[KeyInstr][Clang] If stmt atom

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/CodeGenFunction.cpp

@@ -2099,6 +2099,8 @@ void CodeGenFunction::EmitBranchOnBoolExpr(
 
   llvm::Instruction *BrInst = Builder.CreateCondBr(CondV, TrueBlock, FalseBlock,
                                                    Weights, Unpredictable);
+  addInstToNewSourceAtom(BrInst, CondV);
+
   switch (HLSLControlFlowAttr) {
   case HLSLControlFlowHintAttr::Microsoft_branch:
   case HLSLControlFlowHintAttr::Microsoft_flatten: {

clang/lib/CodeGen/CodeGenFunction.h

@@ -1768,6 +1768,16 @@ class CodeGenFunction : public CodeGenTypeCache {
       DI->addInstToCurrentSourceAtom(KeyInstruction, Backup);
   }
 
+  /// Add \p KeyInstruction and an optional \p Backup instruction to a new atom
+  /// group (See ApplyAtomGroup for more info).
+  void addInstToNewSourceAtom(llvm::Instruction *KeyInstruction,
+                              llvm::Value *Backup) {
+    if (CGDebugInfo *DI = getDebugInfo()) {
+      ApplyAtomGroup Grp(getDebugInfo());
+      DI->addInstToCurrentSourceAtom(KeyInstruction, Backup);
+    }
+  }
+
   /// See CGDebugInfo::addRetToOverrideOrNewSourceAtom.
   void addRetToOverrideOrNewSourceAtom(llvm::ReturnInst *Ret,
                                        llvm::Value *Backup) {

clang/test/KeyInstructions/if.c

@@ -0,0 +1,46 @@
+// RUN: %clang -gkey-instructions -x c++ -std=c++17 %s -gmlt -S -emit-llvm -o - \
+// RUN: | FileCheck %s --implicit-check-not atomGroup --implicit-check-not atomRank --check-prefixes=CHECK,CHECK-CXX
+
+// RUN: %clang -gkey-instructions -x c %s -gmlt -S -emit-llvm -o -  \
+// RUN: | FileCheck %s --implicit-check-not atomGroup --implicit-check-not atomRank
+
+int g;
+void a(int A) {
+// The branch is a key instruction, with the condition being its backup.
+// CHECK: entry:
+// CHECK: %tobool = icmp ne i32 %0, 0{{.*}}, !dbg [[G1R2:!.*]]
+// CHECK: br i1 %tobool, label %if.then, label %if.end{{.*}}, !dbg [[G1R1:!.*]]
+    if (A)
+        ;
+
+// The assignment in the if gets a distinct source atom group.
+// CHECK: if.end:
+// CHECK: %1 = load i32, ptr %A.addr{{.*}}, !dbg [[G2R2:!.*]]
+// CHECK: store i32 %1, ptr @g{{.*}}, !dbg [[G2R1:!.*]]
+// CHECK: %tobool1 = icmp ne i32 %1, 0{{.*}}, !dbg [[G3R2:!.*]]
+// CHECK: br i1 %tobool1, label %if.then2, label %if.end3{{.*}}, !dbg [[G3R1:!.*]]
+    if ((g = A))
+        ;
+
+#ifdef __cplusplus
+// The assignment in the if gets a distinct source atom group.
+// CHECK-CXX: if.end3:
+// CHECK-CXX: %2 = load i32, ptr %A.addr{{.*}}, !dbg [[G4R2:!.*]]
+// CHECK-CXX: store i32 %2, ptr %B{{.*}}, !dbg [[G4R1:!.*]]
+// CHECK-CXX: %tobool4 = icmp ne i32 %3, 0{{.*}}, !dbg [[G5R2:!.*]]
+// CHECK-CXX: br i1 %tobool4, label %if.then5, label %if.end6{{.*}}, !dbg [[G5R1:!.*]]
+    if (int B = A; B)
+        ;
+#endif 
+}
+
+// CHECK: [[G1R2]] = !DILocation({{.*}}, atomGroup: 1, atomRank: 2)
+// CHECK: [[G1R1]] = !DILocation({{.*}}, atomGroup: 1, atomRank: 1)
+// CHECK: [[G2R2]] = !DILocation({{.*}}, atomGroup: 2, atomRank: 2)
+// CHECK: [[G2R1]] = !DILocation({{.*}}, atomGroup: 2, atomRank: 1)
+// CHECK: [[G3R2]] = !DILocation({{.*}}, atomGroup: 3, atomRank: 2)
+// CHECK: [[G3R1]] = !DILocation({{.*}}, atomGroup: 3, atomRank: 1)
+// CHECK-CXX: [[G4R2]] = !DILocation({{.*}}, atomGroup: 4, atomRank: 2)
+// CHECK-CXX: [[G4R1]] = !DILocation({{.*}}, atomGroup: 4, atomRank: 1)
+// CHECK-CXX: [[G5R2]] = !DILocation({{.*}}, atomGroup: 5, atomRank: 2)
+// CHECK-CXX: [[G5R1]] = !DILocation({{.*}}, atomGroup: 5, atomRank: 1)