llvm dev - Jul 2013 - [LLVMdev] Eliminating PHI with Identical Inputs

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

-- John T.

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.

Ciao, Duncan.

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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