[llvm-dev] Where to find the list of passes run by clang/opt with -O3 (original) (raw)

Martin J. O'Riordan via llvm-dev llvm-dev at lists.llvm.org
Tue Sep 5 13:24:21 PDT 2017

I had asked a similar question earlier this year regarding which passes are involved at a particular optimisation level (lists.llvm.org/pipermail/llvm-dev/2017-June/114543.html), and the most helpful suggestion I received was to invoke the compiler with the following options:

-O3 -mllvm -debug-pass=Structure

So I ran the compiler with each of '-O0' .. '-O3' to get the answer I was looking for.

I don't know how to generally disable a particular pass from the command line with LLVM v4.0, but the pass you want to disable identifies itself as:

Loop Strength Reduction

and can be disabled with the option (see 'lib/CodeGen/TargetPassConfig.cpp'):

-mllvm -disable-lsr

I haven't yet started working with LLVM v5.0 and the new pass manager though so this may already be an obsolete suggestion.

All the best,

MartinO

-----Original Message----- From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of Davide Italiano via llvm-dev Sent: 05 September 2017 20:54 To: Nitish Srivastava <nks45 at cornell.edu> Cc: llvm-dev <llvm-dev at lists.llvm.org> Subject: Re: [llvm-dev] Where to find the list of passes run by clang/opt with -O3

On Tue, Sep 5, 2017 at 12:51 PM, Davide Italiano <davide at freebsd.org> wrote:

On Tue, Sep 5, 2017 at 8:16 AM, Nitish Srivastava via llvm-dev <llvm-dev at lists.llvm.org> wrote: Hi, I am trying to locate the passes run by clang/opt when it is passed the option -O3. Can someone point me where to look at? Eventually, I want to turn off just the LoopStrengthReduction pass in the -O3 set of default passes.

Also, the easiest hack to turn off LSR is that of probably just commenting out the line that adds the pass in PassManagerBuilder.cpp. If you use opt + the new pass manager there's a flag for testing to specify a custom pipeline, but that will be more work.

-- Davide

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