[SimplifyCFG] Delete the unnecessary range check for small mask operation

When the small mask value little than 64, we can eliminate the checking
for upper limit of the range by enlarge the lookup table size to the maximum
index value. (Then the final table size grows to the next pow2 value)
```
bool f(unsigned x) {
    switch (x % 8) {
        case 0: return 1;
        case 1: return 0;
        case 2: return 0;
        case 3: return 1;
        case 4: return 1;
        case 5: return 0;
        case 6: return 1;

        // This would remove the range check: case 7: return 0;
    }
    return 0;
}
```
Use WouldFitInRegister instead of fitsInLegalInteger to support
more result type beside bool.

Fixes #65120
Reviewed By: zmodem, nikic, RKSimon

Author: vfdff

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

@@ -6598,9 +6598,8 @@ static bool SwitchToLookupTable(SwitchInst *SI, IRBuilder<> &Builder,
   // 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 =
+  bool DefaultIsReachable =
       !isa<UnreachableInst>(SI->getDefaultDest()->getFirstNonPHIOrDbg());
-  const bool GeneratingCoveredLookupTable = (MaxTableSize == TableSize);
 
   // Create the BB that does the lookups.
   Module &Mod = *CommonDest->getParent()->getParent();
@@ -6631,6 +6630,25 @@ static bool SwitchToLookupTable(SwitchInst *SI, IRBuilder<> &Builder,
 
   BranchInst *RangeCheckBranch = nullptr;
 
+  // Grow the table to cover all possible index values to avoid the range check.
+  if (UseSwitchConditionAsTableIndex) {
+    ConstantRange CR = computeConstantRange(TableIndex, /* ForSigned */ false);
+    // Grow the table shouldn't have any size impact by checking
+    // WouldFitInRegister.
+    // TODO: Consider growing the table also when it doesn't fit in a register
+    // if no optsize is specified.
+    if (all_of(ResultTypes, [&](const auto &KV) {
+          return SwitchLookupTable::WouldFitInRegister(
+              DL, CR.getUpper().getLimitedValue(), KV.second /* ResultType */);
+        })) {
+      // The default branch is unreachable when we enlarge the lookup table.
+      // Adjust DefaultIsReachable to reuse code path.
+      TableSize = CR.getUpper().getZExtValue();
+      DefaultIsReachable = false;
+    }
+  }
+
+  const bool GeneratingCoveredLookupTable = (MaxTableSize == TableSize);
   if (!DefaultIsReachable || GeneratingCoveredLookupTable) {
     Builder.CreateBr(LookupBB);
     if (DTU)

llvm/test/Transforms/SimplifyCFG/switch_mask.ll

@@ -8,13 +8,11 @@ define i1 @switch_lookup_with_small_i1(i64 %x) {
 ; CHECK-LABEL: @switch_lookup_with_small_i1(
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    [[AND:%.*]] = and i64 [[X:%.*]], 15
-; CHECK-NEXT:    [[TMP0:%.*]] = icmp ult i64 [[AND]], 11
-; CHECK-NEXT:    [[SWITCH_CAST:%.*]] = trunc i64 [[AND]] to i11
-; CHECK-NEXT:    [[SWITCH_SHIFTAMT:%.*]] = mul nuw nsw i11 [[SWITCH_CAST]], 1
-; CHECK-NEXT:    [[SWITCH_DOWNSHIFT:%.*]] = lshr i11 -1018, [[SWITCH_SHIFTAMT]]
-; CHECK-NEXT:    [[SWITCH_MASKED:%.*]] = trunc i11 [[SWITCH_DOWNSHIFT]] to i1
-; CHECK-NEXT:    [[TMP1:%.*]] = select i1 [[TMP0]], i1 [[SWITCH_MASKED]], i1 false
-; CHECK-NEXT:    ret i1 [[TMP1]]
+; CHECK-NEXT:    [[SWITCH_CAST:%.*]] = trunc i64 [[AND]] to i16
+; CHECK-NEXT:    [[SWITCH_SHIFTAMT:%.*]] = mul nuw nsw i16 [[SWITCH_CAST]], 1
+; CHECK-NEXT:    [[SWITCH_DOWNSHIFT:%.*]] = lshr i16 1030, [[SWITCH_SHIFTAMT]]
+; CHECK-NEXT:    [[SWITCH_MASKED:%.*]] = trunc i16 [[SWITCH_DOWNSHIFT]] to i1
+; CHECK-NEXT:    ret i1 [[SWITCH_MASKED]]
 ;
 entry:
   %and = and i64 %x, 15
@@ -37,13 +35,11 @@ define i8 @switch_lookup_with_small_i8(i64 %x) {
 ; CHECK-LABEL: @switch_lookup_with_small_i8(
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    [[REM:%.*]] = urem i64 [[X:%.*]], 5
-; CHECK-NEXT:    [[TMP0:%.*]] = icmp ult i64 [[REM]], 3
-; CHECK-NEXT:    [[SWITCH_CAST:%.*]] = trunc i64 [[REM]] to i24
-; CHECK-NEXT:    [[SWITCH_SHIFTAMT:%.*]] = mul nuw nsw i24 [[SWITCH_CAST]], 8
-; CHECK-NEXT:    [[SWITCH_DOWNSHIFT:%.*]] = lshr i24 460303, [[SWITCH_SHIFTAMT]]
-; CHECK-NEXT:    [[SWITCH_MASKED:%.*]] = trunc i24 [[SWITCH_DOWNSHIFT]] to i8
-; CHECK-NEXT:    [[TMP1:%.*]] = select i1 [[TMP0]], i8 [[SWITCH_MASKED]], i8 0
-; CHECK-NEXT:    ret i8 [[TMP1]]
+; CHECK-NEXT:    [[SWITCH_CAST:%.*]] = trunc i64 [[REM]] to i40
+; CHECK-NEXT:    [[SWITCH_SHIFTAMT:%.*]] = mul nuw nsw i40 [[SWITCH_CAST]], 8
+; CHECK-NEXT:    [[SWITCH_DOWNSHIFT:%.*]] = lshr i40 460303, [[SWITCH_SHIFTAMT]]
+; CHECK-NEXT:    [[SWITCH_MASKED:%.*]] = trunc i40 [[SWITCH_DOWNSHIFT]] to i8
+; CHECK-NEXT:    ret i8 [[SWITCH_MASKED]]
 ;
 entry:
   %rem = urem i64 %x, 5