[Bug target/80905] New: ARM: Useless initialization of struct passed by value

[gergo.barany at inria dot fr]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80905

            Bug ID: 80905
           Summary: ARM: Useless initialization of struct passed by value
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gergo.barany at inria dot fr
  Target Milestone: ---

Created attachment 41432
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=41432&action=edit
Input C file for triggering the issue

Input program:

$ cat tst.c
struct S0 {
  int f0;
  int f1;
  int f2;
  int f3;
};

int f1(struct S0 p) {
    return p.f0;
}

int f2(struct S0 p) {
    return p.f0 + p.f3;
}



When entering the function, GCC copies the entire struct from registers to the
stack, even fields that are never used. Fields that *are* used are then
reloaded from the stack even if they are still available in the very same
registers:

$ gcc tst.c -Wall -W -O3 -S -o -
        .arch armv7-a
        .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, 1
        .eabi_attribute 30, 2
        .eabi_attribute 34, 1
        .eabi_attribute 18, 4
        .file   "tst.c"
        .text
        .align  2
        .global f1
        .syntax unified
        .arm
        .fpu vfpv3-d16
        .type   f1, %function
f1:
        @ args = 0, pretend = 0, frame = 16
        @ frame_needed = 0, uses_anonymous_args = 0
        @ link register save eliminated.
        sub     sp, sp, #16
        add     ip, sp, #16
        stmdb   ip, {r0, r1, r2, r3}
        ldr     r0, [sp]
        add     sp, sp, #16
        @ sp needed
        bx      lr
        .size   f1, .-f1
        .align  2
        .global f2
        .syntax unified
        .arm
        .fpu vfpv3-d16
        .type   f2, %function
f2:
        @ args = 0, pretend = 0, frame = 16
        @ frame_needed = 0, uses_anonymous_args = 0
        @ link register save eliminated.
        sub     sp, sp, #16
        add     ip, sp, #16
        stmdb   ip, {r0, r1, r2, r3}
        ldr     r0, [sp]
        ldr     r3, [sp, #12]
        add     r0, r0, r3
        add     sp, sp, #16
        @ sp needed
        bx      lr
        .size   f2, .-f2
        .ident  "GCC: (GNU) 8.0.0 20170527 (experimental)"

Target: armv7a-eabihf
Configured with: --target=armv7a-eabihf --with-arch=armv7-a
--with-fpu=vfpv3-d16 --with-float-abi=hard --with-float=hard
gcc version 8.0.0 20170527 (experimental) (GCC)


This seems to be specific to ARM as I cannot reproduce this behavior on x86-64
or PowerPC.


For comparison, LLVM generates the following code for ARM:

f1:
        .fnstart
@ BB#0:
        bx      lr

f2:
        .fnstart
@ BB#0:
        add     r0, r0, r3
        bx      lr