[llvm-dev] [RFC] [tools] Changing Behavior of LLVM binutils When No File Is Specified

Wed Jun 26 02:31:27 PDT 2019

I agree with others that the a.out behaviour is weird (I've even thought
this about the linker output being called a.out since I started
programming, but perhaps that's a different story). The use-case I can
imagine is something like:

$ ld.lld test.o
$ llvm-objdump -d

I.e. using a tool immediately after generating the linker output. However,
I don't think this is likely actually done by anybody, and I think it's
probably safe to change the behaviour here. I'd rather be universal across
the tools (e.g. always read from stdin) than do this frankly. An error
message about a missing input file might be nice in some cases
(llvm-readelf, llvm-objdump, llvm-nm), but I think we definitely want to
support the option of stdin redirection if nothing else but for test
purposes. If it's complicated to achieve both the error under
non-redirection cases, and no error in redirection cases then I'd prefer
the latter.

Regards,

James

On Wed, 26 Jun 2019 at 08:28, Alex Brachet-Mialot via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Sounds good, I'll work on removing these in favor of defaulting to stdin
> then?
>
> > I think it would be much friendlier to just print "file or pipe
> expected" and then print the help.
> Do you have thoughts on this? I'm not sure there is a clean way to do
> this, the cleanest would be through getFileOrSTDIN(), but I'm not sure all
> of its users want this behavior. I don't think we need it personally, but
> if you think its a good quality of life change to the tools and worth
> working on, I'd be happy to do so.
>
> On Wed, Jun 26, 2019 at 1:41 AM Fāng-ruì Sòng <maskray at google.com> wrote:
>
>> GNU addr2line, nm, objdump, and size default to a.out when no input file
>> is specified.
>> Among llvm binary utilities, llvm-nm llvm-objdump llvm-size
>> llvm-dwarfdump default to a.out.
>>
>> I agree with Michael that a.out behavior may not be used by many people.
>> If people don't care too much
>> about these utilities' compatibility with GNU, deleting the default a.out
>> LGTM.
>>
>> (I am opposed to make more utilities default to a.out)
>>
>> On Wed, Jun 26, 2019 at 9:02 AM Alex Brachet-Mialot via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>
>>> > Does anyone actually use the default to a.out behavior?
>>> This is a good point. What bugs me is continuity across the tools, I
>>> agree with you that it isn't particularly useful to default to a.out. But
>>> we are kind of stuck with the weird way that GNU's binutils do things. I am
>>> not in favor of llvm-objdump defaulting to a.out, but llvm-readelf giving
>>> this warning message. My guess is that moving away from using a.out by
>>> default for llvm-objdump, llvm-nm and others whose gnu counter parts do
>>> this would be more disruptive than my proposal.
>>>
>>> As Jake pointed out we use "-" to describe stdin/stdout, GNU's tools do
>>> not do this, so I think there is some precedence for us slightly modifying
>>> behavior when we can reasonably say things will not happen, like the file
>>> "-" existing.
>>>
>>> On Tue, Jun 25, 2019 at 8:28 PM Michael Spencer <bigcheesegs at gmail.com>
>>> wrote:
>>>
>>>> On Tue, Jun 25, 2019 at 4:58 PM Alex Brachet-Mialot via llvm-dev <
>>>> llvm-dev at lists.llvm.org> wrote:
>>>>
>>>>> Some binutils, nm comes to mind, when to input file is specified will
>>>>> default to a.out. Others do not do this and read from stdin by default. The
>>>>> rest of this email will be specifically referencing these which read from
>>>>> stdin, and not the tools which for various reasons do things differently.
>>>>>
>>>>> I propose that we change the behavior of these tools to use a.out when
>>>>> appropriate. By appropriate I mean firstly, no input file specified and
>>>>> also no stream redirection to stdin. These are the file types of stdin in
>>>>> these scenarios:
>>>>> $ writes-to-stdout | prog # named pipe (fifo)
>>>>> $ prog < file # regular file
>>>>> $ prog # reading from tty, character device
>>>>>
>>>>> Perhaps if stdin is a named pipe or regular file, then the default
>>>>> behavior should be as it always was, to read from stdin, but if stdin is a
>>>>> tty then we should use a.out as a default file. This lets these tools act
>>>>> the same as their gnu counterparts (when meaningful) but also adds what I
>>>>> think is convenient to not have to specify a.out.
>>>>>
>>>>> This proposal would look something like this:
>>>>> $ llvm-strings # not meanigful to read from stdin here, look for a.out
>>>>> $ llvm-strings < file # use stdin
>>>>>
>>>>> I may have got the behavior of stream redirection wrong here, or
>>>>> missed a situation when reading from the terminal is useful. I would love
>>>>> some feedback.
>>>>>
>>>>> Best,
>>>>> Alex
>>>>>
>>>>>
>>>> Does anyone actually use the default to a.out behavior?  I think it
>>>> would be much friendlier to just print "file or pipe expected" and then
>>>> print the help.
>>>>
>>>> - Michael Spencer
>>>>
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>
>>
>>
>> --
>> 宋方睿
>>
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