Re: [PATCH v5 16/30] tcg/loongarch64: Implement shl/shr/sar/rotl/rotr ops

[Richard Henderson]

On 9/25/21 6:05 AM, Philippe Mathieu-Daudé wrote:
> > +    case INDEX_op_shl_i32:
> > +        if (c2) {
>
> Why can't we use:
>
>                 tcg_debug_assert(a2 <= 0x1f);
>                 tcg_out_opc_slli_w(s, a0, a1, a2);
>
> ?

Because tcg/optimize.c can produce out-of-range values.
We have this same masking in tcg/sparc/ starting as far back as 1fd95946657.

Officially, the tcg backend generator must accept this, with UNSPECIFIED behaviour. 
Generally, such out-of-range shifts will be followed by a conditional move that overwrites 
the undefined result.  The tcg backend is not allowed to trap or assert.


r~