[llvm] Revert "[SimplifyCFG] `switch`: Do Not Transform the Default Case if the Condition is Too Wide" (PR #78469)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: None (alexfh)

<details>
<summary>Changes</summary>

Reverts llvm/llvm-project#<!-- -->77831, which depends on #<!-- -->76669, which seriously regresses compilation time / memory usage see https://github.com/llvm/llvm-project/pull/76669#issuecomment-1889271710.

---
Full diff: https://github.com/llvm/llvm-project/pull/78469.diff


2 Files Affected:

- (modified) llvm/lib/Transforms/Utils/SimplifyCFG.cpp (-5) 
- (modified) llvm/test/Transforms/SimplifyCFG/switch-dead-default.ll (-33) 


``````````diff
diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index fe36fd84e0439b..7515e539e7fb78 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -5623,11 +5623,6 @@ static bool eliminateDeadSwitchCases(SwitchInst *SI, DomTreeUpdater *DTU,
     // optimization, such as lookup tables.
     if (SI->getNumCases() == AllNumCases - 1) {
       assert(NumUnknownBits > 1 && "Should be canonicalized to a branch");
-      IntegerType *CondTy = cast<IntegerType>(Cond->getType());
-      if (CondTy->getIntegerBitWidth() > 64 ||
-          !DL.fitsInLegalInteger(CondTy->getIntegerBitWidth()))
-        return false;
-
       uint64_t MissingCaseVal = 0;
       for (const auto &Case : SI->cases())
         MissingCaseVal ^= Case.getCaseValue()->getValue().getLimitedValue();
diff --git a/llvm/test/Transforms/SimplifyCFG/switch-dead-default.ll b/llvm/test/Transforms/SimplifyCFG/switch-dead-default.ll
index 4a457cc177e85c..e30a535c523237 100644
--- a/llvm/test/Transforms/SimplifyCFG/switch-dead-default.ll
+++ b/llvm/test/Transforms/SimplifyCFG/switch-dead-default.ll
@@ -313,39 +313,6 @@ default:
 
 declare void @llvm.assume(i1)
 
-define zeroext i1 @test8(i128 %a) {
-; We should not transform conditions wider than 64 bit.
-; CHECK-LABEL: define zeroext i1 @test8(
-; CHECK-SAME: i128 [[A:%.*]]) {
-; CHECK-NEXT:  entry:
-; CHECK-NEXT:    [[TMP0:%.*]] = and i128 [[A]], 3894222643901120721397872246915072
-; CHECK-NEXT:    switch i128 [[TMP0]], label [[LOR_RHS:%.*]] [
-; CHECK-NEXT:      i128 1298074214633706907132624082305024, label [[LOR_END:%.*]]
-; CHECK-NEXT:      i128 2596148429267413814265248164610048, label [[LOR_END]]
-; CHECK-NEXT:      i128 3894222643901120721397872246915072, label [[LOR_END]]
-; CHECK-NEXT:    ]
-; CHECK:       lor.rhs:
-; CHECK-NEXT:    br label [[LOR_END]]
-; CHECK:       lor.end:
-; CHECK-NEXT:    [[TMP1:%.*]] = phi i1 [ true, [[ENTRY:%.*]] ], [ false, [[LOR_RHS]] ], [ true, [[ENTRY]] ], [ true, [[ENTRY]] ]
-; CHECK-NEXT:    ret i1 [[TMP1]]
-;
-entry:
-  %0 = and i128 %a, 3894222643901120721397872246915072
-  switch i128 %0, label %lor.rhs [
-  i128 1298074214633706907132624082305024, label %lor.end
-  i128 2596148429267413814265248164610048, label %lor.end
-  i128 3894222643901120721397872246915072, label %lor.end
-  ]
-
-lor.rhs:                                          ; preds = %entry
-  br label %lor.end
-
-lor.end:                                          ; preds = %entry, %entry, %entry, %lor.rhs
-  %1 = phi i1 [ true, %entry ], [ false, %lor.rhs ], [ true, %entry ], [ true, %entry ]
-  ret i1 %1
-}
-
 !0 = !{!"branch_weights", i32 8, i32 4, i32 2, i32 1}
 ;.
 ; CHECK: [[PROF0]] = !{!"branch_weights", i32 0, i32 4, i32 2, i32 1, i32 8}

``````````

</details>


https://github.com/llvm/llvm-project/pull/78469