[IRGen] Allow lazy emission of coverage mapped functions

We previously eagerly emitted such functions to
ensure that their name data is emitted through the
profiler increment. Now that are able to emit the
profile name data separately, this is unnecessary,
and we can avoid emitting their definitions.

Author: hamishknight

lib/IRGen/GenCoverage.cpp

@@ -192,3 +192,8 @@ void IRGenModule::emitCoverageMapping() {
   CovData->setAlignment(llvm::Align(8));
   addUsedGlobal(CovData);
 }
+
+void IRGenerator::emitCoverageMapping() {
+  for (auto &IGM : *this)
+    IGM.second->emitCoverageMapping();
+}

lib/IRGen/GenDecl.cpp

@@ -1158,23 +1158,15 @@ void IRGenModule::emitGlobalLists() {
                  /*isConstant*/false, /*asContiguousArray*/true);
 }
 
-static bool hasCodeCoverageInstrumentation(SILFunction &f, SILModule &m) {
-  return f.getProfiler() && m.getOptions().EmitProfileCoverageMapping;
-}
-
-// Eagerly emit functions that are externally visible. Functions with code
-// coverage instrumentation must also be eagerly emitted. So must functions
-// that are a dynamic replacement for another.
+// Eagerly emit functions that are externally visible. Functions that are
+// dynamic replacements must also be eagerly emitted.
 static bool isLazilyEmittedFunction(SILFunction &f, SILModule &m) {
   if (f.isPossiblyUsedExternally())
     return false;
 
   if (f.getDynamicallyReplacedFunction())
     return false;
 
-  if (hasCodeCoverageInstrumentation(f, m))
-    return false;
-
   // Needed by lldb to print global variables which are propagated by the
   // mandatory GlobalOpt.
   if (m.getOptions().OptMode == OptimizationMode::NoOptimization &&
@@ -1258,12 +1250,6 @@ void IRGenerator::emitGlobalTopLevel(
     IGM->emitSILDifferentiabilityWitness(&dw);
   }
 
-  // Emit code coverage mapping data for all modules
-  for (auto Iter : *this) {
-    IRGenModule *IGM = Iter.second;
-    IGM->emitCoverageMapping();
-  }
-
   for (auto Iter : *this) {
     IRGenModule *IGM = Iter.second;
     IGM->finishEmitAfterTopLevel();

lib/IRGen/IRGen.cpp

@@ -1358,6 +1358,11 @@ GeneratedModule IRGenRequest::evaluate(Evaluator &evaluator,
       irgen.emitDynamicReplacements();
     }
 
+    // Emit coverage mapping info. This needs to happen after we've emitted
+    // any lazy definitions, as we need to know whether or not we emitted a
+    // profiler increment for a given coverage map.
+    IGM.emitCoverageMapping();
+
     // Emit symbols for eliminated dead methods.
     IGM.emitVTableStubs();
 
@@ -1608,6 +1613,11 @@ static void performParallelIRGeneration(IRGenDescriptor desc) {
   // Emit reflection metadata for builtin and imported types.
   irgen.emitBuiltinReflectionMetadata();
 
+  // Emit coverage mapping info. This needs to happen after we've emitted
+  // any lazy definitions, as we need to know whether or not we emitted a
+  // profiler increment for a given coverage map.
+  irgen.emitCoverageMapping();
+
   IRGenModule *PrimaryGM = irgen.getPrimaryIGM();
 
   // Emit symbols for eliminated dead methods.

lib/IRGen/IRGenModule.h

@@ -427,6 +427,9 @@ class IRGenerator {
   // Emit info that describes the entry point to the module, if it has one.
   void emitEntryPointInfo();
 
+  /// Emit coverage mapping info.
+  void emitCoverageMapping();
+
   /// Checks if metadata for this type can be emitted lazily. This is true for
   /// non-public types as well as imported types, except for classes and
   /// protocols which are always emitted eagerly.

test/Profiler/coverage_dead_code_elim_onone.swift

@@ -0,0 +1,14 @@
+// RUN: %target-swift-frontend -emit-sil  -profile-generate -profile-coverage-mapping -module-name coverage_deadcode %s | %FileCheck %s -check-prefix SIL
+// RUN: %target-swift-frontend -emit-ir -profile-generate -profile-coverage-mapping -module-name coverage_deadcode %s | %FileCheck %s -check-prefix IR
+
+// This function needs to be present in the SIL for the mandatory passes, but
+// we can drop it in IRGen. We still need to emit its coverage map though.
+func unused() -> Int { 5 }
+
+// SIL: sil hidden @$s17coverage_deadcode6unusedSiyF : $@convention(thin) () -> Int
+// SIL: sil_coverage_map {{.*}} "$s17coverage_deadcode6unusedSiyF"
+
+// IR: @__covrec
+// IR: @__llvm_coverage_mapping
+// IR: @__llvm_prf_nm
+// IR-NOT: define {{.*}} @"$s17coverage_deadcode6unusedSiyF"