BAD TEST CASE REDUCTION ON MY END

clang.s

        .file   "foo.c"
        .globl  foo                             // -- Begin function foo
        .p2align        2
        .type   foo,@function
foo:                                    // @foo
        .cfi_startproc
// %bb.0:
        movi    v0.2d, #0000000000000000
        //APP
        //first use v0
        //NO_APP
        mov     v1.16b, v0.16b
        //APP
        //clobber
        //NO_APP
        mov     v0.16b, v1.16b
        //APP
        //last use v0
        //NO_APP
        ret
.Lfunc_end0:
        .size   foo, .Lfunc_end0-foo
        .cfi_endproc
                                        // -- End function
        .ident  "clang version 18.1.8 (Fedora 18.1.8-1.fc40)"
        .section        ".note.GNU-stack","",@progbits
        .addrsig

gcc.s

# gcc -march=armv8-a+crypto -O2 reduced_case.c -S
o
        .file   "foo.c"
        .text
        .align  2
        .p2align 5,,15
        .global foo
        .type   foo, %function
foo:
.LFB3916:
        .cfi_startproc
        movi    v0.4s, 0
#APP
// 11 "foo.c" 1
         #first use v0
// 0 "" 2
// 14 "foo.c" 1
         #clobber
// 0 "" 2
// 17 "foo.c" 1
         #last use v0
// 0 "" 2
#NO_APP
        ret
        .cfi_endproc
.LFE3916:
        .size   foo, .-foo
        .ident  "GCC: (GNU) 14.2.1 20240912 (Red Hat 14.2.1-3)"
        .section        .note.GNU-stack,"",@progbits

reduced_case.c

#include <arm_neon.h>

typedef uint64x2_t v128;

v128 foo()
{
    // We'll hint to the compiler that this value should go in v0.
    register v128 x asm("v0") = { 0 };

    // Confirm that it is in v0
    asm volatile(" #first use %0" : "+w"(x));

    // Clobber v0... surely there must be a backup/restore sequence?
    // XXX https://hachyderm.io/@pinskia/113256478102232611
    // This is actually WAI.
    asm volatile(" #clobber" ::: "v0");

    // Confirm that the value is restored in v0
    asm volatile(" #last use %0" : "+w"(x));

    // Use the value.
    return x;
}

I tried to use asm variables to reduce a test case, but that just results in a surprisingly invalid program https://hachyderm.io/@pinskia/113256478102232611