     x2, [x1]
        ldr     x0, [x1,8]
        add     w0, w2, w0
        ret
bar:
        adrp    x1, .LANCHOR0
        add     x1, x1, :lo12:.LANCHOR0
        ldr     w2, [x1,16]
        ldr     w0, [x1,20]
        add     w0, w2, w0
        ret

Note that the ldr x2 and ldr x0 could have been merged into an ldp, in foo(). 
Similarly, the ldr w2 and ldr w0 (32-bit loads) could have been merged into an
ldp in bar().

The same optimization applies to stores as well.

I am not sure if this would be handled by the proposed (but apparently not
accepted) patch from March 2013:

http://gcc.gnu.org/ml/gcc-patches/2013-03/msg01051.html