[LLVMdev] How to visualise Clang optimisation phases

Pedro Ferreira Pedro.Ferreira at imgtec.com
Mon Jul 19 01:23:13 PDT 2010

Having a look at clang's source, you can find in
"lib/CodeGen/BackendUtil.cpp" the functions where clang builds the
passes to emit code. The optimization passes used are there and you can
simulate them via the "opt" utility, by running each pass one at a time.
LLVM also declares standard module/function  passes on
include/llvm/Support/StandardPasses.h. Have a look and see if that's
what you want.

> -----Original Message-----
> From: llvmdev-bounces at cs.uiuc.edu 
> [mailto:llvmdev-bounces at cs.uiuc.edu] On Behalf Of Edmund Grimley-Evans
> Sent: 16 July 2010 11:25
> To: llvmdev at cs.uiuc.edu
> Subject: [LLVMdev] How to visualise Clang optimisation phases
> When I'm trying to understand how or why the back end does 
> something I've found the -view-* options to llc to be useful. 
> It's not ideal, because "clang" doesn't take those options, 
> so I have to invoke first "clang" and then "llc", and 
> sometimes that combination gives different final code from 
> invoking just "clang", but usually I can construct a simple 
> example in which "clang" and "clang | llc" give the same 
> output and then use -view-legalize-dags, etc with "llc".
> However, when I'm trying to understand an optimisation that 
> happens (or doesn't happen) earlier on in the compiler I've 
> found this approach sometimes doesn't work at all. If I 
> invoke "clang" with no optimisation options then the 
> optimisation I'm interested in has already happened before 
> the bitcode is generated, but if I invoke "clang" with -O0 
> then "llc" doesn't do it either and the optimisation doesn't 
> happen at all.
> Is there a way of tracing what "clang" actually does and 
> extracting the intermediate representation between the phases?
> Or is this a question for cfe-dev?
> (I'm currently trying to understand how some conditional 
> expressions are optimised.)
> Thanks for any advice.
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