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From: "palchak at google dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug rtl-optimization/104372] New: [ARM] Unnecessary writes to stack
 when passing aggregate in registers
Date: Thu, 03 Feb 2022 19:20:41 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D104372

            Bug ID: 104372
           Summary: [ARM] Unnecessary writes to stack when passing
                    aggregate in registers
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: palchak at google dot com
  Target Milestone: ---

Missed optimization when an aggregate is passed by value entirely in regist=
ers:

struct Bar {};

struct Foo {
    Bar *addr;
    int size;
};

Bar* MakeBar1(Bar *addr, int) noexcept {
    return addr;
}

Bar* MakeBar2(Foo foo) noexcept {
    return foo.addr;
}


When compiled with '-O2' using 'arm-unknown-linux-gnueabihf-g++ (GCC) 12.0.=
0'
generates:

MakeBar1(Bar*, int):
        bx      lr
MakeBar2(Foo):
        sub     sp, sp, #8
        add     r3, sp, #8
        stmdb   r3, {r0, r1}
        add     sp, sp, #8
        bx      lr


The creation of a stack frame in MakeBar2 is completely unnecessary. For
comparison, Clang 11.0.1 generates identical code for both functions that
matches MakeBar1 shown here.=