[Profiler] Don't run PGO on unmapped decls

This avoids an unnecessary walk, as we don't have
any interesting profile data for such decls.

Author: hamishknight

lib/SIL/IR/SILProfiler.cpp

@@ -97,7 +97,12 @@ static bool isUnmapped(ASTNode N) {
 
 namespace swift {
 bool doesASTRequireProfiling(SILModule &M, ASTNode N) {
-  return M.getOptions().GenerateProfile && !isUnmapped(N);
+  // If profiling isn't enabled, don't profile anything.
+  auto &Opts = M.getOptions();
+  if (Opts.UseProfile.empty() && !Opts.GenerateProfile)
+    return false;
+
+  return !isUnmapped(N);
 }
 } // namespace swift
 
@@ -152,7 +157,7 @@ static bool canCreateProfilerForAST(ASTNode N, SILDeclRef forDecl) {
 
 SILProfiler *SILProfiler::create(SILModule &M, ASTNode N, SILDeclRef Ref) {
   const auto &Opts = M.getOptions();
-  if (!doesASTRequireProfiling(M, N) && Opts.UseProfile.empty())
+  if (!doesASTRequireProfiling(M, N))
     return nullptr;
 
   if (!canCreateProfilerForAST(N, Ref)) {

test/Profiler/pgo_with_coverage.swift

@@ -0,0 +1,24 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-build-swift %s -profile-generate -Xfrontend -disable-incremental-llvm-codegen -module-name pgo_with_coverage -o %t/main
+
+// This unusual use of 'sh' allows the path of the profraw file to be
+// substituted by %target-run.
+// RUN: %target-codesign %t/main
+// RUN: %target-run sh -c 'env LLVM_PROFILE_FILE=$1 $2' -- %t/default.profraw %t/main
+
+// RUN: %llvm-profdata merge %t/default.profraw -o %t/default.profdata
+
+// RUN: %target-swift-frontend %s -Xllvm -sil-full-demangle -profile-generate -profile-coverage-mapping -profile-use=%t/default.profdata -emit-ir -module-name pgo_with_coverage | %FileCheck %s --check-prefix=IR
+// RUN: %target-swift-frontend %s -Xllvm -sil-full-demangle -profile-generate -profile-coverage-mapping -profile-use=%t/default.profdata -O -emit-ir -module-name pgo_with_coverage | %FileCheck %s --check-prefix=IR
+
+// REQUIRES: profile_runtime
+// REQUIRES: executable_test
+// REQUIRES: OS=macosx
+
+// Make sure we don't try and profile implicit decls during PGO.
+// IR-NOT: no profile data available
+
+struct S {
+  var x = .random() ? 2 : 3
+}
+print(S().x)