[LLVMdev] Enforcing execution order of passes

[John Criswell]

On 2/17/2011 10:42 PM, nullnull wrote:
> I'd like to run my own passes (implemented as .so) via the opt tool on 
> a linked bitcode.
> >opt -load=my_own_pass.so -do_my_passes  ... test.bc > test.output.bc
> However, my passes require other LLVM's passes before and after. For 
> example, the ordering of passes that I want:
> (1) A couple of LLVM's passes ==> (2) my_loop_pass ==> (3) 
> my_module_pass ==> (4) Other LLVM's standard O3 optimizations (a bunch 
> of passes).
> Can I enfore such complex pass ordering easily? I'd like to pass just 
> a single argument in opt, which trigers the whole pipeline of passes.

In general, a pass cannot force a transform pass to always be executed 
before it is run.  A pass can only indicate the need for an analysis 
pass via addRequired() in its getAnalysisUsage() method.  This is due to 
the fact that requiring some transform passes can cause create a set of 
pass dependencies and pass invalidations that make the passes impossible 
to schedule(*).

To do what you want, you must either:

1) Run opt and specify the transforms in the proper order.
2) Write a script that runs opt as described in 1); or
3) Write a special command-line tool that automatically schedules the 
transforms to be run in the correct order

-- John T.

(*) There are a few cases in the LLVM source tree where one transform 
pass successfully requires another transform pass in 
getAnalysisUsage().  However, in a past conversation with PassManager's 
author, I have learned that this is not supported in the general case.


> I tried to use getAnalysisUsage and addRequired. But, the 
> documentation is somewhat vague, I wasn't able even to enforce the 
> pass in (1) to be executed before (2) my loop pass.
> Also, I'm wondering there is a way to run other passes after my pass.
> Actually, I did it by hard coding in clang and LLVM source code. But, 
> I'd like to avoid the changes in clang/LLVM as much as possible so 
> that I don't need to compile LLVM suite.
> Thank you!