Inline assembly instruction on ARM64 (iOS) fails

Question

I'm trying out the new arm64 instructions on iOS and I'm having a peculiar issue. I hope someone can help me with this.

In particular this fails with 'Invalid operand for instruction'

void test()
{
    register long long signed int r=0,c=0,d=0;
    register signed int a=0,b=0,e=0,f=0;

    // this fails
    asm volatile("smaddl %0, %1, %2, %3" : "=r"(r) : "r"(a), "r"(b), "r"(c));
};

I'm not sure what I'm doing wrong, to the best that I can tell, I'm following the instruction and syntax correctly. Here's how it is defined in the docs:

"SMADDL Xd, Wn, Wm, Xa Signed Multiply-Add Long: Xd = Xa + (Wn × Wm), treating source operands as signed."

where X denotes a 64bit register and W denotes a 32 bit one.

Any help will be appreciated.

Thx

Answer 1

I was able to fix it by using recommendation in this post:

asm volatile("smaddl %x0, %w1, %w2, %x3" : "=r"(r) : "r"(a), "r"(b), "r"(c));

This produces the following assembly:

_test:                                  ; @test
; BB#0:
    sub sp, sp, #48
    movz    w8, #0
    movz    x9, #0
    stp x9, x9, [sp, #32]
    str x9, [sp, #24]
    stp w8, w8, [sp, #16]
    stp w8, w8, [sp, #8]
    ldp w10, w8, [sp, #16]
    ldr x9, [sp, #32]
    ; InlineAsm Start
    smaddl x9, w8, w10, x9
    ; InlineAsm End
    str x9, [sp, #40]
    add sp, sp, #48
    ret lr

It seems you need to use 'w' to specifically mark 32-bit registers.

See also aarch64-inline-asm.c for a few more inline asm examples.