[llvm] Revert "[SimplifyCFG] Delete the unnecessary range check for small mask operation (PR #70324)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: Allen (vfdff)

<details>
<summary>Changes</summary>

This reverts commit 5e07481d4240b5e8fd85f9b92df30849606c2af0.

---
Full diff: https://github.com/llvm/llvm-project/pull/70324.diff


2 Files Affected:

- (modified) llvm/lib/Transforms/Utils/SimplifyCFG.cpp (+2-20) 
- (modified) llvm/test/Transforms/SimplifyCFG/switch_mask.ll (+14-10) 


``````````diff
diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index 18187bcdedf0982..68b5b1a78a3460e 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -6598,8 +6598,9 @@ 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.
-  bool DefaultIsReachable =
+  const bool DefaultIsReachable =
       !isa<UnreachableInst>(SI->getDefaultDest()->getFirstNonPHIOrDbg());
+  const bool GeneratingCoveredLookupTable = (MaxTableSize == TableSize);
 
   // Create the BB that does the lookups.
   Module &Mod = *CommonDest->getParent()->getParent();
@@ -6630,25 +6631,6 @@ 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)
diff --git a/llvm/test/Transforms/SimplifyCFG/switch_mask.ll b/llvm/test/Transforms/SimplifyCFG/switch_mask.ll
index 123519bc6921147..8c97a0660d07074 100644
--- a/llvm/test/Transforms/SimplifyCFG/switch_mask.ll
+++ b/llvm/test/Transforms/SimplifyCFG/switch_mask.ll
@@ -8,11 +8,13 @@ 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:    [[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]]
+; 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]]
 ;
 entry:
   %and = and i64 %x, 15
@@ -35,11 +37,13 @@ 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:    [[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]]
+; 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]]
 ;
 entry:
   %rem = urem i64 %x, 5

``````````

</details>


https://github.com/llvm/llvm-project/pull/70324