[llvm-commits] [PATCH] Make EarlyCSE understand commutativity

[Michael Ilseman]

I don't think that teaching EarlyCSE to do full on value numbering the way GVN does is needed or desired. I was able to factor out GVN's Expression class and its shared functionality, though, and EarlyCSE can use it instead of its SimpleValue as a canonicalized representation of an Instruction.

Attached is the new Expression.h (to reside in llvm/Transforms/Utils/), which contains the shared functionality. Also attached are GVN.patch and EarlyCSE.patch, which modify GVN and EarlyCSE to use it.
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On Sep 24, 2012, at 1:51 PM, Owen Anderson <resistor at mac.com> wrote:

> 
> On Sep 24, 2012, at 12:55 PM, Evan Cheng <evan.cheng at apple.com> wrote:
> 
>> 
>> On Sep 24, 2012, at 9:46 AM, Michael Ilseman <milseman at apple.com> wrote:
>> 
>>> 
>>> On Sep 22, 2012, at 3:01 AM, Duncan Sands <baldrick at free.fr> wrote:
>>>> urgh, I really don't like creating an instruction just to look it up, then
>>>> deleting it again.  Can you make the lookup logic more flexible instead?
>>>> By the way, how GVN takes care of commutativity is: when computing the value
>>>> number for an instruction, it sorts the operands of commutative instructions
>>>> (its sorts the operands according to their value number) before calculating
>>>> a value number from them, ensuring that if two commutative instructions are
>>>> the same except for a different operand order then they get the same value
>>>> number.  I don't know how EarlyCSE works, but maybe something along these lines
>>>> would also be possible?
>>>> 
>>> 
>>> Thanks for the feedback!
>>> 
>>> I also don't like making a new instruction, but considered it the lesser of evils. Essentially, EarlyCSE uses a DenseMap over the instructions, but overloading hash to hash the instruction itself rather than its address. It also overloads isEqual to call Instruction::isIdenticalTo.
>>> 
>>> As far as changing the lookup mechanism, that would involve both sorting operand hashes on getHashValue, and extending Instruction::isIdenticalToWhenDefined to also calculate equality modulo commutativity (or, essentially inlining its body into the overloaded isEqual function rather than making the call). I don't know what is more desirable, but the original patch to EarlyCSE at least kept it very localized and would only trigger when a commutative op was encountered.
>>> 
>>> Another option is to restructure EarlyCSE to do expression numbering similarly to GVN, as you mentioned. Again, I don't know if this is more desirable over a localized call to clone(), as this would require a re-encapsulation of Instruction's data that would affect all of EarlyCSE.
>> 
>> I would strong advise against the new instruction approach. That's considered a definite no-no in LLVM. Do you have a good sense how much effort it is to implement expression numbering?
> 
> It's not so hard.  GVN already does it, and the code is relatively independent of the details of GVN.  You could probably extract it to a set of utility files and share it between both passes.
> 
> --Owen