llvm dev - Oct 2015 - Matching instruction patterns from a FunctionPass

Hi all,

I'm working on a pass that would need to match a pattern of instructions. In
pseudo-IR:

%0 = sub %a, %b
%1 = xor %a, %b
%2 = xor %0, %a
%3 = and %1, %2

After this instruction sequence, %3's most significant bit tells whether the
subtraction overflowed. I would like to find occurrences of this pattern from a
FunctionPass.

However, this has a couple of possible permutations. And and Xor are both
commutative, meaning that there are 8 valid ways to express this. Since I might
not be the first person to run into this kind of issue, I was wondering if there
is a common way to handle this.

Thanks!

Félix

Looks like llvm/IR/PatternMatch.h is what I'm looking for.

Félix
> Le 23 oct. 2015 à 23:53:44, Félix Cloutier <felixcca at yahoo.ca> a
écrit :
> 
> Hi all,
> 
> I'm working on a pass that would need to match a pattern of
instructions. In pseudo-IR:
> 
> %0 = sub %a, %b
> %1 = xor %a, %b
> %2 = xor %0, %a
> %3 = and %1, %2
> 
> After this instruction sequence, %3's most significant bit tells
whether the subtraction overflowed. I would like to find occurrences of this
pattern from a FunctionPass.
> 
> However, this has a couple of possible permutations. And and Xor are both
commutative, meaning that there are 8 valid ways to express this. Since I might
not be the first person to run into this kind of issue, I was wondering if there
is a common way to handle this.
> 
> Thanks!
> 
> Félix