[llvm-dev] Is -disable-llvm-passes and -disable-llvm-optzns different?

[Craig Topper via llvm-dev]

For some stupid reason you need to give clang a .c or .cc file to get it to
print the help.

~Craig

On Mon, Jan 8, 2018 at 11:05 PM, toddy wang <wenwangtoddy at gmail.com> wrote:

> Quick results:
>
> [twang15 at c89 temp]$ clang -mllvm -help
> clang-5.0: error: no input files
> [twang15 at c89 temp]$ clang -mllvm -help-hidden
> clang-5.0: error: no input files
> [twang15 at c89 temp]$ clang -mllvm --help-hidden
> clang-5.0: error: no input files
> [twang15 at c89 temp]$ clang -mllvm --help
> clang-5.0: error: no input files
>
> Anything wrong?
>
> On Tue, Jan 9, 2018 at 1:56 AM, Craig Topper <craig.topper at gmail.com>
> wrote:
>
>> There's a hack in lib/Driver/ToolChains/Clang.cpp to accept "-mllvm
>> -disable-llvm-optzns" as a hack for legacy compatibility. It's deprecated
>> and the -Xclang spelling should be used.
>>
>> There does appear to be a bug with "-mllvm -disable-llvm-optzns" and
>> -save-temps. -save-temps causes clang to be invoked separately to text
>> assembly into binary code, and it looks like that call to clang gets -mllvm
>> -disable-llvm-optzns passed to it but it isn't capable of handling it.  You
>> can see this if you add "-v" to your command line so you can see all the
>> subprocesses that get invoked.
>>
>> For the -help question, when clang printed "Try: 'clang (LLVM option
>> parsing) -help'" it was trying and failing to tell you to type "clang
>> -mllvm -help" or "clang -mllvm -help-hidden". This will print the help from
>> the llvm components.
>>
>> ~Craig
>>
>> On Mon, Jan 8, 2018 at 10:40 PM, toddy wang via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>
>>> Hi guys,
>>>
>>> According to this patch review:
>>> *Title: Remove the '-disable-llvm-passes' flag (which I didn't even know
>>> existed, and I suspect many others aren't aware of either) and strength
>>> '-disable-llvm-optzns' to do the same thing*
>>> Link: https://reviews.llvm.org/D28047
>>>
>>> *-disable-llvm-optzns and -disable-llvm-passes are aliasing each other.*
>>>
>>> *1. Both  can be passed to cc1 w/ -Xclang when compilation is run to
>>> finish.*
>>> E.g
>>> [twang15 at c89 temp]$ clang++ -O2 -Xclang -disable-llvm-optzns
>>> -save-temps LULESH.cc
>>> [twang15 at c89 temp]$ cp a.out a.out.1
>>> [twang15 at c89 temp]$ clang++ -O2 -Xclang -disable-llvm-passes
>>> -save-temps LULESH.cc
>>> [twang15 at c89 temp]$ diff a.out a.out.1
>>> [twang15 at c89 temp]$
>>>
>>> *2. Both cannot be passed w/ -mllvm without -emit-llvm -c*
>>>
>>> [twang15 at c89 temp]$ clang++ -O2 -mllvm -disable-llvm-optzns -save-temps
>>>  LULESH.cc
>>> clang (LLVM option parsing): Unknown command line argument
>>> '-disable-llvm-optzns'.  Try: 'clang (LLVM option parsing) -help'
>>> clang (LLVM option parsing): Did you mean '-disable-cgp-gc-opts'?
>>>
>>> [twang15 at c89 temp]$ clang++ -O2 -mllvm -disable-llvm-passes -save-temps
>>>  LULESH.cc
>>> clang (LLVM option parsing): Unknown command line argument
>>> '-disable-llvm-passes'.  Try: 'clang (LLVM option parsing) -help'
>>> clang (LLVM option parsing): Did you mean '-disable-cleanups'?
>>>
>>> *3. However, -disable-llvm-optzns can be passed w/ -mllvm when
>>> -emit-llvm is also passed, but -disable-llvm-passes cannot *
>>>
>>> [twang15 at c89 temp]$ clang++ -O2 -mllvm -disable-llvm-optzns -save-temps
>>> -emit-llvm -c LULESH.cc
>>>
>>> [twang15 at c89 temp]$ clang++ -O2 -mllvm -disable-llvm-passes -save-temps
>>> -emit-llvm -c LULESH.cc
>>> clang (LLVM option parsing): Unknown command line argument
>>> '-disable-llvm-passes'.  Try: 'clang (LLVM option parsing) -help'
>>> clang (LLVM option parsing): Did you mean '-disable-cleanups'?
>>>
>>> *My questions: *
>>> 1. Is there a bug?
>>>
>>> 2. Lastly, try as clang instructs me: Try: 'clang (LLVM option parsing)
>>> -help'
>>>
>>> [twang15 at c89 temp]$ clang (LLVM option parsing) -help
>>> -bash: syntax error near unexpected token `LLVM'
>>>
>>> Can clang give me more helpful information on which flags can be passed
>>> with mllvm and which can be passed with -Xclang?
>>>
>>> 3. It seems to me clang cc1 has at least two major stages to handle *bc
>>> file.
>>> The first stage is to transform c/c++ to bc without any or w/ little
>>> optimizations, and -disable-llvm-passes is enabled by default.
>>>
>>> The second stage is to optimize bc files and output .s assembly. And by
>>> default, -disable-llvm-passes is disabled for this stage. -Xclang
>>> -disable-llvm-passes or -Xclang disable-llvm-optzns will affect this stage.
>>>
>>> Is this a correct understanding? or Partially correct? Where -mllvm
>>> options come into play?
>>>
>>> Thanks a lot!
>>>
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>
>>>
>>
>
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