llvm dev - Dec 2015 - Is there a way to pass Optimization passes to clang?

On Wed, Dec 2, 2015 at 10:17 AM, David Chisnall <David.Chisnall at
cl.cam.ac.uk
> wrote:
> On 2 Dec 2015, at 17:56, Phil Tomson via llvm-dev <llvm-dev at
lists.llvm.org>
> wrote:
> >
> > Now my intent is to figure out which optimization pass is causing the
> problem by trying each one until I hit the same problem as with -O1, but
> clang itself doesn't seem to allow these commandline options, for
example:
> >
> > clang -loops -lcssa ....
> >
> >
>
> If you want to pass LLVM arguments to clang, then you must prefix them
> with -llvm (e.g. -mllvm -loops -mllvm lcssa).
>
>
I just tried this:
CFLAGS += -mllvm adce -mllvm loops -mllvm loop-simplify -mllvm lcssa -mllvm
simplify-libcalls

And get:

clang (LLVM option parsing): Unknown command line argument 'adce'.  Try:
'clang (LLVM option parsing) -help'
clang (LLVM option parsing): Unknown command line argument 'loops'. 
Try:
'clang (LLVM option parsing) -help'
clang (LLVM option parsing): Unknown command line argument
'loop-simplify'.  Try: 'clang (LLVM option parsing) -help'
clang (LLVM option parsing): Unknown command line argument 'lcssa'. 
Try:
'clang (LLVM option parsing) -help'
clang (LLVM option parsing): Unknown command line argument
'simplify-libcalls'.  Try: 'clang (LLVM option parsing) -help'


> David
>
>
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On 2 Dec 2015, at 18:30, Phil Tomson <phil.a.tomson at gmail.com>
wrote:
> 
>> If you want to pass LLVM arguments to clang, then you must prefix them
with -llvm (e.g. -mllvm -loops -mllvm lcssa).
> 
> 
> I just tried this:
> CFLAGS += -mllvm adce -mllvm loops -mllvm loop-simplify -mllvm lcssa -mllvm
simplify-libcalls
> 
> And get:
> 
> clang (LLVM option parsing): Unknown command line argument 'adce'. 
Try: 'clang (LLVM option parsing) -help'
> clang (LLVM option parsing): Unknown command line argument 'loops'.
Try: 'clang (LLVM option parsing) -help'
> clang (LLVM option parsing): Unknown command line argument
'loop-simplify'.  Try: 'clang (LLVM option parsing) -help'
> clang (LLVM option parsing): Unknown command line argument 'lcssa'.
Try: 'clang (LLVM option parsing) -help'
> clang (LLVM option parsing): Unknown command line argument
'simplify-libcalls'.  Try: 'clang (LLVM option parsing) -help'
Sorry, I wasn’t thinking.  These are options provided by opt, not by LLVM
libraries.  Clang has made an explicit design decision not to expose the
optimisation pipeline details to consumers (because it’s far from stable and
there is no desire to maintain backwards compatibility for things that are
implementation details and because it’s very hard for someone to use correctly).
Your best bet if you *really* want this is probably to wrap clang in a shell
script that does something along the lines of clang -c -emit-llvm && opt
-whatever | llc, with appropriate handling / replacing of the last -o argument
to clang.

David

On Wed, Dec 2, 2015 at 10:39 AM, David Chisnall <David.Chisnall at
cl.cam.ac.uk
> wrote:
> On 2 Dec 2015, at 18:30, Phil Tomson <phil.a.tomson at gmail.com>
wrote:
> >
> >> If you want to pass LLVM arguments to clang, then you must prefix
them
> with -llvm (e.g. -mllvm -loops -mllvm lcssa).
> >
> >
> > I just tried this:
> > CFLAGS += -mllvm adce -mllvm loops -mllvm loop-simplify -mllvm lcssa
> -mllvm simplify-libcalls
> >
> > And get:
> >
> > clang (LLVM option parsing): Unknown command line argument
'adce'.  Try:
> 'clang (LLVM option parsing) -help'
> > clang (LLVM option parsing): Unknown command line argument
'loops'.
> Try: 'clang (LLVM option parsing) -help'
> > clang (LLVM option parsing): Unknown command line argument
> 'loop-simplify'.  Try: 'clang (LLVM option parsing) -help'
> > clang (LLVM option parsing): Unknown command line argument
'lcssa'.
> Try: 'clang (LLVM option parsing) -help'
> > clang (LLVM option parsing): Unknown command line argument
> 'simplify-libcalls'.  Try: 'clang (LLVM option parsing)
-help'
>
> Sorry, I wasn’t thinking.  These are options provided by opt, not by LLVM
> libraries.  Clang has made an explicit design decision not to expose the
> optimisation pipeline details to consumers (because it’s far from stable
> and there is no desire to maintain backwards compatibility for things that
> are implementation details and because it’s very hard for someone to use
> correctly).  Your best bet if you *really* want this is probably to wrap
> clang in a shell script that does something along the lines of clang -c
> -emit-llvm && opt -whatever | llc, with appropriate handling /
replacing of
> the last -o argument to clang.
>
I have a feeling this would get rather tricky given that there are several
libraries linked in to the final executable as well. It would have to look
something like:

clang -c file.c -emit-llvm > opt -... > llc ... > as > ld (libs to
link in)

Someone above mentioned modifying PassManagerBuilder and rebuilding clang -
I'd guess there's a list of optimization passes performed somewhere that
PassManagerBuilder references, but I haven't been in the optimization pass
area of LLVM much yet. Any pointers on that approach?

Phil


>
> David
>
>
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