update comments and warning info, bitmaps section in llvm profdata shouldn't be used in PGO

Author: TianlongLiang

core/iwasm/aot/aot_runtime.c

@@ -4887,7 +4887,7 @@ aot_dump_pgo_prof_data_to_buf(AOTModuleInstance *module_inst, char *buf,
     prof_header.num_prof_counters = num_prof_counters;
     prof_header.names_size = prof_names_size;
     prof_header.value_kind_last = 1;
-    /* TODO: add __llvm_prf_bits support in the future */
+    /* __llvm_prf_bits won't be used in PGO, set dummy value here */
     prof_header.num_prof_bitmaps = 0;
     prof_header.bitmap_delta = 0;
 
@@ -4921,7 +4921,7 @@ aot_dump_pgo_prof_data_to_buf(AOTModuleInstance *module_inst, char *buf,
             prof_data_64->func_ptr = prof_data->func_ptr;
             prof_data_64->values = (uint64)(uintptr_t)prof_data->values;
             prof_data_64->num_counters = prof_data->num_counters;
-            /* TODO: add __llvm_prf_bits support in the future */
+            /* __llvm_prf_bits won't be used in PGO, set dummy value here */
             prof_data_64->num_bitmaps = 0;
             prof_data_64->num_value_sites[0] = prof_data->num_value_sites[0];
             prof_data_64->num_value_sites[1] = prof_data->num_value_sites[1];

core/iwasm/aot/aot_runtime.h

@@ -437,9 +437,9 @@ typedef struct AOTFrame {
 } AOTFrame;
 
 #if WASM_ENABLE_STATIC_PGO != 0
-/* TODO: bitmaps fields in LLVMProfileRawHeader, LLVMProfileData,
- * LLVMProfileData_64 all dummy fields, remove this comment when __llvm_prf_bits
- * are actual supported */
+/* The bitmaps fields in LLVMProfileRawHeader, LLVMProfileData,
+ * LLVMProfileData_64 all dummy fields, it's used in MC/DC code coverage
+ * instead of PGO. See https://llvm.org/docs/InstrProfileFormat.html#bitmap */
 typedef struct LLVMProfileRawHeader {
     uint64 magic;
     uint64 version;

core/iwasm/compilation/aot_emit_aot_file.c

@@ -3380,7 +3380,8 @@ aot_resolve_object_data_sections(AOTObjectData *obj_data)
                 }
                 else if (obj_data->comp_ctx->enable_llvm_pgo
                          && !strcmp(name, "__llvm_prf_bits")) {
-                    LOG_WARNING("__llvm_prf_bits section is not supported yet");
+                    LOG_WARNING("__llvm_prf_bits section is not supported and "
+                                "shouldn't be used in PGO.");
                     return false;
                 }