[llvm-commits] [Patch] Switch-to-lookup tables: do simple constant propagation

[Evan Cheng]

LGTM

evan

On Oct 29, 2012, at 10:54 AM, Hans Wennborg <hans at chromium.org> wrote:

> On Wed, Oct 10, 2012 at 9:55 AM, Hans Wennborg <hans at chromium.org> wrote:
>> On Wed, Oct 3, 2012 at 1:51 PM, Duncan Sands <baldrick at free.fr> wrote:
>>> 
>>>> A patch [1] by Craig earlier this week pointed out that Clang fails to
>>>> build lookup tables for switches like this:
>>>> 
>>>>   switch(x) {
>>>>     case 1: return 5;
>>>>     case 2: return 42;
>>>>     case 3: case 4: case 5:
>>>>       return x - 123;
>>>>     default: return 123;
>>>>   }
>>>> 
>>>> Where the resulting value for the 3, 4, 5 cases is "x - 123", rather
>>>> than a constant.
>>> 
>>> 
>>> maybe it is better to do this as part of some more generic switch
>>> simplification
>>> logic rather than during table construction.
>> 
>> We chatted about this on IRC last week, and I think we concluded that
>> it seemed hard to determine when it would be profitable to do this
>> transformation in general, and that it probably made sense to just do
>> it when forming lookup tables.
>> 
>> If so, I'd like to move forward with this patch.
>> 
>> Does anyone have comments on the constant propagation code itself?
>> Does it look sensible, or are there any utility functions I could
>> reuse?
> 
> Ping? Attaching a rebased patch for convenience.
> 
> Thanks,
> Hans
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