Re: [Qemu-devel] [PATCH 1/6] target-arm: Fix rounding constant addition for Neon shift instructions.

[Christophe Lyon <christophe.lyon@st.com>]

On 14.02.2011 19:12, Peter Maydell wrote:
> On 11 February 2011 15:10,  <christophe.lyon@st.com> wrote:
>> +uint32_t HELPER(neon_rshl_s32)(uint32_t valop, uint32_t shiftop)
>> +{
>> +    int32_t dest;
>> +    int32_t val = (int32_t)valop;
>> +    int8_t shift = (int8_t)shiftop;
>> +    if (shift >= 32) {
>> +        dest = 0;
>> +    } else if (shift < -32) {
>> +        dest = val >> 31;
> 
> This is the wrong answer: large rounding right shifts give zero.
> 
>> +    } else if (shift == -32) {
>> +        dest = val >> 31;
>> +        dest++;
>> +        dest >>= 1;
> 
> These three lines will always result in dest becoming
> 0 regardless of the input value.
> 

You are right. Actually, I just intended to fix the case where
-32 < shift < 0, and merely re-instanciated the preceding macro with a known size of 32.

You comments also apply to the 8 and 16 bits variants in that macro.

I am too respectful of existing code :-)

Christophe.