[llvm] [DebugInfo] When merging locations prefer unannotated empty locs (PR #157707)

[Stephen Tozer via llvm-commits]

https://github.com/SLTozer created https://github.com/llvm/llvm-project/pull/157707

When merging DILocations, we prefer to use DebugLoc::getMergedLocation when possible to better preserve DebugLoc coverage tracking information through transformations (as conversion to DILocations drops all coverage tracking data). Currently, DebugLoc::getMergedLocation checks to see if either DebugLoc is empty and returns it directly if so, to propagate that DebugLoc's coverage tracking data to the merged location; however, it only checks whether either location is valid, not whether they are annotated.

This is significant because an annotated location is not a bug, while an empty unannotated location may be one; therefore, we check to see if either location is unannotated, and prefer to return that location if it exists rather than an annotated one.

This change is NFC outside of DebugLoc coverage tracking builds.

>From e6b9834f3a0a4fe4292ad50d55b07abccbb795e1 Mon Sep 17 00:00:00 2001
From: Stephen Tozer <stephen.tozer at sony.com>
Date: Tue, 9 Sep 2025 17:37:25 +0100
Subject: [PATCH] [DebugInfo] When merging locations prefer unannotated empty
 locs

When merging DILocations, we prefer to use DebugLoc::getMergedLocation when
possible to better preserve DebugLoc coverage tracking information through
transformations (as conversion to DILocations drops all coverage tracking
data). Currently, DebugLoc::getMergedLocation checks to see if either
DebugLoc is empty and returns it directly if so, to propagate that
DebugLoc's coverage tracking data to the merged location; however, it only
checks whether either location is valid, not whether they are annotated.

This is significant because an annotated location is not a bug, while an
empty unannotated location may be one; therefore, we check to see if either
location is unannotated, and prefer to return that location if it exists
rather than an annotated one.

This change is NFC outside of DebugLoc coverage tracking builds.
---
 llvm/lib/IR/DebugLoc.cpp | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/llvm/lib/IR/DebugLoc.cpp b/llvm/lib/IR/DebugLoc.cpp
index 79c5b896f8f25..01dafcab94ce9 100644
--- a/llvm/lib/IR/DebugLoc.cpp
+++ b/llvm/lib/IR/DebugLoc.cpp
@@ -181,10 +181,19 @@ DebugLoc DebugLoc::getMergedLocations(ArrayRef<DebugLoc> Locs) {
   return Merged;
 }
 DebugLoc DebugLoc::getMergedLocation(DebugLoc LocA, DebugLoc LocB) {
-  if (!LocA)
-    return LocA;
-  if (!LocB)
+  if (!LocA || !LocB) {
+    // If coverage tracking is enabled, prioritize returning empty non-annotated
+    // locations to empty annotated locations.
+#if LLVM_ENABLE_DEBUGLOC_TRACKING_COVERAGE
+    if (!LocA && LocA.getKind() == DebugLocKind::Normal)
+      return LocA;
+    if (!LocB && LocB.getKind() == DebugLocKind::Normal)
+      return LocB;
+#endif // LLVM_ENABLE_DEBUGLOC_TRACKING_COVERAGE
+    if (!LocA)
+      return LocA;
     return LocB;
+  }
   return DILocation::getMergedLocation(LocA, LocB);
 }