[LLVMdev] Efficient Pattern matching in Instruction Combine

[David Majnemer]

On Thu, Aug 7, 2014 at 9:46 AM, suyog sarda <sardask01 at gmail.com> wrote:

> Hi,
>
> All, Duncan, Rafael, David, Nick.
>
> This is regarding pattern matching in InstructionCombine pass.
>
> We use 'match' functions many times, but it doesn't do the pattern matching
> effectively.
>
> e.x. Lets take pattern :
>
> (A ^ B) | ((B ^ C) ^ A) -> (A ^ B) | C
>
> (B ^ A) | ((B ^ C) ^ A) -> (A ^ B) | C
>
> Both the patterns above are same, since ^ is commutative in Op0.
>
> But, 'match' pattern has to be written for both the patterns separately
> since 'match' identifies pattern as it (like regular expression) and
> doesn't
> have the logic to determine that 'A^B' and 'B^A' are same.
>
> I propose to improve 'match' functions where we can include the logic
> to determine if an operation is commutative or not. If it is commutative,
> then a single 'match' call should be able to detect both 'A^B' and 'B^A'.
> So, basically we will modify the commutative 'match' functions.
>
> There will be various permutations of it where one of the operand might be
> a constant (I guess this is handled already as constant are re-associated
> to
> RHS).
>
> I will try to dig more on this.
>
> Inputs/suggestions/comments on improving match functions are most awaited.
> :)
>
> Regards,
> Suyog
>
> --
> With regards,
> Suyog Sarda
>


It seems to me that we could rejigger Reassociate to reduce the number of
possibilities that InstCombine could expect.
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