[DLCov] Implement DebugLoc coverage tracking (#107279)

This is part of a series of patches that tries to improve DILocation bug
detection in Debugify; see the review for more details. This is the patch
that adds the main feature, adding a set of `DebugLoc::get<Kind>`
functions that can be used for instructions with intentionally empty
DebugLocs to prevent Debugify from treating them as bugs, removing the
currently-pervasive false positives and allowing us to use Debugify (in
its original DI preservation mode) to reliably detect existing bugs and
regressions. This patch does not add uses of these functions, except for
once in Clang before optimizations, and in
`Instruction::dropLocation()`, since that is an obvious case that
immediately removes a set of false positives.

1 files changed, 5 insertions, 3 deletions

diff --git a/llvm/lib/IR/DebugInfo.cpp b/llvm/lib/IR/DebugInfo.cpp
index 7a133b2..a6a88a2 100644
--- a/llvm/lib/IR/DebugInfo.cpp
+++ b/llvm/lib/IR/DebugInfo.cpp
@@ -987,8 +987,10 @@ void Instruction::updateLocationAfterHoist() { dropLocation(); }
 
 void Instruction::dropLocation() {
   const DebugLoc &DL = getDebugLoc();
-  if (!DL)
+  if (!DL) {
+    setDebugLoc(DebugLoc::getDropped());
     return;
+  }
 
   // If this isn't a call, drop the location to allow a location from a
   // preceding instruction to propagate.
@@ -1000,7 +1002,7 @@ void Instruction::dropLocation() {
   }
 
   if (!MayLowerToCall) {
-    setDebugLoc(DebugLoc());
+    setDebugLoc(DebugLoc::getDropped());
     return;
   }
 
@@ -1019,7 +1021,7 @@ void Instruction::dropLocation() {
     //
     // One alternative is to set a line 0 location with the existing scope and
     // inlinedAt info. The location might be sensitive to when inlining occurs.
-    setDebugLoc(DebugLoc());
+    setDebugLoc(DebugLoc::getDropped());
 }
 
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