[KeyInstr][debugify] Add --debugify-atoms to add key instructions metadata (#133483)

RFC:
https://discourse.llvm.org/t/rfc-improving-is-stmt-placement-for-better-interactive-debugging/82668

Author: OCHyams

llvm/lib/Transforms/Utils/Debugify.cpp

@@ -35,6 +35,8 @@ using namespace llvm;
 
 namespace {
 
+cl::opt<bool> ApplyAtomGroups("debugify-atoms", cl::init(false));
+
 cl::opt<bool> Quiet("debugify-quiet",
                     cl::desc("Suppress verbose debugify output"));
 
@@ -142,8 +144,13 @@ bool llvm::applyDebugifyMetadata(
 
     for (BasicBlock &BB : F) {
       // Attach debug locations.
-      for (Instruction &I : BB)
-        I.setDebugLoc(DILocation::get(Ctx, NextLine++, 1, SP));
+      for (Instruction &I : BB) {
+        uint64_t AtomGroup = ApplyAtomGroups ? NextLine : 0;
+        uint8_t AtomRank = ApplyAtomGroups ? 1 : 0;
+        uint64_t Line = NextLine++;
+        I.setDebugLoc(DILocation::get(Ctx, Line, 1, SP, nullptr, false,
+                                      AtomGroup, AtomRank));
+      }
 
       if (DebugifyLevel < Level::LocationsAndVariables)
         continue;

llvm/test/DebugInfo/KeyInstructions/debugify.ll

@@ -0,0 +1,46 @@
+; RUN: opt -passes=debugify --debugify-atoms -S -o - < %s \
+; RUN: | FileCheck %s
+
+;; Mirrors llvm/test/DebugInfo/debugify.ll. Split out here because the
+;; test is only supported if LLVM_EXPERIMENTAL_KEY_INSTRUCTIONS is enabled
+;; (which is a condition for running this test directory). Once the conditional
+;; compilation of the feature is removed this can be merged into the original.
+
+; CHECK-LABEL: define void @foo
+define void @foo() {
+; CHECK: ret void, !dbg ![[RET1:.*]]
+  ret void
+}
+
+; CHECK-LABEL: define i32 @bar
+define i32 @bar() {
+; CHECK: call void @foo(), !dbg ![[CALL1:.*]]
+  call void @foo()
+
+; CHECK: add i32 0, 1, !dbg ![[ADD1:.*]]
+  %sum = add i32 0, 1
+
+; CHECK: ret i32 0, !dbg ![[RET2:.*]]
+  ret i32 0
+}
+
+; CHECK-LABEL: define weak_odr zeroext i1 @baz
+define weak_odr zeroext i1 @baz() {
+; CHECK-NOT: !dbg
+  ret i1 false
+}
+
+; CHECK-LABEL: define i32 @boom
+define i32 @boom() {
+; CHECK: [[result:%.*]] = musttail call i32 @bar(), !dbg ![[musttail:.*]]
+  %retval = musttail call i32 @bar()
+; CHECK-NEXT: ret i32 [[result]], !dbg ![[musttailRes:.*]]
+  ret i32 %retval
+}
+
+; CHECK-DAG: ![[RET1]] = !DILocation(line: 1, {{.*}}, atomGroup: 1, atomRank: 1
+; CHECK-DAG: ![[CALL1]] = !DILocation(line: 2, {{.*}}, atomGroup: 2, atomRank: 1
+; CHECK-DAG: ![[ADD1]] = !DILocation(line: 3, {{.*}}, atomGroup: 3, atomRank: 1
+; CHECK-DAG: ![[RET2]] = !DILocation(line: 4, {{.*}}, atomGroup: 4, atomRank: 1
+; CHECK-DAG: ![[musttail]] = !DILocation(line: 5, {{.*}}, atomGroup: 5, atomRank: 1
+; CHECK-DAG: ![[musttailRes]] = !DILocation(line: 6, {{.*}}, atomGroup: 6, atomRank: 1