CVP: Improve handling of Selects used as incoming PHI values

Summary:
If the branch that leads to the PHI node and the Select instruction
depend on correlated conditions, we might be able to directly use the
corresponding value from the Select instruction as the incoming value
for the PHI node, allowing later removal of the select instruction.

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D9051

llvm-svn: 237201

diff --git a/llvm/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp b/llvm/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp
index 912d527..d1302c6 100644 (file)
--- a/llvm/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp
+++ b/llvm/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp
@@ -103,24 +103,43 @@ bool CorrelatedValuePropagation::processPHI(PHINode *P) {
 
     Value *V = LVI->getConstantOnEdge(Incoming, P->getIncomingBlock(i), BB, P);
 
-    // Look if the incoming value is a select with a constant but LVI tells us
-    // that the 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.
+    // Look if the incoming value is a select with a scalar condition for which
+    // LVI can tells us the value. In that case replace the incoming value with
+    // the appropriate value of the select. This often allows us to remove the
+    // select later.
     if (!V) {
       SelectInst *SI = dyn_cast<SelectInst>(Incoming);
       if (!SI) continue;
 
-      Constant *C = dyn_cast<Constant>(SI->getFalseValue());
-      if (!C) continue;
+      Value *Condition = SI->getCondition();
+      if (!Condition->getType()->isVectorTy()) {
+        if (Constant *C = LVI->getConstantOnEdge(Condition, P->getIncomingBlock(i), BB, P)) {
+          if (C == ConstantInt::getTrue(Condition->getType())) {
+            V = SI->getTrueValue();
+          } else {
+            V = SI->getFalseValue();
+          }
+          // Once LVI learns to handle vector types, we could also add support
+          // for vector type constants that are not all zeroes or all ones.
+        }
+      }
 
-      if (LVI->getPredicateOnEdge(ICmpInst::ICMP_EQ, SI, C,
-                                  P->getIncomingBlock(i), BB, P) !=
-          LazyValueInfo::False)
-        continue;
+      // Look if the select has a constant but LVI tells us that the 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.
+      if (!V) {
+        Constant *C = dyn_cast<Constant>(SI->getFalseValue());
+        if (!C) continue;
+
+        if (LVI->getPredicateOnEdge(ICmpInst::ICMP_EQ, SI, C,
+              P->getIncomingBlock(i), BB, P) !=
+            LazyValueInfo::False)
+          continue;
+        V = SI->getTrueValue();
+      }
 
       DEBUG(dbgs() << "CVP: Threading PHI over " << *SI << '\n');
-      V = SI->getTrueValue();
     }
 
     P->setIncomingValue(i, V);

diff --git a/llvm/test/Transforms/CorrelatedValuePropagation/select.ll b/llvm/test/Transforms/CorrelatedValuePropagation/select.ll
new file mode 100644 (file)
index 0000000..5501438
--- /dev/null
+++ b/llvm/test/Transforms/CorrelatedValuePropagation/select.ll
@@ -0,0 +1,53 @@
+; RUN: opt < %s -correlated-propagation -S | FileCheck %s
+
+; CHECK-LABEL: @simple(
+define i8 @simple(i1) {
+entry:
+  %s = select i1 %0, i8 0, i8 1
+  br i1 %0, label %then, label %else
+
+then:
+; CHECK: ret i8 0
+  %a = phi i8 [ %s, %entry ]
+  ret i8 %a
+
+else:
+; CHECK: ret i8 1
+  %b = phi i8 [ %s, %entry ]
+  ret i8 %b
+}
+
+; CHECK-LABEL: @loop(
+define void @loop(i32) {
+entry:
+  br label %loop
+
+loop:
+  %idx = phi i32 [ %0, %entry ], [ %sel, %loop ]
+; CHECK: %idx = phi i32 [ %0, %entry ], [ %2, %loop ]
+  %1 = icmp eq i32 %idx, 0
+  %2 = add i32 %idx, -1
+  %sel = select i1 %1, i32 0, i32 %2
+  br i1 %1, label %out, label %loop
+
+out:
+  ret void
+}
+
+; CHECK-LABEL: @not_correlated(
+define i8 @not_correlated(i1, i1) {
+entry:
+  %s = select i1 %0, i8 0, i8 1
+  br i1 %1, label %then, label %else
+
+then:
+; CHECK: ret i8 %s
+  %a = phi i8 [ %s, %entry ]
+  ret i8 %a
+
+else:
+; CHECK: ret i8 %s
+  %b = phi i8 [ %s, %entry ]
+  ret i8 %b
+}
+