[Transforms] Enhance CorrelatedValuePropagation to handle both values of select

The "Correlated Value Propagation" pass was missing a case when handling select instructions. It was only handling the "false" constant value, while in NVPTX the select may have the condition (and thus the branches) inverted, for example:
```
loop:
	%phi = phi i32* [ %sel, %loop ], [ %x, %entry ]
	%f = tail call i32* @f(i32* %phi)
	%cmp1 = icmp ne i32* %f, %y
	%sel = select i1 %cmp1, i32* %f, i32* null
	%cmp2 = icmp eq i32* %sel, null
	br i1 %cmp2, label %return, label %loop
```
But the select condition can be inverted:
```
	%cmp1 = icmp eq i32* %f, %y
	%sel = select i1 %cmp1, i32* null, i32* %f
```
The fix is to enhance "Correlated Value Propagation" to handle both branches of the select instruction.

Reviewed By: nikic, lebedev.ri

Differential Revision: https://reviews.llvm.org/D119643

diff --git a/llvm/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp b/llvm/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp
index cc90be9..dc28a1c 100644 (file)
--- a/llvm/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp
+++ b/llvm/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp
@@ -245,11 +245,20 @@ static Value *getValueOnEdge(LazyValueInfo *LVI, Value *Incoming,
   // value can never be that constant. In that case replace the incoming
   // value with the other value of the select. This often allows us to
   // remove the select later.
+
+  // The "false" case
   if (auto *C = dyn_cast<Constant>(SI->getFalseValue()))
     if (LVI->getPredicateOnEdge(ICmpInst::ICMP_EQ, SI, C, From, To, CxtI) ==
         LazyValueInfo::False)
       return SI->getTrueValue();
 
+  // The "true" case,
+  // similar to the select "false" case, but try the select "true" value
+  if (auto *C = dyn_cast<Constant>(SI->getTrueValue()))
+    if (LVI->getPredicateOnEdge(ICmpInst::ICMP_EQ, SI, C, From, To, CxtI) ==
+        LazyValueInfo::False)
+      return SI->getFalseValue();
+
   return nullptr;
 }
 

diff --git a/llvm/test/Transforms/CorrelatedValuePropagation/basic.ll b/llvm/test/Transforms/CorrelatedValuePropagation/basic.ll
index 024ff29..55ae559 100644 (file)
--- a/llvm/test/Transforms/CorrelatedValuePropagation/basic.ll
+++ b/llvm/test/Transforms/CorrelatedValuePropagation/basic.ll
@@ -179,9 +179,8 @@ define void @loop2(i32* %x, i32* %y) {
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    br label [[LOOP:%.*]]
 ; CHECK:       loop:
-;; CorrelatedValuePropagation should handle inverted select condition, but it does not yet.
-;; CHECK-NEXT:    [[PHI:%.*]] = phi i32* [ [[F:%.*]], [[LOOP]] ], [ [[X:%.*]], [[ENTRY:%.*]] ]
-; CHECK-NEXT:    [[F:%.*]] = tail call i32* @f(i32* [[PHI]])
+; CHECK-NEXT:    [[PHI:%.*]] = phi i32* [ [[F:%.*]], [[LOOP]] ], [ [[X:%.*]], [[ENTRY:%.*]] ]
+; CHECK-NEXT:    [[F]] = tail call i32* @f(i32* [[PHI]])
 ; CHECK-NEXT:    [[CMP1:%.*]] = icmp eq i32* [[F]], [[Y:%.*]]
 ; CHECK-NEXT:    [[SEL:%.*]] = select i1 [[CMP1]], i32* null, i32* [[F]]
 ; CHECK-NEXT:    [[CMP2:%.*]] = icmp eq i32* [[SEL]], null