SimplifyCFG: Teach switch generation some patterns that instcombine forms.

This allows us to create switches even if instcombine has munged two of the
incombing compares into one and some bit twiddling. This was motivated by enum
compares that are common in clang.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185632 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 27 insertions, 1 deletions

diff --git a/lib/Transforms/Utils/SimplifyCFG.cpp b/lib/Transforms/Utils/SimplifyCFG.cpp
index 6d12f7a..d8878f2 100644
--- a/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -40,12 +40,14 @@
 #include "llvm/Support/ConstantRange.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/NoFolder.h"
+#include "llvm/Support/PatternMatch.h"
 #include "llvm/Support/raw_ostream.h"
 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
 #include <algorithm>
 #include <map>
 #include <set>
 using namespace llvm;
+using namespace PatternMatch;
 
 static cl::opt<unsigned>
 PHINodeFoldingThreshold("phi-node-folding-threshold", cl::Hidden, cl::init(1),
@@ -432,7 +434,24 @@ GatherConstantCompares(Value *V, std::vector<ConstantInt*> &Vals, Value *&Extra,
   // If this is an icmp against a constant, handle this as one of the cases.
   if (ICmpInst *ICI = dyn_cast<ICmpInst>(I)) {
     if (ConstantInt *C = GetConstantInt(I->getOperand(1), TD)) {
+      Value *RHSVal;
+      ConstantInt *RHSC;
+
       if (ICI->getPredicate() == (isEQ ? ICmpInst::ICMP_EQ:ICmpInst::ICMP_NE)) {
+        // (x & ~2^x) == y --> x == y || x == y|2^x
+        // This undoes a transformation done by instcombine to fuse 2 compares.
+        if (match(ICI->getOperand(0),
+                  m_And(m_Value(RHSVal), m_ConstantInt(RHSC)))) {
+          APInt Not = ~RHSC->getValue();
+          if (Not.isPowerOf2()) {
+            Vals.push_back(C);
+            Vals.push_back(
+                ConstantInt::get(C->getContext(), C->getValue() | Not));
+            UsedICmps++;
+            return RHSVal;
+          }
+        }
+
         UsedICmps++;
         Vals.push_back(C);
         return I->getOperand(0);
@@ -443,6 +462,13 @@ GatherConstantCompares(Value *V, std::vector<ConstantInt*> &Vals, Value *&Extra,
       ConstantRange Span =
         ConstantRange::makeICmpRegion(ICI->getPredicate(), C->getValue());
 
+      // Shift the range if the compare is fed by an add. This is the range
+      // compare idiom as emitted by instcombine.
+      bool hasAdd =
+          match(I->getOperand(0), m_Add(m_Value(RHSVal), m_ConstantInt(RHSC)));
+      if (hasAdd)
+        Span = Span.subtract(RHSC->getValue());
+
       // If this is an and/!= check then we want to optimize "x ugt 2" into
       // x != 0 && x != 1.
       if (!isEQ)
@@ -455,7 +481,7 @@ GatherConstantCompares(Value *V, std::vector<ConstantInt*> &Vals, Value *&Extra,
       for (APInt Tmp = Span.getLower(); Tmp != Span.getUpper(); ++Tmp)
         Vals.push_back(ConstantInt::get(V->getContext(), Tmp));
       UsedICmps++;
-      return I->getOperand(0);
+      return hasAdd ? RHSVal : I->getOperand(0);
     }
     return 0;
   }