[KeyInstr][Clang] For range stmt atoms

This patch is part of a stack that teaches Clang to generate Key Instructions
metadata for C and C++.

The Key Instructions project is introduced, including a "quick summary" section
at the top which adds context for this PR, here:
https://discourse.llvm.org/t/rfc-improving-is-stmt-placement-for-better-interactive-debugging/82668

The feature is only functional in LLVM if LLVM is built with CMake flag
LLVM_EXPERIMENTAL_KEY_INSTRUCTIONs. Eventually that flag will be removed.

The Clang-side work is demoed here:
#130943

Author: OCHyams

clang/lib/CodeGen/CGStmt.cpp

@@ -1477,7 +1477,14 @@ CodeGenFunction::EmitCXXForRangeStmt(const CXXForRangeStmt &S,
   if (!Weights && CGM.getCodeGenOpts().OptimizationLevel)
     BoolCondVal = emitCondLikelihoodViaExpectIntrinsic(
         BoolCondVal, Stmt::getLikelihood(S.getBody()));
-  Builder.CreateCondBr(BoolCondVal, ForBody, ExitBlock, Weights);
+  auto *I = Builder.CreateCondBr(BoolCondVal, ForBody, ExitBlock, Weights);
+  // Key Instructions: Emit the condition and branch as separate atoms to
+  // match existing loop stepping behaviour. FIXME: We could have the branch as
+  // the backup location for the condition, which would probably be a better
+  // experience.
+  if (auto *I = dyn_cast<llvm::Instruction>(BoolCondVal))
+    addInstToNewSourceAtom(I, nullptr);
+  addInstToNewSourceAtom(I, nullptr);
 
   if (ExitBlock != LoopExit.getBlock()) {
     EmitBlock(ExitBlock);
@@ -1526,6 +1533,10 @@ CodeGenFunction::EmitCXXForRangeStmt(const CXXForRangeStmt &S,
 
   if (CGM.shouldEmitConvergenceTokens())
     ConvergenceTokenStack.pop_back();
+
+  // We want the for closing brace to be step-able on to match existing
+  // behaviour. 
+  addInstToNewSourceAtom(ForBody->getTerminator(), nullptr);
 }
 
 void CodeGenFunction::EmitReturnOfRValue(RValue RV, QualType Ty) {

clang/test/KeyInstructions/for-range.cpp

@@ -0,0 +1,72 @@
+// NOTE: Assertions have been autogenerated by utils/update_cc_test_checks.py UTC_ARGS: --version 5
+// RUN: %clang -gkey-instructions %s -gmlt -S -emit-llvm -o - \
+// RUN: | FileCheck %s --implicit-check-not atomGroup --implicit-check-not atomRank
+
+// Perennial quesiton: should the inc be its own source atom or not
+// (currently it is).
+
+// FIXME: See do.c and while.c regarding cmp and cond br groups.
+
+// The stores in the setup (stores to __RANGE1, __BEGIN1, __END1) are all
+// marked as Key. Unclear whether that's desirable. Keep for now as that's
+// least risky.
+
+// Check the conditional branch (and the condition) in FOR_COND and
+// unconditional branch in FOR_BODY are Key Instructions.
+
+struct Range {
+    int *begin();
+    int *end();
+} r;
+
+// CHECK-LABEL: define dso_local void @_Z1av(
+// CHECK-SAME: ) #[[ATTR0:[0-9]+]] !dbg [[DBG10:![0-9]+]] {
+// CHECK-NEXT:  [[ENTRY:.*:]]
+// CHECK-NEXT:    [[__RANGE1:%.*]] = alloca ptr, align 8
+// CHECK-NEXT:    [[__BEGIN1:%.*]] = alloca ptr, align 8
+// CHECK-NEXT:    [[__END1:%.*]] = alloca ptr, align 8
+// CHECK-NEXT:    [[I:%.*]] = alloca i32, align 4
+// CHECK-NEXT:    store ptr @r, ptr [[__RANGE1]], align 8, !dbg [[DBG14:![0-9]+]]
+// CHECK-NEXT:    [[CALL:%.*]] = call noundef ptr @_ZN5Range5beginEv(ptr noundef nonnull align 1 dereferenceable(1) @r), !dbg [[DBG15:![0-9]+]]
+// CHECK-NEXT:    store ptr [[CALL]], ptr [[__BEGIN1]], align 8, !dbg [[DBG16:![0-9]+]]
+// CHECK-NEXT:    [[CALL1:%.*]] = call noundef ptr @_ZN5Range3endEv(ptr noundef nonnull align 1 dereferenceable(1) @r), !dbg [[DBG17:![0-9]+]]
+// CHECK-NEXT:    store ptr [[CALL1]], ptr [[__END1]], align 8, !dbg [[DBG18:![0-9]+]]
+// CHECK-NEXT:    br label %[[FOR_COND:.*]], !dbg [[DBG19:![0-9]+]]
+// CHECK:       [[FOR_COND]]:
+// CHECK-NEXT:    [[TMP0:%.*]] = load ptr, ptr [[__BEGIN1]], align 8, !dbg [[DBG19]]
+// CHECK-NEXT:    [[TMP1:%.*]] = load ptr, ptr [[__END1]], align 8, !dbg [[DBG19]]
+// CHECK-NEXT:    [[CMP:%.*]] = icmp ne ptr [[TMP0]], [[TMP1]], !dbg [[DBG20:![0-9]+]]
+// CHECK-NEXT:    br i1 [[CMP]], label %[[FOR_BODY:.*]], label %[[FOR_END:.*]], !dbg [[DBG21:![0-9]+]]
+// CHECK:       [[FOR_BODY]]:
+// CHECK-NEXT:    [[TMP2:%.*]] = load ptr, ptr [[__BEGIN1]], align 8, !dbg [[DBG19]]
+// CHECK-NEXT:    [[TMP3:%.*]] = load i32, ptr [[TMP2]], align 4, !dbg [[DBG22:![0-9]+]]
+// CHECK-NEXT:    store i32 [[TMP3]], ptr [[I]], align 4, !dbg [[DBG23:![0-9]+]]
+// CHECK-NEXT:    br label %[[FOR_INC:.*]], !dbg [[DBG24:![0-9]+]]
+// CHECK:       [[FOR_INC]]:
+// CHECK-NEXT:    [[TMP4:%.*]] = load ptr, ptr [[__BEGIN1]], align 8, !dbg [[DBG19]]
+// CHECK-NEXT:    [[INCDEC_PTR:%.*]] = getelementptr inbounds nuw i32, ptr [[TMP4]], i32 1, !dbg [[DBG25:![0-9]+]]
+// CHECK-NEXT:    store ptr [[INCDEC_PTR]], ptr [[__BEGIN1]], align 8, !dbg [[DBG26:![0-9]+]]
+// CHECK-NEXT:    br label %[[FOR_COND]], !dbg [[DBG19]], !llvm.loop
+// CHECK:       [[FOR_END]]:
+// CHECK-NEXT:    ret void, !dbg [[DBG30:![0-9]+]]
+//
+void a() {
+    for (int i: r)
+        ;
+}
+//.
+// CHECK: [[DBG14]] = !DILocation({{.*}}, atomGroup: 1, atomRank: 1)
+// CHECK: [[DBG15]] = !DILocation({{.*}}, atomGroup: 2, atomRank: 2)
+// CHECK: [[DBG16]] = !DILocation({{.*}}, atomGroup: 2, atomRank: 1)
+// CHECK: [[DBG17]] = !DILocation({{.*}}, atomGroup: 3, atomRank: 2)
+// CHECK: [[DBG18]] = !DILocation({{.*}}, atomGroup: 3, atomRank: 1)
+// CHECK: [[DBG19]] = !DILocation({{.*}})
+// CHECK: [[DBG20]] = !DILocation({{.*}}, atomGroup: 4, atomRank: 1)
+// CHECK: [[DBG21]] = !DILocation({{.*}}, atomGroup: 5, atomRank: 1)
+// CHECK: [[DBG22]] = !DILocation({{.*}}, atomGroup: 6, atomRank: 2)
+// CHECK: [[DBG23]] = !DILocation({{.*}}, atomGroup: 6, atomRank: 1)
+// CHECK: [[DBG24]] = !DILocation({{.*}} atomGroup: 8, atomRank: 1)
+// CHECK: [[DBG25]] = !DILocation({{.*}}, atomGroup: 7, atomRank: 2)
+// CHECK: [[DBG26]] = !DILocation({{.*}}, atomGroup: 7, atomRank: 1)
+// CHECK: [[DBG30]] = !DILocation({{.*}})
+//.