mardi 28 juin 2016

RGBA to ABGR: Inline arm neon asm for iOS/XCode

This code(very similar code, haven't tried exactly this code) compiles using Android NDK, but not with XCode/armv7+arm64/iOS

Errors in comments:

uint32_t *src;
uint32_t *dst;

#ifdef __ARM_NEON
__asm__ volatile(
    "vld1.32 {d0, d1}, [%[src]] n" // error: Vector register expected
    "vrev32.8 q0, q0            n" // error: Unrecognized instruction mnemonic
    "vst1.32 {d0, d1}, [%[dst]] n" // error: Vector register expected
    :
    : [src]"r"(src), [dst]"r"(dst)
    : "d0", "d1"
    );
#endif

What's wrong with this code?

EDIT1:

I rewrote the code using intrinsics:

uint8x16_t x = vreinterpretq_u8_u32(vld1q_u32(src));
uint8x16_t y = vrev32q_u8(x);
vst1q_u32(dst, vreinterpretq_u32_u8(y));

After disassembling, I get the following, which is a variation I have already tried:

vld1.32 {d16, d17}, [r0]!
vrev32.8    q8, q8
vst1.32 {d16, d17}, [r1]!

So my code looks like this now, but gives the exact same errors:

__asm__ volatile("vld1.32 {d0, d1}, [%0]! n"
                 "vrev32.8 q0, q0         n"
                 "vst1.32 {d0, d1}, [%1]! n"
                 :
                 : "r"(src), "r"(dst)
                 : "d0", "d1"
                 );

EDIT2:

Reading through the disassembly, I actually found a second version of the function. It turns out that arm64 uses a slightly different instruction set. For example, the arm64 assembly uses rev32.16b v0, v0 instead. The whole function listing(which I can't make heads or tails of) is below:

_My_Function:
cmp     w2, #0
add w9, w2, #3
csel    w8, w9, w2, lt
cmp     w9, #7
b.lo    0x3f4
asr w9, w8, #2
ldr     x8, [x0]
mov  w9, w9
lsl x9, x9, #2
ldr q0, [x8], #16
rev32.16b   v0, v0
str q0, [x1], #16
sub x9, x9, #16
cbnz    x9, 0x3e0
ret

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