[PATCH] D43835: Simplify more cases of logical ops of masked icmps.

[Hiroshi Yamauchi via Phabricator via llvm-commits]

yamauchi added inline comments.


================
Comment at: lib/Transforms/InstCombine/InstCombineAndOrXor.cpp:499
+  // For example,
+  // (icmp ne (A & 12), 0) & (icmp eq (A & 7), 1) -> (icmp eq (A & 15), 9)
+  // (icmp ne (A & 15), 0) & (icmp eq (A & 7), 0) -> (icmp eq (A & 15), 8)
----------------
davidxl wrote:
> This check can probably be extended:
> 
> 1) B's bitset subtracts the intersection of B and D contains only one bit
> 2) B and D's intersection have zero values
> 
> In other words, D does not need to be power of 2
Is there a chance that those two conditions you are referring to *are* what the current code is already checking here?

To be more specific, 

Isn't the condition 1 equivalent to "(B & (B ^ D)).isPowerOf2()" that the current code checks?

(I meant "(B & (B ^ D))" as the bits that are 1 in B but 0 in D.)

For example, suppose B = 1100 and D = 0101. The condition 1 would say 1100 - (1100 & 0101) = 1100 - 0100 = 1000 while (B & (B ^ D)) = (1100 & (1100 ^ 0101)) = (1100 & 1001) = 1000. 

Also, isn't the condition 2 equivalent to "((B & D) & E) == 0" that the current code checks?

(Note that in the current code, it's not D, but (B & (B ^ D)) that needs to be a power of 2.)

Do you have an example that isn't handled by the current code but is handled by the suggested extension?


Repository:
  rL LLVM

https://reviews.llvm.org/D43835