[llvm] [llvm-objdump] Support --mcpu=help/--mattr=help without -d (PR #165661)

[Ruoyu Qiu via llvm-commits]

================
@@ -3826,20 +3853,33 @@ int llvm_objdump_main(int argc, char **argv, const llvm::ToolContext &) {
       !DisassembleSymbols.empty())
     Disassemble = true;
 
-  if (!ArchiveHeaders && !Disassemble && DwarfDumpType == DIDT_Null &&
-      !DynamicRelocations && !FileHeaders && !PrivateHeaders && !RawClangAST &&
-      !Relocations && !SectionHeaders && !SectionContents && !SymbolTable &&
-      !DynamicSymbolTable && !UnwindInfo && !FaultMapSection && !Offloading &&
-      !(MachOOpt &&
-        (Bind || DataInCode || ChainedFixups || DyldInfo || DylibId ||
-         DylibsUsed || ExportsTrie || FirstPrivateHeader ||
-         FunctionStartsType != FunctionStartsMode::None || IndirectSymbols ||
-         InfoPlist || LazyBind || LinkOptHints || ObjcMetaData || Rebase ||
-         Rpaths || UniversalHeaders || WeakBind || !FilterSections.empty()))) {
+  const bool PrintCpuHelp = (MCPU == "help" || is_contained(MAttrs, "help"));
+
+  const bool ShouldDump =
+      ArchiveHeaders || Disassemble || DwarfDumpType != DIDT_Null ||
+      DynamicRelocations || FileHeaders || PrivateHeaders || RawClangAST ||
+      Relocations || SectionHeaders || SectionContents || SymbolTable ||
+      DynamicSymbolTable || UnwindInfo || FaultMapSection || Offloading ||
+      (MachOOpt &&
+       (Bind || DataInCode || ChainedFixups || DyldInfo || DylibId ||
+        DylibsUsed || ExportsTrie || FirstPrivateHeader ||
+        FunctionStartsType != FunctionStartsMode::None || IndirectSymbols ||
+        InfoPlist || LazyBind || LinkOptHints || ObjcMetaData || Rebase ||
+        Rpaths || UniversalHeaders || WeakBind || !FilterSections.empty()));
+
+  if (!ShouldDump && !PrintCpuHelp) {
     T->printHelp(ToolName);
     return 2;
   }
 
+  // If the target triple is derived from the files, we display help message
+  // when disassembling them.
+  if (!Disassemble && PrintCpuHelp) {
----------------
cabbaken wrote:

Yes, that is correct. The output from `PrintCpuHelp` is identical to `!Disassemble && PrintCpuHelp`.
Furthermore, `llvm-objdump --triple=x86_64 --mcpu=help arm.o` will output the X86 help message using `PrintCpuHelp`.

https://github.com/llvm/llvm-project/pull/165661