[SimplifyCFG] Make the value type of the hole check bitmask a power-o…

…f-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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222168 91177308-0d34-0410-b5e6-96231b3b80d8

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() -

test/Transforms/SimplifyCFG/X86/switch_to_lookup_table.ll

@@ -856,10 +856,10 @@ return:
 ; CHECK: entry:
 ; CHECK: br i1 %{{.*}}, label %switch.hole_check, label %sw.default
 ; CHECK: switch.hole_check:
-; CHECK-NEXT: %switch.maskindex = trunc i32 %switch.tableidx to i6
-; CHECK-NEXT: %switch.shifted = lshr i6 -17, %switch.maskindex
+; CHECK-NEXT: %switch.maskindex = trunc i32 %switch.tableidx to i8
+; CHECK-NEXT: %switch.shifted = lshr i8 47, %switch.maskindex
 ; The mask is binary 101111.
-; CHECK-NEXT: %switch.lobit = trunc i6 %switch.shifted to i1
+; CHECK-NEXT: %switch.lobit = trunc i8 %switch.shifted to i1
 ; CHECK-NEXT: br i1 %switch.lobit, label %switch.lookup, label %sw.default
 ; CHECK-NOT: switch i32
 }