[llvm-commits] DIV -> U/S/FDiv Patch For Review (Followup #2)

[Chris Lattner]

On Oct 24, 2006, at 9:45 PM, Reid Spencer wrote:

> On Mon, 2006-10-23 at 22:52 -0700, Chris Lattner wrote:
>
>> I strongly recommend adding a transformation to instcombine that will
>> produce more divs with such operands.  For example, changing  
>> something
>> like:
>>
>>   %X = cast uint %A to int
>>   %Y = sdiv int %X, 1234
>>   %Z = cast int %Y to uint
>> into:
>>    %Z = sdiv uint %A, 1234
>> Fortunately, instcombine already does this for other operations
>> (mul/add/and/or/xor, etc) at line 5658 of Instcombine in CVS.  Please
>> add a case there for UDIV/SDIV.  This will hopefully help flush out
>> bugs in the patch that I didn't catch.
>
> I took a look at doing this but there are corner cases I'm dealing  
> with.
> If the input is:
>
>   %X = cast uint %A to int
>   %Y = sdiv int %X, -1234
>   %Z = cast int %Y to uint
>
> then its a little more challenging to make the sdiv work with unsigned
> operands.  So I'm assuming this only works with positive constants.
> There are other corner cases too, like if the 2nd operand isn't a
> constant but an instruction.
>
> I would rather not risk breaking things now by trying to add a new
> transform. After implementing all your other suggestions the patch is
> passing all the tests.

I don't follow.  The existing code should transform this into:

%Z = sdiv uint %A, 4294966062

This is exactly what you want.  "4294966062" treated as a signed  
value (which is what sdiv does) is -1234.

-Chris