[BOLT][NFC] Fix for a dangling reference UB

[noclowns]

Fix UB caused by accessing the top element of the stack via a dangling reference after a call to .pop() This is tripping static analysis.

No functional changes. Performance impact is negligible, but alt. implementation of the fix is possible if needed.

Testing: Both functional and unit tests are passing.

[llvmbot]

@llvm/pr-subscribers-bolt

Author: Slava Gurevich (noclowns)

Changes

Fix UB caused by accessing the top element of the stack via a dangling reference after a call to .pop() This is tripping static analysis.

No functional changes. Performance impact is negligible, but alt. implementation of the fix is possible if needed.

Testing: Both functional and unit tests are passing.

---

Full diff: https://github.com/llvm/llvm-project/pull/163344.diff

2 Files Affected:

• (modified) bolt/lib/Passes/FrameAnalysis.cpp (+1-1)
• (modified) bolt/lib/Passes/ShrinkWrapping.cpp (+1-1)

diff --git a/bolt/lib/Passes/FrameAnalysis.cpp b/bolt/lib/Passes/FrameAnalysis.cpp
index f568039bbf163..0b26da3371234 100644
--- a/bolt/lib/Passes/FrameAnalysis.cpp
+++ b/bolt/lib/Passes/FrameAnalysis.cpp
@@ -198,7 +198,7 @@ class FrameAccessAnalysis {
         if (CFIStack.empty())
           dbgs() << "Assertion is about to fail: " << BF.getPrintName() << "\n";
         assert(!CFIStack.empty() && "Corrupt CFI stack");
-        std::pair<int64_t, uint16_t> &Elem = CFIStack.top();
+        std::pair<int64_t, uint16_t> Elem = CFIStack.top();
         CFIStack.pop();
         CfaOffset = Elem.first;
         CfaReg = Elem.second;
diff --git a/bolt/lib/Passes/ShrinkWrapping.cpp b/bolt/lib/Passes/ShrinkWrapping.cpp
index 4ea60f388e2fa..fe342ccd38a67 100644
--- a/bolt/lib/Passes/ShrinkWrapping.cpp
+++ b/bolt/lib/Passes/ShrinkWrapping.cpp
@@ -402,7 +402,7 @@ void StackLayoutModifier::classifyCFIs() {
         break;
       case MCCFIInstruction::OpRestoreState: {
         assert(!CFIStack.empty() && "Corrupt CFI stack");
-        std::pair<int64_t, uint16_t> &Elem = CFIStack.top();
+        std::pair<int64_t, uint16_t> Elem = CFIStack.top();
         CFIStack.pop();
         CfaOffset = Elem.first;
         CfaReg = Elem.second;

[bgergely0]

LGTM!