Re: [PATCH 1/4] target/arm: Vectorize USHL and SSHL

[Richard Henderson]

On 10/17/19 9:01 AM, Alex Bennée wrote:
>> +    /*
>> +     * Rely on the TCG guarantee that out of range shifts produce
>> +     * unspecified results, not undefined behaviour (i.e. no trap).
>> +     * Discard out-of-range results after the fact.
>> +     */
>> +    tcg_gen_ext8s_i32(lsh, b);
>> +    tcg_gen_neg_i32(rsh, lsh);
>> +    tcg_gen_shl_i32(lval, a, lsh);
>> +    tcg_gen_shr_i32(rval, a, rsh);
>> +    tcg_gen_movcond_i32(TCG_COND_LTU, d, lsh, max, lval, zero);
>> +    tcg_gen_movcond_i32(TCG_COND_LTU, d, rsh, max, rval, d);
> 
> Do these get dead coded away if the shift is a const?

They would, although because of the form of the instruction, as a variable
shift from a vector element, I don't expect it to ever be const.  We will have
just read the value from env memory.


r~