Use TargetTransformInfo to control switch-to-lookup table transformation

When the switch-to-lookup tables transform landed in SimplifyCFG, it
was pointed out that this could be inappropriate for some targets.
Since there was no way at the time for the pass to know anything about
the target, an awkward reverse-transform was added in CodeGenPrepare
that turned lookup tables back into switches for some targets.

This patch uses the new TargetTransformInfo to determine if a
switch should be transformed, and removes
CodeGenPrepare::ConvertLoadToSwitch.

llvm-svn: 167011

1 files changed, 15 insertions, 6 deletions

diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index f05a8bae600..04ffc90a23a 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -14,6 +14,7 @@
 #define DEBUG_TYPE "simplifycfg"
 #include "llvm/Transforms/Utils/Local.h"
 #include "llvm/Constants.h"
+#include "llvm/DataLayout.h"
 #include "llvm/DerivedTypes.h"
 #include "llvm/GlobalVariable.h"
 #include "llvm/IRBuilder.h"
@@ -39,7 +40,7 @@
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/NoFolder.h"
 #include "llvm/Support/raw_ostream.h"
-#include "llvm/DataLayout.h"
+#include "llvm/TargetTransformInfo.h"
 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
 #include <algorithm>
 #include <set>
@@ -82,6 +83,7 @@ namespace {
 
 class SimplifyCFGOpt {
   const DataLayout *const TD;
+  const TargetTransformInfo *const TTI;
 
   Value *isValueEqualityComparison(TerminatorInst *TI);
   BasicBlock *GetValueEqualityComparisonCases(TerminatorInst *TI,
@@ -101,7 +103,8 @@ class SimplifyCFGOpt {
   bool SimplifyCondBranch(BranchInst *BI, IRBuilder <>&Builder);
 
 public:
-  explicit SimplifyCFGOpt(const DataLayout *td) : TD(td) {}
+  SimplifyCFGOpt(const DataLayout *td, const TargetTransformInfo *tti)
+      : TD(td), TTI(tti) {}
   bool run(BasicBlock *BB);
 };
 }
@@ -3459,8 +3462,13 @@ static bool ShouldBuildLookupTable(SwitchInst *SI,
 /// replace the switch with lookup tables.
 static bool SwitchToLookupTable(SwitchInst *SI,
                                 IRBuilder<> &Builder,
-                                const DataLayout* TD) {
+                                const DataLayout* TD,
+                                const TargetTransformInfo *TTI) {
   assert(SI->getNumCases() > 1 && "Degenerate switch?");
+
+  if (TTI && !TTI->getScalarTargetTransformInfo()->shouldBuildLookupTables())
+    return false;
+
   // FIXME: Handle unreachable cases.
 
   // FIXME: If the switch is too sparse for a lookup table, perhaps we could
@@ -3619,7 +3627,7 @@ bool SimplifyCFGOpt::SimplifySwitch(SwitchInst *SI, IRBuilder<> &Builder) {
   if (ForwardSwitchConditionToPHI(SI))
     return SimplifyCFG(BB) | true;
 
-  if (SwitchToLookupTable(SI, Builder, TD))
+  if (SwitchToLookupTable(SI, Builder, TD, TTI))
     return SimplifyCFG(BB) | true;
 
   return false;
@@ -3916,6 +3924,7 @@ bool SimplifyCFGOpt::run(BasicBlock *BB) {
 /// eliminates unreachable basic blocks, and does other "peephole" optimization
 /// of the CFG.  It returns true if a modification was made.
 ///
-bool llvm::SimplifyCFG(BasicBlock *BB, const DataLayout *TD) {
-  return SimplifyCFGOpt(TD).run(BB);
+bool llvm::SimplifyCFG(BasicBlock *BB, const DataLayout *TD,
+                       const TargetTransformInfo *TTI) {
+  return SimplifyCFGOpt(TD, TTI).run(BB);
 }