[PATCH] CVP: Improve handling of Selects used as incoming PHI values

[Björn Steinbrink]

Updated the patch to only apply the optimization to selects with a scalar condition.


http://reviews.llvm.org/D9051

Files:
  lib/Transforms/Scalar/CorrelatedValuePropagation.cpp
  test/Transforms/CorrelatedValuePropagation/select.ll

Index: lib/Transforms/Scalar/CorrelatedValuePropagation.cpp
===================================================================
--- lib/Transforms/Scalar/CorrelatedValuePropagation.cpp
+++ lib/Transforms/Scalar/CorrelatedValuePropagation.cpp
@@ -103,24 +103,46 @@
 
     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()) {
+        switch (LVI->getPredicateOnEdge(ICmpInst::ICMP_EQ, Condition,
+              ConstantInt::getTrue(Condition->getType()),
+              P->getIncomingBlock(i), BB, P)) {
+          case LazyValueInfo::True:
+            V = SI->getTrueValue();
+            break;
+          case LazyValueInfo::False:
+            V = SI->getFalseValue();
+            break;
+          default:
+            break;
+        }
+      }
 
-      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);
Index: test/Transforms/CorrelatedValuePropagation/select.ll
===================================================================
--- /dev/null
+++ test/Transforms/CorrelatedValuePropagation/select.ll
@@ -0,0 +1,18 @@
+; RUN: opt < %s -correlated-propagation -S | FileCheck %s
+
+; CHECK-LABEL: @test(
+define void @test(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
+}

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