SimplifyCFG: Omit range checks for switch lookup tables when default is unreachable

They would get optimized away later, but we might as well not emit them.

llvm-svn: 223051

2 files changed, 37 insertions, 7 deletions

diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index 48d6a81364f..91127b27046 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -4134,19 +4134,20 @@ static bool SwitchToLookupTable(SwitchInst *SI,
          "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 fully 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.
+  // If the default destination is unreachable, or if the lookup table covers
+  // all values of the conditional variable, branch directly to the lookup table
+  // BB. Otherwise, check that the condition is within the case range.
+  const bool DefaultIsReachable =
+      !isa<UnreachableInst>(SI->getDefaultDest()->getFirstNonPHIOrDbg());
+  const bool GeneratingCoveredLookupTable = (MaxTableSize == TableSize);
   BranchInst *RangeCheckBranch = nullptr;
 
-  const bool GeneratingCoveredLookupTable = MaxTableSize == TableSize;
-  if (GeneratingCoveredLookupTable) {
+  if (!DefaultIsReachable || GeneratingCoveredLookupTable) {
     Builder.CreateBr(LookupBB);
     // We cached PHINodes in PHIs, to avoid accessing deleted PHINodes later,
     // do not delete PHINodes here.
     SI->getDefaultDest()->removePredecessor(SI->getParent(),
-                                            true/*DontDeleteUselessPHIs*/);
+                                            true /*DontDeleteUselessPHIs*/);
   } else {
     Value *Cmp = Builder.CreateICmpULT(TableIndex, ConstantInt::get(
                                        MinCaseVal->getType(), TableSize));
diff --git a/llvm/test/Transforms/SimplifyCFG/X86/switch_to_lookup_table.ll b/llvm/test/Transforms/SimplifyCFG/X86/switch_to_lookup_table.ll
index 27c4839846e..29f1a35fb14 100644
--- a/llvm/test/Transforms/SimplifyCFG/X86/switch_to_lookup_table.ll
+++ b/llvm/test/Transforms/SimplifyCFG/X86/switch_to_lookup_table.ll
@@ -781,6 +781,35 @@ return:
 ; CHECK: getelementptr inbounds [9 x i32]* @switch.table6, i32 0, i32 %switch.tableidx
 }
 
+define i32 @unreachable_default(i32 %x)  {
+entry:
+  switch i32 %x, label %default [
+    i32 0, label %bb0
+    i32 1, label %bb1
+    i32 2, label %bb2
+    i32 3, label %bb3
+  ]
+
+bb0: br label %return
+bb1: br label %return
+bb2: br label %return
+bb3: br label %return
+default: unreachable
+
+return:
+  %retval = phi i32 [ 42, %bb0 ], [ 52, %bb1 ], [ 1, %bb2 ], [ 2, %bb3 ]
+  ret i32 %retval
+
+; CHECK-LABEL: @unreachable_default(
+; CHECK: entry:
+; CHECK-NEXT: %switch.tableidx = sub i32 %x, 0
+; CHECK-NOT: icmp
+; CHECK-NOT: br 1i
+; CHECK-NEXT: %switch.gep = getelementptr inbounds [4 x i32]* @switch.table7, i32 0, i32 %switch.tableidx
+; CHECK-NEXT: %switch.load = load i32* %switch.gep
+; CHECK-NEXT: ret i32 %switch.load
+}
+
 ; Don't create a table with illegal type
 define i96 @illegaltype(i32 %c) {
 entry: