[Qemu-devel] Why some ARM NEON helper functions need mask?

[陳韋任]

Hi, all

  I am looking into QEMU's implementation for ARM NEON instructions
(target-arm/neon_helper.c). Some helper functions will do mask
operation, neon_add_u8, for example. I thought simply adding a and b
is enough and can't figure out why the mask operation is needed.

---
uint32_t HELPER(neon_add_u8)(uint32_t a, uint32_t b)
{
    uint32_t mask;
    mask = (a ^ b) & 0x80808080u;
    a &= ~0x80808080u;
    b &= ~0x80808080u;
    return (a + b) ^ mask;
}
---

  Any help is appreciated.

Regards,
chenwj

-- 
Wei-Ren Chen (陳韋任)
Computer Systems Lab, Institute of Information Science,
Academia Sinica, Taiwan (R.O.C.)
Tel:886-2-2788-3799 #1667