CVP: If we have a PHI with an incoming select, try to skip the select.

This is a common pattern with dyn_cast and similar constructs, when the
PHI no longer depends on the select it can often be turned into a simpler
construct or even get hoisted out of the loop.

PR15340.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@175995 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Scalar/CorrelatedValuePropagation.cpp

@@ -21,6 +21,8 @@
 #include "llvm/IR/Instructions.h"
 #include "llvm/Pass.h"
 #include "llvm/Support/CFG.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/Support/raw_ostream.h"
 #include "llvm/Transforms/Utils/Local.h"
 using namespace llvm;
 
@@ -97,12 +99,29 @@ bool CorrelatedValuePropagation::processPHI(PHINode *P) {
     Value *Incoming = P->getIncomingValue(i);
     if (isa<Constant>(Incoming)) continue;
 
-    Constant *C = LVI->getConstantOnEdge(P->getIncomingValue(i),
-                                         P->getIncomingBlock(i),
-                                         BB);
-    if (!C) continue;
+    Value *V = LVI->getConstantOnEdge(Incoming, P->getIncomingBlock(i), BB);
 
-    P->setIncomingValue(i, C);
+    // 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.
+    if (!V) {
+      SelectInst *SI = dyn_cast<SelectInst>(Incoming);
+      if (!SI) continue;
+
+      Constant *C = dyn_cast<Constant>(SI->getFalseValue());
+      if (!C) continue;
+
+      if (LVI->getPredicateOnEdge(ICmpInst::ICMP_EQ, SI, C,
+                                  P->getIncomingBlock(i), BB) !=
+          LazyValueInfo::False)
+        continue;
+
+      DEBUG(dbgs() << "CVP: Threading PHI over " << *SI << '\n');
+      V = SI->getTrueValue();
+    }
+
+    P->setIncomingValue(i, V);
     Changed = true;
   }
 

test/Transforms/CorrelatedValuePropagation/basic.ll

@@ -81,6 +81,26 @@ LessThanOrEqualToTwo:
   ret i32 0
 }
 
+declare i32* @f(i32*)
+define void @test5(i32* %x, i32* %y) {
+; CHECK: @test5
+entry:
+  %pre = icmp eq i32* %x, null
+  br i1 %pre, label %return, label %loop
+
+loop:
+  %phi = phi i32* [ %sel, %loop ], [ %x, %entry ]
+; CHECK: %phi = phi i32* [ %f, %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
+
+return:
+  ret void
+}
+
 define i32 @switch1(i32 %s) {
 ; CHECK: @switch1
 entry: