[llvm-dev] Significant code difference with a split call to opt

[Hiroshi Yamauchi via llvm-dev]

Passing -print-after-all to opt should print the IR after each pass. That
may help figure out what's going on.

On Mon, Jun 17, 2019 at 1:30 PM Sébastien Michelland via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi,
>
> I reproduced the test on many individual files and got very variable
> results... it seems the computer's workload when running the test suite
> influenced the execution speed a lot more than standard deviation shows.
> I'll withdraw the performance claim until I can get consistent results
> (changed subject line), apologies for the confusion.
>
> What I can still show easily is that the code generated by these two
> methods is different (which is already weird). For a simple example,
> grab a copy of bilateral_grid.bc:
>
>
> <
> https://github.com/llvm/llvm-test-suite/blob/master/Bitcode/Benchmarks/Halide/bilateral_grid/bilateral_grid.bc
> >
>
> Then you can generate my sequences with [opt -O3 -debug-pass=Arguments]
> and diff the outputs. Please see the attached script.
>
> The differences seem to be mainly on variable indices (are they
> randomized?); on some test (namely jacobi-2d-imper) I have seen calling
> convention differences.
>
> I'd like to optimize programs by greedily selecting optimizations,
> making a call to opt at each step. If I don't have equality between the
> two methods, I can't be sure that the sequence I'm building will make
> much sense.
>
> Sébastien Michelland
>
> On 6/14/19 4:49 PM, David Greene wrote:
> > Do you have more information?  What were the exact command lines you
> > used?  Do you have an example program that demonstrates the difference
> > than you can share?
> >
> >                        -David
> >
> > Sébastien Michelland via llvm-dev <llvm-dev at lists.llvm.org> writes:
> >
> >> Hello list,
> >>
> >> This is a follow-up from a question I asked last month. I'm evaluating
> >> the performance of two pass sequences that resemble (but are not) -O3.
> >>
> >> With -O3, -debug-pass=Structure prints several independent blocks that
> >> seem to represent several calls to opt. I focused on two of these
> >> blocks, say S1 and S2, and compared the following optimization
> >> methods:
> >>
> >> 1. Executing them separately, ie. opt -S1 | opt -S2
> >> 2. Executing them in a single call, ie. opt -S1 -S2
> >>
> >> I built the test suite with each of these configurations, then
> >> measured the performance of the compiled programs with perf, over 10
> >> runs.
> >>
> >> I'm attaching a plot of the speedup of method 1 over method 2. The
> >> intervals represent the standard deviation of the performance
> >> measures.
> >>
> >> As you can see, programs compiled with method 1 are significantly
> >> slower than their counterparts compiled with method 2. However, if
> >> passes were applied in order using function composition, their
> >> performance should be the same.
> >>
> >> I'd like to know if there is a way to recover this property in the
> >> pass manager, or at least explain the difference. If needed, I can
> >> provide scripts to reproduce the measurements.
> >>
> >> Thanks,
> >> Sébastien Michelland
> >>
> >>
> >> _______________________________________________
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> >> llvm-dev at lists.llvm.org
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