[Bug target/81012] ARM: Spill instead of register copy / dead store on int-to-double conversion

["gergo.barany at inria dot fr" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81012

--- Comment #2 from Gergö Barany <gergo.barany at inria dot fr> ---
Created attachment 41672
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=41672&action=edit
Smaller test case

Added a smaller test case:

int fn3(int p1, int p2) {
  int a = p2;
  if (p1)
    a *= 10.0;
  return a;
}

It compiles to the following:

fn3:
        @ args = 0, pretend = 0, frame = 8
        @ frame_needed = 0, uses_anonymous_args = 0
        @ link register save eliminated.
        sub     sp, sp, #8
        cmp     r0, #0
        str     r1, [sp, #4]
        beq     .L2
        vmov.f64        d6, #1.0e+1
        vmov    s15, r1 @ int
        vcvt.f64.s32    d7, s15
        vmul.f64        d7, d7, d6
        vcvt.s32.f64    s15, d7
        vstr.32 s15, [sp, #4]   @ int
.L2:
        ldr     r0, [sp, #4]
        add     sp, sp, #8
        @ sp needed
        bx      lr
        .size   fn3, .-fn3
        .ident  "GCC: (GNU) 8.0.0 20170626 (experimental)"

Instead of the first store, r1 should be moved to r0. The second store should
then be a vmov r0, s15. No spills needed.

This is done correctly on x86-64:

fn3:
.LFB0:
        .cfi_startproc
        testl   %edi, %edi
        movl    %esi, %eax
        je      .L2
        pxor    %xmm0, %xmm0
        cvtsi2sd        %esi, %xmm0
        mulsd   .LC0(%rip), %xmm0
        cvttsd2si       %xmm0, %eax
.L2:
        rep ret