[DLCov][NFC] Annotate intentionally-blank DebugLocs in existing code (#136192)

Following the work in PR #107279, this patch applies the annotative
DebugLocs, which indicate that a particular instruction is intentionally
missing a location for a given reason, to existing sites in the compiler
where their conditions apply. This is NFC in ordinary LLVM builds (each
function `DebugLoc::getFoo()` is inlined as `DebugLoc()`), but marks the
instruction in coverage-tracking builds so that it will be ignored by
Debugify, allowing only real errors to be reported. From a developer
standpoint, it also communicates the intentionality and reason for a
missing DebugLoc.

Some notes for reviewers:

- The difference between `I->dropLocation()` and
`I->setDebugLoc(DebugLoc::getDropped())` is that the former _may_ decide
to keep some debug info alive, while the latter will always be empty; in
this patch, I always used the latter (even if the former could
technically be correct), because the former could result in some
(barely) different output, and I'd prefer to keep this patch purely NFC.
- I've generally documented the uses of `DebugLoc::getUnknown()`, with
the exception of the vectorizers - in summary, they are a huge cause of
dropped source locations, and I don't have the time or the domain
knowledge currently to solve that, so I've plastered it all over them as
a form of "fixme".

1 files changed, 6 insertions, 4 deletions

diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index e221022..975ce3b 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -1137,7 +1137,7 @@ static void cloneInstructionsIntoPredecessorBlockAndUpdateSSAUses(
         // branch, drop it. When we fold the bonus instructions we want to make
         // sure we reset their debug locations in order to avoid stepping on
         // dead code caused by folding dead branches.
-        NewBonusInst->setDebugLoc(DebugLoc());
+        NewBonusInst->setDebugLoc(DebugLoc::getDropped());
       } else if (const DebugLoc &DL = NewBonusInst->getDebugLoc()) {
         mapAtomInstance(DL, VMap);
       }
@@ -2821,7 +2821,8 @@ static void mergeCompatibleInvokesImpl(ArrayRef<InvokeInst *> Invokes,
       // so just form a new block with unreachable terminator.
       BasicBlock *MergedNormalDest = BasicBlock::Create(
           Ctx, II0BB->getName() + ".cont", Func, InsertBeforeBlock);
-      new UnreachableInst(Ctx, MergedNormalDest);
+      auto *UI = new UnreachableInst(Ctx, MergedNormalDest);
+      UI->setDebugLoc(DebugLoc::getTemporary());
       MergedInvoke->setNormalDest(MergedNormalDest);
     }
 
@@ -3389,7 +3390,7 @@ bool SimplifyCFGOpt::speculativelyExecuteBB(BranchInst *BI,
     if (!SpeculatedStoreValue || &I != SpeculatedStore) {
       // Don't update the DILocation of dbg.assign intrinsics.
       if (!isa<DbgAssignIntrinsic>(&I))
-        I.setDebugLoc(DebugLoc());
+        I.setDebugLoc(DebugLoc::getDropped());
     }
     I.dropUBImplyingAttrsAndMetadata();
 
@@ -5707,7 +5708,8 @@ static void createUnreachableSwitchDefault(SwitchInst *Switch,
   BasicBlock *NewDefaultBlock = BasicBlock::Create(
       BB->getContext(), BB->getName() + ".unreachabledefault", BB->getParent(),
       OrigDefaultBlock);
-  new UnreachableInst(Switch->getContext(), NewDefaultBlock);
+  auto *UI = new UnreachableInst(Switch->getContext(), NewDefaultBlock);
+  UI->setDebugLoc(DebugLoc::getTemporary());
   Switch->setDefaultDest(&*NewDefaultBlock);
   if (DTU) {
     SmallVector<DominatorTree::UpdateType, 2> Updates;