[Bug middle-end/69008] gcc emits unneeded memory access when passing trivial structs by value (ARM)

[david.forgeas at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69008

David Forgeas <david.forgeas at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |david.forgeas at gmail dot com

--- Comment #7 from David Forgeas <david.forgeas at gmail dot com> ---
The use case that made me see this is std::string_view, or equivalently:

namespace my {
        struct string_view {
                const char *data;
                unsigned long length;
                const char & back() const {
                        return data[length - 1];
                }
        };
}
const char *back(my::string_view const sv)
{
        return &sv.back();
}

pi@raspberrypi:~ $ g++ -O3 -pipe -S string_view.cpp -std=c++1z
pi@raspberrypi:~ $ cat string_view.s 
        .arch armv6
        .eabi_attribute 28, 1
        .eabi_attribute 20, 1
        .eabi_attribute 21, 1
        .eabi_attribute 23, 3
        .eabi_attribute 24, 1
        .eabi_attribute 25, 1
        .eabi_attribute 26, 2
        .eabi_attribute 30, 2
        .eabi_attribute 34, 1
        .eabi_attribute 18, 4
        .file   "string_view.cpp"
        .text
        .align  2
        .global _Z4backN2my11string_viewE
        .syntax unified
        .arm
        .fpu vfp
        .type   _Z4backN2my11string_viewE, %function
_Z4backN2my11string_viewE:
        .fnstart
.LFB1:
        @ 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}
        ldm     sp, {r0, r3}
        sub     r3, r3, #1
        add     r0, r0, r3
        add     sp, sp, #8
        @ sp needed
        bx      lr
        .cantunwind
        .fnend
        .size   _Z4backN2my11string_viewE, .-_Z4backN2my11string_viewE
        .ident  "GCC: (Raspbian 6.3.0-18+rpi1+deb9u1) 6.3.0 20170516"


Also seen in a more recent version: https://godbolt.org/z/3LJ9SN