PGO: new -debug-info-for-profiling frontend flag

This achieves the same as clang's `-fdebug-info-for-profiling`, which
emits DWARF discriminators to aid in narrowing-down which basic block
corresponds to a particular instruction address. This is particularly
useful for sampling-based profiling.

rdar://135443278

include/swift/AST/IRGenOptions.h

@@ -504,6 +504,9 @@ public:
   /// or the empty string.
   std::string UseSampleProfile = "";
 
+  /// Controls whether DWARF discriminators are added to the IR.
+  unsigned DebugInfoForProfiling : 1;
+
   /// List of backend command-line options for -embed-bitcode.
   std::vector<uint8_t> CmdArgs;
 
@@ -588,7 +591,8 @@ public:
         ColocateTypeDescriptors(true), UseRelativeProtocolWitnessTables(false),
         UseFragileResilientProtocolWitnesses(false), EnableHotColdSplit(false),
         EmitAsyncFramePushPopMetadata(false), EmitYieldOnce2AsYieldOnce(true),
-        AsyncFramePointerAll(false), UseProfilingMarkerThunks(false), CmdArgs(),
+        AsyncFramePointerAll(false), UseProfilingMarkerThunks(false),
+        DebugInfoForProfiling(false), CmdArgs(),
         SanitizeCoverage(llvm::SanitizerCoverageOptions()),
         TypeInfoFilter(TypeInfoDumpFilter::All),
         PlatformCCallingConvention(llvm::CallingConv::C), UseCASBackend(false),

include/swift/Option/Options.td

@@ -1477,6 +1477,10 @@ def profile_generate : Flag<["-"], "profile-generate">,
   Flags<[FrontendOption, NoInteractiveOption]>,
   HelpText<"Generate instrumented code to collect execution counts">;
 
+def debug_info_for_profiling : Flag<["-"], "debug-info-for-profiling">,
+  Flags<[FrontendOption, NoInteractiveOption]>,
+  HelpText<"Emit extra debug info (DWARF discriminators) to make sampling-based profiling more accurate">;
+
 def profile_use : CommaJoined<["-"], "profile-use=">,
   Flags<[FrontendOption, NoInteractiveOption, ArgumentIsPath]>,
   MetaVarName<"<profdata>">,

lib/Frontend/CompilerInvocation.cpp

@@ -3180,6 +3180,8 @@ static bool ParseIRGenArgs(IRGenOptions &Opts, ArgList &Args,
   const Arg *ProfileSampleUse = Args.getLastArg(OPT_profile_sample_use);
   Opts.UseSampleProfile = ProfileSampleUse ? ProfileSampleUse->getValue() : "";
 
+  Opts.DebugInfoForProfiling |= Args.hasArg(OPT_debug_info_for_profiling);
+
   Opts.PrintInlineTree |= Args.hasArg(OPT_print_llvm_inline_tree);
   // Always producing all outputs when caching is enabled.
   Opts.AlwaysCompile |= Args.hasArg(OPT_always_compile_output_files) ||

lib/IRGen/IRGen.cpp

@@ -212,7 +212,22 @@ static void populatePGOOptions(std::optional<PGOOptions> &Out,
       /*Action=*/ PGOOptions::SampleUse,
       /*CSPGOAction=*/ PGOOptions::NoCSAction,
       /*ColdType=*/ PGOOptions::ColdFuncOpt::Default,
-      /*DebugInfoForProfiling=*/ false
+      /*DebugInfoForProfiling=*/ Opts.DebugInfoForProfiling
+    );
+    return;
+  }
+
+  if (Opts.DebugInfoForProfiling) {
+    Out = PGOOptions(
+        /*ProfileFile=*/ "",
+        /*CSProfileGenFile=*/ "",
+        /*ProfileRemappingFile=*/ "",
+        /*MemoryProfile=*/ "",
+        /*FS=*/ nullptr,
+        /*Action=*/ PGOOptions::NoAction,
+        /*CSPGOAction=*/ PGOOptions::NoCSAction,
+        /*ColdType=*/ PGOOptions::ColdFuncOpt::Default,
+        /*DebugInfoForProfiling=*/ true
     );
     return;
   }

lib/IRGen/IRGenDebugInfo.cpp

@@ -839,6 +839,7 @@ private:
                               TheCU->getProducer(), true, StringRef(), 0,
                               RemappedASTFile, llvm::DICompileUnit::FullDebug,
                               Signature);
+        // NOTE: not setting DebugInfoForProfiling here
         DIB.finalize();
       }
     }
@@ -2504,7 +2505,7 @@ IRGenDebugInfoImpl::IRGenDebugInfoImpl(const IRGenOptions &Opts,
           ? llvm::DICompileUnit::FullDebug
           : llvm::DICompileUnit::LineTablesOnly,
       /* DWOId */ 0, /* SplitDebugInlining */ true,
-      /* DebugInfoForProfiling */ false,
+      /* DebugInfoForProfiling */ Opts.DebugInfoForProfiling,
       llvm::DICompileUnit::DebugNameTableKind::Default,
       /* RangesBaseAddress */ false, DebugPrefixMap.remapPath(Sysroot), SDK);
 

test/DebugInfo/discriminators_for_profiling.swift

@@ -0,0 +1,16 @@
+// RUN: %target-swift-frontend %s -emit-ir -g -debug-info-for-profiling -o - | %FileCheck %s
+
+// CHECK: distinct !DICompileUnit(language: DW_LANG_Swift
+// CHECK-SAME: debugInfoForProfiling: true
+
+// CHECK: !DILexicalBlockFile({{.*}} discriminator: 2)
+
+public func lobster(_ n: Int) -> Bool {
+  guard n > 0 else { fatalError("too cold!") }
+
+  if n > 100 {
+    print("warm but ok")
+  }
+
+  return n < 120
+}