Support using sample profiles with partial debug info (driver)

[dnovillo]

Summary

When using a profile, we used to require the use -gmlt so that we could
get access to the line locations. This is used to match line numbers in
the input profile to the line numbers in the function's IR.

But this is actually not necessary. The driver can provide source
location tracking without the emission of debug information. In these
cases, the annotation 'llvm.dbg.cu' is missing from the IR, but the
actual line location annotations are still present.

This patch tells the driver to only emit source location tracking
when -fprofile-sample-use is present in the command line.

[dblaikie]

(driver code review should be sent to the cfe-commits list - just abandon
this CR and create a new one (because you can never get Phab to resend the
initial, informative (with patch attached, summary, etc) email again to the
list - so just switching mailing lists won't produce a good experience for
Clang developers reading their mailing list who aren't subscribed to this
one))

[echristo]

OK.

-eric

[dnovillo]

• Update comment for LocTrackingOnly.

[dnovillo]

Closed by commit rL220383 (authored by @dnovillo).

lib/Frontend/CompilerInvocation.cpp

static bool ParseCodeGenArgs(CodeGenOptions &Opts, ArgList &Args, InputKind IK,
   }
 
   if (Arg *A = Args.getLastArg(OPT_Rpass_analysis_EQ)) {
     Opts.OptimizationRemarkAnalysisPattern =
         GenerateOptimizationRemarkRegex(Diags, Args, A);
     NeedLocTracking = true;
   }
 
+  // If the user requested to use a sample profile for PGO, then the
+  // backend will need to track source location information so the profile
+  // can be incorporated into the IR.
+  if (!Opts.SampleProfileFile.empty())
+    NeedLocTracking = true;
+
   // If the user requested one of the flags in the -Rpass family, make sure
   // that the backend tracks source location information.
   if (NeedLocTracking && Opts.getDebugInfo() == CodeGenOptions::NoDebugInfo)
     Opts.setDebugInfo(CodeGenOptions::LocTrackingOnly);
 
   return Success;
 }
 

test/Frontend/Inputs/profile-sample-use-loc-tracking.prof

+bar:100:100
+1: 2000

test/Frontend/profile-sample-use-loc-tracking.c

+// This file tests that -fprofile-sample-use enables location tracking
+// generation in the same way that -Rpass does. The sample profiler needs
+// to associate line locations in the profile to the code, so it needs the
+// frontend to emit source location annotations.
+
+// RUN: %clang_cc1 %s -fprofile-sample-use=%S/Inputs/profile-sample-use-loc-tracking.prof -emit-llvm -o - 2>/dev/null | FileCheck %s
+
+// -fprofile-sample-use should produce source location annotations, exclusively
+// (just like -gmlt).
+// CHECK: , !dbg !
+// CHECK-NOT: DW_TAG_base_type
+
+// But llvm.dbg.cu should be missing (to prevent writing debug info to
+// the final output).
+// CHECK-NOT: !llvm.dbg.cu = !{
+
+int bar(int j) {
+  return (j + j - 2) * (j - 2) * j;
+}