[llvm] [LoopRotate] Use `poison` instead of `undef` as placeholder (PR #119135)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: Pedro Lobo (pedroclobo)

<details>
<summary>Changes</summary>

The `poison` values are used to substitute debug information of values moved from the original header into the preheader that are no longer available in the former.

---
Full diff: https://github.com/llvm/llvm-project/pull/119135.diff


2 Files Affected:

- (modified) llvm/lib/Transforms/Utils/LoopRotationUtils.cpp (+4-4) 
- (modified) llvm/test/Transforms/LoopRotate/dbgvalue.ll (+3-3) 


``````````diff
diff --git a/llvm/lib/Transforms/Utils/LoopRotationUtils.cpp b/llvm/lib/Transforms/Utils/LoopRotationUtils.cpp
index 04042e71a2b82e..fffff295ba926d 100644
--- a/llvm/lib/Transforms/Utils/LoopRotationUtils.cpp
+++ b/llvm/lib/Transforms/Utils/LoopRotationUtils.cpp
@@ -171,14 +171,14 @@ static void RewriteUsesOfClonedInstructions(BasicBlock *OrigHeader,
       // Users in the OrigPreHeader need to use the value to which the
       // original definitions are mapped and anything else can be handled by
       // the SSAUpdater. To avoid adding PHINodes, check if the value is
-      // available in UserBB, if not substitute undef.
+      // available in UserBB, if not substitute poison.
       Value *NewVal;
       if (UserBB == OrigPreheader)
         NewVal = OrigPreHeaderVal;
       else if (SSA.HasValueForBlock(UserBB))
         NewVal = SSA.GetValueInMiddleOfBlock(UserBB);
       else
-        NewVal = UndefValue::get(OrigHeaderVal->getType());
+        NewVal = PoisonValue::get(OrigHeaderVal->getType());
       DbgValue->replaceVariableLocationOp(OrigHeaderVal, NewVal);
     }
 
@@ -194,14 +194,14 @@ static void RewriteUsesOfClonedInstructions(BasicBlock *OrigHeader,
       // Users in the OrigPreHeader need to use the value to which the
       // original definitions are mapped and anything else can be handled by
       // the SSAUpdater. To avoid adding PHINodes, check if the value is
-      // available in UserBB, if not substitute undef.
+      // available in UserBB, if not substitute poison.
       Value *NewVal;
       if (UserBB == OrigPreheader)
         NewVal = OrigPreHeaderVal;
       else if (SSA.HasValueForBlock(UserBB))
         NewVal = SSA.GetValueInMiddleOfBlock(UserBB);
       else
-        NewVal = UndefValue::get(OrigHeaderVal->getType());
+        NewVal = PoisonValue::get(OrigHeaderVal->getType());
       DVR->replaceVariableLocationOp(OrigHeaderVal, NewVal);
     }
   }
diff --git a/llvm/test/Transforms/LoopRotate/dbgvalue.ll b/llvm/test/Transforms/LoopRotate/dbgvalue.ll
index b58d821fbca5d7..026306a4637f0e 100644
--- a/llvm/test/Transforms/LoopRotate/dbgvalue.ll
+++ b/llvm/test/Transforms/LoopRotate/dbgvalue.ll
@@ -113,7 +113,7 @@ return:                                           ; preds = %if.end
 
 ; Check that the dbg.values move up to being immediately below the PHIs,
 ; using their Values. However once we exit the loop, the x and y values
-; become irrelevant and undef, only z gets an LCSSA PHI to preserve it.
+; become irrelevant and poison, only z gets an LCSSA PHI to preserve it.
 ;
 ; Note that while the icmp is initially undominated by any dbg.value and thus
 ; shouldn't get a variable location, the first iteration is peeled off into the
@@ -138,8 +138,8 @@ define i32 @tak2(i32 %x, i32 %y, i32 %z) nounwind ssp !dbg !21 {
 ; CHECK:      tail call i32 @tak(i32
 ; CHECK: if.end:
 ; CHECK-NEXT: z.tr.lcssa = phi i32
-; CHECK-NEXT: #dbg_value(i32 undef
-; CHECK-NEXT: #dbg_value(i32 undef
+; CHECK-NEXT: #dbg_value(i32 poison
+; CHECK-NEXT: #dbg_value(i32 poison
 ; CHECK-NEXT: #dbg_value(i32 %z.tr.lcssa
 entry:
   br label %tailrecurse

``````````

</details>


https://github.com/llvm/llvm-project/pull/119135