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

[Craig Topper via llvm-dev]

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!
>
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