[llvm] Fix performance bug in buildLocationList (PR #109343)

[Jeremy Morse via llvm-commits]

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@@ -1824,22 +1821,27 @@ bool DwarfDebug::buildLocationList(SmallVectorImpl<DebugLocEntry> &DebugLoc,
     RangeMBB = &Asm->MF->front();
   else
     RangeMBB = Entries.begin()->getInstr()->getParent();
+  auto RangeIt = Asm->MBBSectionRanges.find(RangeMBB->getSectionID());
+  assert(RangeIt != Asm->MBBSectionRanges.end() &&
+         "Range MBB not found in MBBSectionRanges!");
   auto *CurEntry = DebugLoc.begin();
   auto *NextEntry = std::next(CurEntry);
+  auto NextRangeIt = std::next(RangeIt);
   while (NextEntry != DebugLoc.end()) {
-    // Get the last machine basic block of this section.
-    while (!RangeMBB->isEndSection())
-      RangeMBB = RangeMBB->getNextNode();
-    if (!RangeMBB->getNextNode())
+    if (NextRangeIt == Asm->MBBSectionRanges.end())
       return false;
     // CurEntry should end the current section and NextEntry should start
     // the next section and the Values must match for these two ranges to be
-    // merged.
-    if (CurEntry->getEndSym() != RangeMBB->getEndSymbol() ||
-        NextEntry->getBeginSym() != RangeMBB->getNextNode()->getSymbol() ||
+    // merged.  Do not match the section label end if it is the entry block
+    // section.  This is because the end label for the Debug Loc and the
+    // Function end label could be different.
+    if ((RangeIt->second.EndLabel != Asm->getFunctionEnd() &&
+         CurEntry->getEndSym() != RangeIt->second.EndLabel) ||
+        NextEntry->getBeginSym() != NextRangeIt->second.BeginLabel ||
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jmorse wrote:

Stupid question perhaps but: the comment talks about not matching stuff in the entry block, but the new comparison is against `getFunctionEnd()` which I anticipate is the exit block. Shouldn't that be fetching the MCLabel for the end-of-the-entry block instead?

(There's some overlap between the current "Entry" and the "entry" block, so I might have confused myself)

https://github.com/llvm/llvm-project/pull/109343