[SimplifyCFG] Make the value type of the hole check bitmask a power-of-2.

When converting a switch to a lookup table we might have to generate a bitmaks
to encode and check for holes in the original switch statement.

The type of this mask depends on the number of switch statements, which can
result in illegal types for pretty much all architectures.

To avoid unnecessary type legalization and help FastISel this commit increases
the size of the bitmask to next power-of-2 value when necessary.

This fixes rdar://problem/18984639.

llvm-svn: 222168

1 files changed, 5 insertions, 2 deletions

diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index a8a97f0e25c..e3bdd6066be 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -4074,9 +4074,12 @@ static bool SwitchToLookupTable(SwitchInst *SI,
                                   CommonDest->getParent(),
                                   CommonDest);
 
+    // Make the mask's bitwidth at least 8bit and a power-of-2 to avoid
+    // unnecessary illegal types.
+    uint64_t TableSizePowOf2 = NextPowerOf2(std::max(7ULL, TableSize - 1ULL));
+    APInt MaskInt(TableSizePowOf2, 0);
+    APInt One(TableSizePowOf2, 1);
     // Build bitmask; fill in a 1 bit for every case.
-    APInt MaskInt(TableSize, 0);
-    APInt One(TableSize, 1);
     const ResultListTy &ResultList = ResultLists[PHIs[0]];
     for (size_t I = 0, E = ResultList.size(); I != E; ++I) {
       uint64_t Idx = (ResultList[I].first->getValue() -