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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@167011 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Scalar/SimplifyCFGPass.cpp

@@ -35,6 +35,7 @@
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/ADT/Statistic.h"
+#include "llvm/TargetTransformInfo.h"
 using namespace llvm;
 
 STATISTIC(NumSimpl, "Number of blocks simplified");
@@ -293,7 +294,8 @@ static bool mergeEmptyReturnBlocks(Function &F) {
 
 /// iterativelySimplifyCFG - Call SimplifyCFG on all the blocks in the function,
 /// iterating until no more changes are made.
-static bool iterativelySimplifyCFG(Function &F, const DataLayout *TD) {
+static bool iterativelySimplifyCFG(Function &F, const DataLayout *TD,
+                                   const TargetTransformInfo *TTI) {
   bool Changed = false;
   bool LocalChange = true;
   while (LocalChange) {
@@ -302,7 +304,7 @@ static bool iterativelySimplifyCFG(Function &F, const DataLayout *TD) {
     // Loop over all of the basic blocks and remove them if they are unneeded...
     //
     for (Function::iterator BBIt = F.begin(); BBIt != F.end(); ) {
-      if (SimplifyCFG(BBIt++, TD)) {
+      if (SimplifyCFG(BBIt++, TD, TTI)) {
         LocalChange = true;
         ++NumSimpl;
       }
@@ -317,9 +319,11 @@ static bool iterativelySimplifyCFG(Function &F, const DataLayout *TD) {
 //
 bool CFGSimplifyPass::runOnFunction(Function &F) {
   const DataLayout *TD = getAnalysisIfAvailable<DataLayout>();
+  const TargetTransformInfo *TTI =
+      getAnalysisIfAvailable<TargetTransformInfo>();
   bool EverChanged = removeUnreachableBlocksFromFn(F);
   EverChanged |= mergeEmptyReturnBlocks(F);
-  EverChanged |= iterativelySimplifyCFG(F, TD);
+  EverChanged |= iterativelySimplifyCFG(F, TD, TTI);
 
   // If neither pass changed anything, we're done.
   if (!EverChanged) return false;
@@ -333,7 +337,7 @@ bool CFGSimplifyPass::runOnFunction(Function &F) {
     return true;
 
   do {
-    EverChanged = iterativelySimplifyCFG(F, TD);
+    EverChanged = iterativelySimplifyCFG(F, TD, TTI);
     EverChanged |= removeUnreachableBlocksFromFn(F);
   } while (EverChanged);