[PATCH] peephole optimization in switch table lookup: reuse the guarding table comparison if possible

[Hans Wennborg]

On Tue, Nov 18, 2014 at 10:08 AM, Hans Wennborg <hans at chromium.org> wrote:
> Hi Erik,
>
> (Haven't looked much at the patch yet, just thinking about the idea
> itself right now.)
>
> On Tue, Nov 18, 2014 at 7:53 AM, Erik Eckstein <eeckstein at apple.com> wrote:
>> Hi Hans,
>>
>> I have another patch for the switch table lookup generation. It tries to reuse the generated compare instruction, if there is a comparison agains the default value after the switch.
>>
>> Example:
>>
>> switch (x) {
>>   case 0: r = 10; break;
>>   case 1: r = 11; break;
>>   ...
>>   default: r = 0; break; // 0 does not appear in any case value.
>> }
>> if (r == 0) {
>>   do_something;
>> }
>>
>> transforms to:
>>
>> if (x < table_size) {
>>   r = table[x];
>> } else {
>>   r = 0;
>>   do_something;
>> }
>
> Interesting. We're basically exploiting that we have extra knowledge
> about the relationship between x and r. It would be cool if it was a
> bit broader though, e.g. what if it was "if (r < 10)", and that was
> only true if we hit the default case in the switch.
>
> My main concern is whether the benefit is worth the complexity this
> adds to SwitchToLookupTable (or if that's the right place for it).
>
> I also wonder if we could make it broader, based on control-flow and
> value tracking. For example, if we have:
>
>   vars = phi
>   if (expr(vars)) {
>     do stuff
>   }
>
> If we know that expr(vars) is only true for one of the incoming edges
> to the phi, could we try to hoist the if-body into that incoming edge?
> It seems this would cover your example, and could maybe hit more
> cases.

I guess I'm thinking that your example isn't really specific to the
switch transformation. For example, even if we're not building a
lookup table, your code could still save the conditional branch by
transforming to:

 switch (x) {
   case 0: r = 10; break;
   case 1: r = 11; break;
   ...
   default:
     r = 0;
     do_something;
     break; // 0 does not appear in any case value.
 }