Teach simplify-cfg how to correctly create covered lookup tables for switches on iN with N >= 3.

One optimization simplify-cfg performs is the converting of switches to
lookup tables if the switch has > 4 cases. This is done by:

1. Finding the max/min case value and calculating the switch case range.
2. Create a lookup table basic block.
3. Perform a check in the switch's BB to see if the input value is in
the switch's case range. If the input value satisfies said predicate
branch to the lookup table BB, otherwise branch to the switch's default
destination BB using the default value as the result.

The conditional check consists of subtracting the min case value of the
table from any input iN value and then ensuring that said value is
unsigned less than the size of the lookup table represented as an iN
value.

If the lookup table is a covered lookup table, the size of the table will be N
which is 0 as an iN value. Thus the comparison will be an `icmp ult` of an iN
value against 0 which is always false yielding the incorrect result.

This patch fixes this problem by recognizing if we have a covered lookup table
and if we do, unconditionally jumps to the lookup table BB since the covering
property of the lookup table implies no input values could not be handled by
said BB.

rdar://15268442

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@193045 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 28 insertions, 6 deletions

diff --git a/lib/Transforms/Utils/SimplifyCFG.cpp b/lib/Transforms/Utils/SimplifyCFG.cpp
index 0e56904ebd..ad3b92a714 100644
--- a/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -3734,14 +3734,30 @@ static bool SwitchToLookupTable(SwitchInst *SI,
                                             CommonDest->getParent(),
                                             CommonDest);
 
-  // Check whether the condition value is within the case range, and branch to
-  // the new BB.
+  // Compute the table index value.
   Builder.SetInsertPoint(SI);
   Value *TableIndex = Builder.CreateSub(SI->getCondition(), MinCaseVal,
                                         "switch.tableidx");
-  Value *Cmp = Builder.CreateICmpULT(TableIndex, ConstantInt::get(
-      MinCaseVal->getType(), TableSize));
-  Builder.CreateCondBr(Cmp, LookupBB, SI->getDefaultDest());
+
+  // Compute the maximum table size representable by the integer type we are
+  // switching upon.
+  const unsigned CaseSize = MinCaseVal->getType()->getPrimitiveSizeInBits();
+  const uint64_t MaxTableSize = CaseSize > 63? UINT64_MAX : 1ULL << CaseSize;
+  assert(MaxTableSize >= TableSize &&
+         "It is impossible for a switch to have more entries than the max "
+         "representable value of its input integer type's size.");
+
+  // If we have a covered lookup table, unconditionally branch to the lookup table
+  // BB. Otherwise, check if the condition value is within the case range. If it
+  // is so, branch to the new BB. Otherwise branch to SI's default destination.
+  const bool GeneratingCoveredLookupTable = MaxTableSize == TableSize;
+  if (GeneratingCoveredLookupTable) {
+    Builder.CreateBr(LookupBB);
+  } else {
+    Value *Cmp = Builder.CreateICmpULT(TableIndex, ConstantInt::get(
+                                         MinCaseVal->getType(), TableSize));
+    Builder.CreateCondBr(Cmp, LookupBB, SI->getDefaultDest());
+  }
 
   // Populate the BB that does the lookups.
   Builder.SetInsertPoint(LookupBB);
@@ -3772,7 +3788,13 @@ static bool SwitchToLookupTable(SwitchInst *SI,
   // Remove the switch.
   for (unsigned i = 0; i < SI->getNumSuccessors(); ++i) {
     BasicBlock *Succ = SI->getSuccessor(i);
-    if (Succ == SI->getDefaultDest()) continue;
+
+    // If we are not generating a covered lookup table, we will have a
+    // conditional branch from SI's parent BB to SI's default destination if our
+    // input value lies outside of our case range. Thus in that case leave the
+    // default destination BB as a predecessor of SI's parent BB.
+    if (Succ == SI->getDefaultDest() && !GeneratingCoveredLookupTable)
+      continue;
     Succ->removePredecessor(SI->getParent());
   }
   SI->eraseFromParent();