[LLVMdev] Eliminating PHI with Identical Inputs

[John Criswell]

On 7/30/13 9:46 AM, Duncan Sands wrote:
> Hi John,
>
> On 30/07/13 16:12, John Criswell wrote:
>> Dear All,
>>
>> Is there a pass (or set of passes) that will replace a phi whose 
>> input operands
>> all compute the same value with an instruction that computes that 
>> value?  In
>> other words, something that will convert:
>>
>> define internal i32 @function(i32 %x) {
>> ...
>> bb1:
>> %y = add %x, 10
>> ...
>> bb2:
>> %z = add %x, 10
>> ...
>> bb3:
>> %phi = [bb1, %y], [bb2, %z]
>>
>> into
>>
>> define internal i32 @function(i32 %x) {
>> ...
>> bb1:
>> ...
>> bb2:
>> ...
>> bb3:
>> %phi = add %x, 10
>
> yes, GVN should replace %y and %z with the same register, giving 
> something like
>
>  %same = add %x, 10
> ...
>  bb1:
>  ...
>  bb2:
>  ...
>  bb3:
>  %phi = [bb1, %same], [bb2, %same]
>
> at which point the instcombine pass should zap the phi, though maybe 
> GVN will
> get it already.

Odd.  I'm running GVN and then InstCombine, and it doesn't fix the problem.

Is this new behavior added since LLVM 3.2, or is this something that GVN 
has been doing for awhile?

-- John T.

>
> Ciao, Duncan.
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