[SYCL] Implement review comments for PR #1454.

Signed-off-by: Konstantin S Bobrovsky <konstantin.s.bobrovsky@intel.com>

Author: kbobrovs

llvm/test/tools/sycl-post-link/sym_but_no_split.ll

@@ -2,9 +2,9 @@
 ; table and a symbol file for an input module with two kernels when no code
 ; splitting is requested.
 ;
-; RUN: sycl-post-link -symbols -spec-const=rt -S %s -o %T/files.table
-; RUN: FileCheck %s -input-file=%T/files.table --check-prefixes CHECK-TABLE
-; RUN: FileCheck %s -input-file=%T/files_0.sym --match-full-lines --check-prefixes CHECK-SYM
+; RUN: sycl-post-link -symbols -spec-const=rt -S %s -o %t.files.table
+; RUN: FileCheck %s -input-file=%t.files.table --check-prefixes CHECK-TABLE
+; RUN: FileCheck %s -input-file=%t.files_0.sym --match-full-lines --check-prefixes CHECK-SYM
 
 define dso_local spir_kernel void @KERNEL_AAA() {
 ; CHECK-SYM-NOT: {{[a-zA-Z0-9._@]+}}

llvm/tools/sycl-post-link/sycl-post-link.cpp

@@ -122,16 +122,19 @@ static void writeToFile(std::string Filename, std::string Content) {
 }
 
 // Describes scope covered by each entry in the module-kernel map populated by
-// the function below.
+// the collectKernelModuleMap function.
 enum KernelMapEntryScope {
   Scope_PerKernel, // one entry per kernel
   Scope_PerModule, // one entry per module
   Scope_Global     // single entry in the map for all kernels
 };
 
-// Output parameter ResKernelModuleMap is a map containing groups of kernels
-// with same values of the sycl-module-id attribute.
-// The function fills ResKernelModuleMap using input module M.
+// This function decides how kernels of the input module M will be distributed
+// ("split") into multiple modules based on the command options and IR
+// attributes. The desision is recorded in the output map parameter
+// ResKernelModuleMap which maps some key to a group of kernels. Each such group
+// along with IR it depends on (globals, functions from its call graph,...) will
+// constitute a separate module.
 static void collectKernelModuleMap(
     Module &M, std::map<StringRef, std::vector<Function *>> &ResKernelModuleMap,
     KernelMapEntryScope EntryScope) {
@@ -145,6 +148,9 @@ static void collectKernelModuleMap(
       case Scope_PerModule: {
         constexpr char ATTR_SYCL_MODULE_ID[] = "sycl-module-id";
 
+        // TODO It may make sense to group all kernels w/o the attribute into
+        // a separate module rather than issuing an error. Should probably be
+        // controlled by an option.
         if (!F.hasFnAttribute(ATTR_SYCL_MODULE_ID))
           error("no '" + Twine(ATTR_SYCL_MODULE_ID) +
                 "' attribute in kernel '" + F.getName() +