[llvm-dev] Is there a way to pass Optimization passes to clang?

[Phil Tomson via llvm-dev]

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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