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

[Sriraman Tallam via llvm-commits]

================
@@ -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 ||
----------------
tmsri wrote:

Basically, MBBSectionRanges stores the ranges for each section.  For the entry section, the end label is the Function End Label, Asm->getFunctionEnd().  The entry section is actually the original function section.

Now, take a look at this assembly:

```
	.section	.text._Z4testv,"ax", at progbits
	.globl	_Z4testv                        # -- Begin function _Z4testv
	.type	_Z4testv, at function
_Z4testv:                               # @_Z4testv
.Lfunc_begin0:
	.loc	0 3 0                           # ex4.cc:3:0
# %bb.0:                                # %entry
	pushq	%rax
.Ltmp0:
	#DEBUG_VALUE: test:i <- 7
	.loc	0 7 5 prologue_end              # ex4.cc:7:5
	callq	_Z2f1v at PLT
        ...
.LBB_END0_0:
	.cfi_endproc
	.section	.text._Z4testv,"ax", at progbits,unique,1
_Z4testv.__part.1:                      # %if.then
	.section	.text._Z4testv,"ax", at progbits
.Lfunc_end0:
```

The entry block section uses label ".Lfunc_end0" which is Asm->getFunctionEnd(). There is an additional label that is created immediately after the end of the entry section ".LBB_END0_0" which is what the debug loc uses, like an alias.  In this case and this case alone, these two labels won't match even though semantically, they are the same.  So, I exclude that match.  

CurEntry and entry block are different things.  Sorry for the use of the overloaded entry.

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