[SimplifyCFG] delay switch condition forwarding to -latesimplifycfg

As discussed in D39011:
https://reviews.llvm.org/D39011
...replacing constants with a variable is inverting the transform done
by other IR passes, so we definitely don't want to do this early. 
In fact, it's questionable whether this transform belongs in SimplifyCFG 
at all. I'll look at moving this to codegen as a follow-up step.

llvm-svn: 316298

4 files changed, 18 insertions, 10 deletions

diff --git a/llvm/include/llvm/Transforms/Utils/Local.h b/llvm/include/llvm/Transforms/Utils/Local.h
index af00a6c2cce..f66a05eb8c0 100644
--- a/llvm/include/llvm/Transforms/Utils/Local.h
+++ b/llvm/include/llvm/Transforms/Utils/Local.h
@@ -60,14 +60,16 @@ class TargetTransformInfo;
 /// replaced by lookup tables and selects.
 struct SimplifyCFGOptions {
   int BonusInstThreshold;
+  bool ForwardSwitchCondToPhi;
   bool ConvertSwitchToLookupTable;
   bool NeedCanonicalLoop;
   AssumptionCache *AC;
 
-  SimplifyCFGOptions(int BonusThreshold = 1, bool SwitchToLookup = false,
-                     bool CanonicalLoops = true,
+  SimplifyCFGOptions(int BonusThreshold = 1, bool ForwardSwitchCond = false,
+                     bool SwitchToLookup = false, bool CanonicalLoops = true,
                      AssumptionCache *AssumpCache = nullptr)
       : BonusInstThreshold(BonusThreshold),
+        ForwardSwitchCondToPhi(ForwardSwitchCond),
         ConvertSwitchToLookupTable(SwitchToLookup),
         NeedCanonicalLoop(CanonicalLoops), AC(AssumpCache) {}
 };
diff --git a/llvm/lib/Transforms/Scalar/SimplifyCFGPass.cpp b/llvm/lib/Transforms/Scalar/SimplifyCFGPass.cpp
index 723619f46bb..6f38e5d11b5 100644
--- a/llvm/lib/Transforms/Scalar/SimplifyCFGPass.cpp
+++ b/llvm/lib/Transforms/Scalar/SimplifyCFGPass.cpp
@@ -179,8 +179,10 @@ static bool simplifyFunctionCFG(Function &F, const TargetTransformInfo &TTI,
   return true;
 }
 
+// FIXME: The new pass manager always creates a "late" simplifycfg pass using
+// this default constructor.
 SimplifyCFGPass::SimplifyCFGPass()
-    : Options(UserBonusInstThreshold, true, false) {}
+    : Options(UserBonusInstThreshold, true, true, false) {}
 
 SimplifyCFGPass::SimplifyCFGPass(const SimplifyCFGOptions &PassOptions)
     : Options(PassOptions) {}
@@ -200,12 +202,15 @@ namespace {
 struct BaseCFGSimplifyPass : public FunctionPass {
   std::function<bool(const Function &)> PredicateFtor;
   int BonusInstThreshold;
+  bool ForwardSwitchCondToPhi;
   bool ConvertSwitchToLookupTable;
   bool KeepCanonicalLoops;
 
-  BaseCFGSimplifyPass(int T, bool ConvertSwitch, bool KeepLoops,
+  BaseCFGSimplifyPass(int T, bool ForwardSwitchCond, bool ConvertSwitch,
+                      bool KeepLoops,
                       std::function<bool(const Function &)> Ftor, char &ID)
       : FunctionPass(ID), PredicateFtor(std::move(Ftor)),
+        ForwardSwitchCondToPhi(ForwardSwitchCond),
         ConvertSwitchToLookupTable(ConvertSwitch),
         KeepCanonicalLoops(KeepLoops) {
     BonusInstThreshold = (T == -1) ? UserBonusInstThreshold : T;
@@ -219,8 +224,9 @@ struct BaseCFGSimplifyPass : public FunctionPass {
     const TargetTransformInfo &TTI =
         getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
     return simplifyFunctionCFG(F, TTI,
-                               {BonusInstThreshold, ConvertSwitchToLookupTable,
-                                KeepCanonicalLoops, AC});
+                               {BonusInstThreshold, ForwardSwitchCondToPhi,
+                                ConvertSwitchToLookupTable, KeepCanonicalLoops,
+                                AC});
   }
 
   void getAnalysisUsage(AnalysisUsage &AU) const override {
@@ -235,7 +241,7 @@ struct CFGSimplifyPass : public BaseCFGSimplifyPass {
 
   CFGSimplifyPass(int T = -1,
                   std::function<bool(const Function &)> Ftor = nullptr)
-                  : BaseCFGSimplifyPass(T, false, true, Ftor, ID) {
+                  : BaseCFGSimplifyPass(T, false, false, true, Ftor, ID) {
     initializeCFGSimplifyPassPass(*PassRegistry::getPassRegistry());
   }
 };
@@ -245,7 +251,7 @@ struct LateCFGSimplifyPass : public BaseCFGSimplifyPass {
 
   LateCFGSimplifyPass(int T = -1,
                       std::function<bool(const Function &)> Ftor = nullptr)
-                      : BaseCFGSimplifyPass(T, true, false, Ftor, ID) {
+                      : BaseCFGSimplifyPass(T, true, true, false, Ftor, ID) {
     initializeLateCFGSimplifyPassPass(*PassRegistry::getPassRegistry());
   }
 };
diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index 42f2bf267f5..954d6339dc0 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -5563,7 +5563,7 @@ bool SimplifyCFGOpt::SimplifySwitch(SwitchInst *SI, IRBuilder<> &Builder) {
   if (switchToSelect(SI, Builder, DL, TTI))
     return simplifyCFG(BB, TTI, Options) | true;
 
-  if (ForwardSwitchConditionToPHI(SI))
+  if (Options.ForwardSwitchCondToPhi && ForwardSwitchConditionToPHI(SI))
     return simplifyCFG(BB, TTI, Options) | true;
 
   // The conversion from switch to lookup tables results in difficult-to-analyze
diff --git a/llvm/test/Transforms/SimplifyCFG/ForwardSwitchConditionToPHI.ll b/llvm/test/Transforms/SimplifyCFG/ForwardSwitchConditionToPHI.ll
index d51d24ad3cd..4dcccef03d8 100644
--- a/llvm/test/Transforms/SimplifyCFG/ForwardSwitchConditionToPHI.ll
+++ b/llvm/test/Transforms/SimplifyCFG/ForwardSwitchConditionToPHI.ll
@@ -1,5 +1,5 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
-; RUN: opt < %s -simplifycfg -S | FileCheck %s
+; RUN: opt < %s -latesimplifycfg -S | FileCheck %s
 
 ; PR10131