[LiveDebugValues] Add switches for using instr-ref variable locations

This patch adds the -Xclang option
"-fexperimental-debug-variable-locations" and same LLVM CodeGen option,
to pick which variable location tracking solution to use.

Right now all the switch does is pick which LiveDebugValues
implementation to use, the normal VarLoc one or the instruction
referencing one in rGae6f78824031. Over time, the aim is to add fragments
of support in aid of the value-tracking RFC:

  http://lists.llvm.org/pipermail/llvm-dev/2020-February/139440.html

also controlled by this command line switch. That will slowly move
variable locations to be defined by an instruction calculating a value,
and a DBG_INSTR_REF instruction referring to that value. Thus, this is
going to grow into a "use the new kind of variable locations" switch,
rather than just "use the new LiveDebugValues implementation".

Differential Revision: https://reviews.llvm.org/D83048

clang/include/clang/Basic/CodeGenOptions.def

@@ -325,6 +325,9 @@ ENUM_CODEGENOPT(DebuggerTuning, llvm::DebuggerKind, 2,
 /// emitted.
 VALUE_CODEGENOPT(DwarfVersion, 3, 0)
 
+/// Whether to use experimental new variable location tracking.
+CODEGENOPT(ValueTrackingVariableLocations, 1, 0)
+
 /// Whether we should emit CodeView debug information. It's possible to emit
 /// CodeView and DWARF into the same object.
 CODEGENOPT(EmitCodeView, 1, 0)

clang/include/clang/Driver/Options.td

@@ -3870,6 +3870,9 @@ def fdebug_pass_manager : Flag<["-"], "fdebug-pass-manager">,
     HelpText<"Prints debug information for the new pass manager">;
 def fno_debug_pass_manager : Flag<["-"], "fno-debug-pass-manager">,
     HelpText<"Disables debug printing for the new pass manager">;
+def fexperimental_debug_variable_locations : Flag<["-"],
+    "fexperimental-debug-variable-locations">,
+    HelpText<"Use experimental new value-tracking variable locations">;
 // The driver option takes the key as a parameter to the -msign-return-address=
 // and -mbranch-protection= options, but CC1 has a separate option so we
 // don't have to parse the parameter twice.

clang/lib/CodeGen/BackendUtil.cpp

@@ -520,6 +520,8 @@ static void initTargetOptions(DiagnosticsEngine &Diags,
   Options.EmitAddrsig = CodeGenOpts.Addrsig;
   Options.ForceDwarfFrameSection = CodeGenOpts.ForceDwarfFrameSection;
   Options.EmitCallSiteInfo = CodeGenOpts.EmitCallSiteInfo;
+  Options.ValueTrackingVariableLocations =
+      CodeGenOpts.ValueTrackingVariableLocations;
   Options.XRayOmitFunctionIndex = CodeGenOpts.XRayOmitFunctionIndex;
 
   Options.MCOptions.SplitDwarfFile = CodeGenOpts.SplitDwarfFile;

clang/lib/Frontend/CompilerInvocation.cpp

@@ -835,6 +835,9 @@ static bool ParseCodeGenArgs(CodeGenOptions &Opts, ArgList &Args, InputKind IK,
       llvm::is_contained(DebugEntryValueArchs, T.getArch()))
     Opts.EmitCallSiteInfo = true;
 
+  Opts.ValueTrackingVariableLocations =
+      Args.hasArg(OPT_fexperimental_debug_variable_locations);
+
   Opts.DisableO0ImplyOptNone = Args.hasArg(OPT_disable_O0_optnone);
   Opts.DisableRedZone = Args.hasArg(OPT_disable_red_zone);
   Opts.IndirectTlsSegRefs = Args.hasArg(OPT_mno_tls_direct_seg_refs);

clang/test/Driver/debug-var-experimental-switch.c

@@ -0,0 +1,2 @@
+// RUN: %clang -Xclang -fexperimental-debug-variable-locations -fsyntax-only -disable-llvm-passes %s
+int main() {}

llvm/include/llvm/CodeGen/CommandFlags.h

@@ -116,6 +116,8 @@ bool getEmitCallSiteInfo();
 
 bool getEnableDebugEntryValues();
 
+bool getValueTrackingVariableLocations();
+
 bool getForceDwarfFrameSection();
 
 bool getXRayOmitFunctionIndex();

llvm/include/llvm/Target/TargetOptions.h

@@ -126,8 +126,8 @@ namespace llvm {
           EmitStackSizeSection(false), EnableMachineOutliner(false),
           SupportsDefaultOutlining(false), EmitAddrsig(false),
           EmitCallSiteInfo(false), SupportsDebugEntryValues(false),
-          EnableDebugEntryValues(false), ForceDwarfFrameSection(false),
-          XRayOmitFunctionIndex(false),
+          EnableDebugEntryValues(false), ValueTrackingVariableLocations(false),
+          ForceDwarfFrameSection(false), XRayOmitFunctionIndex(false),
           FPDenormalMode(DenormalMode::IEEE, DenormalMode::IEEE) {}
 
     /// DisableFramePointerElim - This returns true if frame pointer elimination
@@ -285,6 +285,11 @@ namespace llvm {
     /// production.
     bool ShouldEmitDebugEntryValues() const;
 
+    // When set to true, use experimental new debug variable location tracking,
+    // which seeks to follow the values of variables rather than their location,
+    // post isel.
+    unsigned ValueTrackingVariableLocations : 1;
+
     /// Emit DWARF debug frame section.
     unsigned ForceDwarfFrameSection : 1;
 

llvm/lib/CodeGen/CommandFlags.cpp

@@ -85,6 +85,7 @@ CGOPT(bool, EnableStackSizeSection)
 CGOPT(bool, EnableAddrsig)
 CGOPT(bool, EmitCallSiteInfo)
 CGOPT(bool, EnableDebugEntryValues)
+CGOPT(bool, ValueTrackingVariableLocations)
 CGOPT(bool, ForceDwarfFrameSection)
 CGOPT(bool, XRayOmitFunctionIndex)
 
@@ -400,6 +401,12 @@ codegen::RegisterCodeGenFlags::RegisterCodeGenFlags() {
       cl::init(false));
   CGBINDOPT(EnableDebugEntryValues);
 
+  static cl::opt<bool> ValueTrackingVariableLocations(
+      "experimental-debug-variable-locations",
+      cl::desc("Use experimental new value-tracking variable locations"),
+      cl::init(false));
+  CGBINDOPT(ValueTrackingVariableLocations);
+
   static cl::opt<bool> ForceDwarfFrameSection(
       "force-dwarf-frame-section",
       cl::desc("Always emit a debug frame section."), cl::init(false));
@@ -475,6 +482,7 @@ TargetOptions codegen::InitTargetOptionsFromCodeGenFlags() {
   Options.EmitAddrsig = getEnableAddrsig();
   Options.EmitCallSiteInfo = getEmitCallSiteInfo();
   Options.EnableDebugEntryValues = getEnableDebugEntryValues();
+  Options.ValueTrackingVariableLocations = getValueTrackingVariableLocations();
   Options.ForceDwarfFrameSection = getForceDwarfFrameSection();
   Options.XRayOmitFunctionIndex = getXRayOmitFunctionIndex();
 

llvm/lib/CodeGen/LiveDebugValues/LiveDebugValues.cpp

@@ -14,6 +14,7 @@
 #include "llvm/CodeGen/Passes.h"
 #include "llvm/InitializePasses.h"
 #include "llvm/Pass.h"
+#include "llvm/Target/TargetMachine.h"
 
 /// \file LiveDebugValues.cpp
 ///
@@ -40,7 +41,10 @@ public:
   static char ID;
 
   LiveDebugValues();
-  ~LiveDebugValues() { delete TheImpl; }
+  ~LiveDebugValues() {
+    if (TheImpl)
+      delete TheImpl;
+  }
 
   /// Calculate the liveness information for the given machine function.
   bool runOnMachineFunction(MachineFunction &MF) override;
@@ -57,6 +61,7 @@ public:
 
 private:
   LDVImpl *TheImpl;
+  TargetPassConfig *TPC;
 };
 
 char LiveDebugValues::ID = 0;
@@ -69,10 +74,24 @@ INITIALIZE_PASS(LiveDebugValues, DEBUG_TYPE, "Live DEBUG_VALUE analysis", false,
 /// Default construct and initialize the pass.
 LiveDebugValues::LiveDebugValues() : MachineFunctionPass(ID) {
   initializeLiveDebugValuesPass(*PassRegistry::getPassRegistry());
-  TheImpl = llvm::makeVarLocBasedLiveDebugValues();
+  TheImpl = nullptr;
 }
 
 bool LiveDebugValues::runOnMachineFunction(MachineFunction &MF) {
-  auto *TPC = getAnalysisIfAvailable<TargetPassConfig>();
+  if (!TheImpl) {
+    TPC = getAnalysisIfAvailable<TargetPassConfig>();
+
+    bool InstrRefBased = false;
+    if (TPC) {
+      auto &TM = TPC->getTM<TargetMachine>();
+      InstrRefBased = TM.Options.ValueTrackingVariableLocations;
+    }
+
+    if (InstrRefBased)
+      TheImpl = llvm::makeInstrRefBasedLiveDebugValues();
+    else
+      TheImpl = llvm::makeVarLocBasedLiveDebugValues();
+  }
+
   return TheImpl->ExtendRanges(MF, TPC);
 }