llvm-objdump --mcpu=help doesn't work without an input file

[JonathonReinhart]

$ llvm-objdump --version
Fuchsia LLVM version 21.0.0git
...

--help indicates that --mcpu=help should provide details for the --mcpu option:

$ llvm-objdump --help
OVERVIEW: llvm object file dumper

USAGE: llvm-objdump [options] <input object files>

OPTIONS:
  ...
  --mcpu=cpu-name         Target a specific cpu type (--mcpu=help for details) 
  ...

If I do only that, it just outputs the normal help again:

$ llvm-objdump --mcpu=help
OVERVIEW: llvm object file dumper

...

But if I provide it an input file, then it provides the CPU help:

$ llvm-objdump --mcpu=help -d a.out >/dev/null
Available CPUs for this target:

  arm1020e       - Select the arm1020e processor.
  ...

It seems like --mcpu=help should bypass the rest of the argument checking.

[llvmbot]

@llvm/issue-subscribers-tools-llvm-objdump

Author: Jonathon Reinhart (JonathonReinhart)

``` $ llvm-objdump --version Fuchsia LLVM version 21.0.0git ... ```

--help indicates that --mcpu=help should provide details for the --mcpu option:

$ llvm-objdump --help
OVERVIEW: llvm object file dumper

USAGE: llvm-objdump [options] <input object files>

OPTIONS:
  ...
  --mcpu=cpu-name         Target a specific cpu type (--mcpu=help for details) 
  ...

If I do only that, it just outputs the normal help again:

$ llvm-objdump --mcpu=help
OVERVIEW: llvm object file dumper

...

But if I provide it an input file, then it provides the CPU help:

$ llvm-objdump --mcpu=help -d a.out >/dev/null
Available CPUs for this target:

  arm1020e       - Select the arm1020e processor.
  ...

It seems like --mcpu=help should bypass the rest of the argument checking.

[jh7370]

To be more accurate, the thing that is needed is the -d, not the a.out: llvm-objdump defaults to a.out, but it requires one of a set of options to proceed past the "printHelp" bit. See

llvm-project/llvm/tools/llvm-objdump/llvm-objdump.cpp

Line 3811 in 76f2afa

if (!ArchiveHeaders && !Disassemble && DwarfDumpType == DIDT_Null &&

.

The --mcpu=help bit is handled by the MCSubtargetInfo class initialisation. This is initialised by llvm-objdump when disassembly is requested for an object file. I'm not sure it can happen much earlier, because it depends on the architecture of the specified object file, which it obviously can't know until we get to the point where we read the object file. Potentially, we could initialise the class without explicitly needing to request disassembly, but certainly llvm-objdump --mcpu=help alone without an a.out on disk wouldn't be possible.

To summarise, there are two possible things we could do to improve the situation:

1. Improve the docs and potentially help text for the --mcpu option to explain the expected behaviour for "help" more accurately.
2. Potentially move the MCSubtargetInfo initialisation up to dumpInput and explicitly check for --mcpu=help in the previously linked set, so that --disassemble (or similar) isn't required.

@JonathonReinhart, does this make sense to you?