[PATCH] D45448: [CVP] simplify phi with constant incoming values that match common variable edge values

[Florian Hahn via Phabricator via llvm-commits]

fhahn added inline comments.


================
Comment at: test/Transforms/CorrelatedValuePropagation/phi-common-val.ll:4
+
+define i8* @simplify_phi_common_value_op0(i8* %ptr, i32* %b) {
+; CHECK-LABEL: @simplify_phi_common_value_op0(
----------------
It looks like the reason we miss those cases is that we are missing the links between the null constants in the comparison and the PHI node. 

Now, if we would have something like the code below, GVN could handle it.
```
define i8* @simplify_phi_common_value_op0(i8* %ptr, i32* %b, i8* %cmp) {
entry:
  %null.v = call i8* @llvm.ssa.copy.p0i8(i8* null)
  %isnull = icmp eq i8* %ptr, %cmp
  br i1 %isnull, label %return, label %else

else:                                             ; preds = %entry
  %lb = load i32, i32* %b
  %add = add nsw i32 %lb, 1
  store i32 %add, i32* %b
  br label %return

return:                                           ; preds = %else, %entry
  ret i8* %ptr
}
```

Unfortunately I don't think we have an easy way to get to that. In theory, I think should be able to annotate constants with the possible variables they are equal to, similar to what PredicateInfo does. Then we could use that to check if a constant is equal to a helpful variable. But that is a long shot and also (much) more work than this solution.


https://reviews.llvm.org/D45448