Jeff Law via Gcc-patches <gcc-patches@gcc.gnu.org> writes:
> On 6/9/23 04:41, juzhe.zh...@rivai.ai wrote:
>> @@ -4342,135 +4510,81 @@ pass_vsetvl::cleanup_insns (void) const
>> }
>> }
>>
>> +/* Return true if the SET result is not used by any instructions. */
>> +static bool
>> +has_no_uses (basic_block cfg_bb, rtx_insn *rinsn, int regno)
>> +{
>> + /* Handle the following case that can not be detected in RTL_SSA. */
>> + /* E.g.
>> + li a5, 100
>> + vsetvli a6, a5...
>> + ...
>> + vadd (use a6)
>> +
>> + The use of "a6" is removed from "vadd" but the information is
>> + not updated in RTL_SSA framework. We don't want to re-new
>> + a new RTL_SSA which is expensive, instead, we use data-flow
>> + analysis to check whether "a6" has no uses. */
> I'm a bit surprised there wasn't a reasonable way to update the RTL SSA
> framework for this case. If we were to remove the entire vadd, then we
> would have to update the uses of a6. If we have that capability, then I
> would expect we could refactor the updating code so that we had an API
> to remove an operand from an instruction.
>
> In fact, if we have a constant propagator in the RTL SSA framework,
> wouldn't it have to have this capability?
RTL-SSA isn't supposed to be feature-complete in its current state.
So yeah, if something is missing, it's better to add it to RTL-SSA
rather than work around it in consumers.
(Responding without fully understanding the context though, sorry.)
Thanks,
Richard
