[llvm-dev] RFC: Add bitcode tests to test-suite

[Hal Finkel via llvm-dev]

----- Original Message -----

> From: "Mehdi Amini via llvm-dev" <llvm-dev at lists.llvm.org>
> To: "Alina Sbirlea" <alina.sbirlea at gmail.com>
> Cc: "llvm-dev" <llvm-dev at lists.llvm.org>
> Sent: Monday, February 29, 2016 7:06:51 PM
> Subject: Re: [llvm-dev] RFC: Add bitcode tests to test-suite

> Sent from my iPhone

> On Feb 29, 2016, at 3:39 PM, Alina Sbirlea < alina.sbirlea at gmail.com
> > wrote:

> > On Mon, Feb 29, 2016 at 2:06 PM, Mehdi Amini <
> > mehdi.amini at apple.com
> > > wrote:
> 

> > > > On Feb 29, 2016, at 1:50 PM, Alina Sbirlea <
> > > > alina.sbirlea at gmail.com
> > > > > wrote:
> > > 
> > 
> 

> > > > On Mon, Feb 29, 2016 at 12:18 PM, Mehdi Amini <
> > > > mehdi.amini at apple.com
> > > > > wrote:
> > > 
> > 
> 

> > > > > > On Feb 29, 2016, at 11:40 AM, Mehdi Amini via llvm-dev <
> > > > > > llvm-dev at lists.llvm.org > wrote:
> > > > > 
> > > > 
> > > 
> > 
> 

> > > > > > > On Feb 29, 2016, at 11:16 AM, Alina Sbirlea via llvm-dev
> > > > > > > <
> > > > > > > llvm-dev at lists.llvm.org > wrote:
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 

> > > > > > > All,
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 

> > > > > > > To get the discussion going in a focused manner, here is
> > > > > > > an
> > > > > > > initial
> > > > > > > patch with a running test. The test is from the Halide
> > > > > > > suite
> > > > > > > and
> > > > > > > is
> > > > > > > checking the correctness of several simd operations.
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > (Notes: the patch is large due to the number of
> > > > > > > operations
> > > > > > > being
> > > > > > > tested;
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > I expect a lot of changes before actually landing it,
> > > > > > > this
> > > > > > > is
> > > > > > > simply
> > > > > > > to continue the discussion using a concrete example.)
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 

> > > > > > > http://reviews.llvm.org/D17726
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 

> > > > > > I can't figure how to download the patch *with the bitcode
> > > > > > files*
> > > > > > from Phabricator. Can you push this on github (or somewhere
> > > > > > else)?
> > > > > > (or if I missed how to proceed...).
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > I was able to figure how get them "one by one", it would
> > > > > still
> > > > > be
> > > > > more convenient to have an archive or a repo to clone
> > > > > somewhere.
> > > > 
> > > 
> > 
> 

> > > > > > > A few questions/todos to start the discussion:
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > 1. What is a good location for these tests? They are in a
> > > > > > > separate
> > > > > > > Bitcode directory atm, but using the llvm_multisource.
> > > > > > > This
> > > > > > > may
> > > > > > > change to more closely model the approach for external
> > > > > > > tests
> > > > > > > (see
> > > > > > > next item).
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > A good location would be their own external repository IMO :)
> > > > 
> > > 
> > 
> 

> > > > > > > 2. There is a single .cpp file testing all operations
> > > > > > > provided
> > > > > > > by
> > > > > > > individual bitcode files. I expect this to change.
> > > > > > > Instead
> > > > > > > of
> > > > > > > using
> > > > > > > llvm_multisource to have the same test run with specific
> > > > > > > arguments,
> > > > > > > each run testing a single operation.
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > 3. The building approach I took is to first link all
> > > > > > > bitcode
> > > > > > > files
> > > > > > > into a single one, then obtain the assembly for it, which
> > > > > > > cmake
> > > > > > > knows to take as an input source.
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > Yeah, so I'd rather have a split-build model, with a split
> > > > > execution
> > > > > model. Having a gigantic bitcode file to debug an issue is
> > > > > not
> > > > > friendly.
> > > > 
> > > 
> > 
> 
> > > > > I'd expect to have a .cpp file that contains the main and the
> > > > > logic
> > > > > to run test, and then every test that is linked-in to be
> > > > > executed,
> > > > > a
> > > > > bit like gtests is doing (there are multiple registering
> > > > > mechanisms
> > > > > that would avoid to declare explicitly a test in the header).
> > > > 
> > > 
> > 
> 
> > > > > -> filters.h and filter_headers.h should just go away.
> > > > 
> > > 
> > 
> 
> > > > I agree, this is related to point 2. The plan here is to update
> > > > the
> > > > current test .cpp file to test each operation individually. In
> > > > this
> > > > model it will be enough to link with a single bitcode file per
> > > > test.
> > > 
> > 
> 

> > > > > Also on the test in general: we should have an idea for each
> > > > > test
> > > > > what it is doing and how.
> > > > 
> > > 
> > 
> 
> > > > > I was expecting your tests to be on the pattern of having an
> > > > > implementation in C++ and an implementation in Halide bitcode
> > > > > of
> > > > > a
> > > > > filters (or whatever) and run both on random data and
> > > > > verifies
> > > > > that
> > > > > the result is matching.
> > > > 
> > > 
> > 
> 
> > > > > Unfortunately from what I can see you are feeding the tests
> > > > > with
> > > > > random data, and the tests are "blackboxes" that set an error
> > > > > flag
> > > > > if they detect an error.
> > > > 
> > > 
> > 
> 
> > > > > This is not super robust: the compiler can mess with the
> > > > > error
> > > > > checking and eliminate it for instance, making any error
> > > > > undetected.
> > > > 
> > > 
> > 
> 
> > > > The Halide bitcode filters compare the result of vectorized
> > > > operations vs scalar runs of the same code. The error code
> > > > against
> > > > which we compare the output will be set to loose tolerance - it
> > > > is
> > > > currently 0. We're interested in codegen bugs that return the
> > > > wrong
> > > > value entirely, not accuracy differences (especially for
> > > > floating
> > > > point tests).
> > > 
> > 
> 
> > > > With the new error threshold, the data fed into may be random
> > > > or
> > > > read
> > > > from provided input files, I can do either.
> > > 
> > 
> 
> > > > The filters will still look somewhat like blackboxes, though
> > > > the
> > > > name
> > > > of the filter says what operation it's being tested and the
> > > > disassembled bitcode files are reasonably readable.
> > > 
> > 
> 
> > > > Using your suggestion, the driver .cpp file will test one
> > > > operation
> > > > at a time (argvs set accordingly) and return right away once an
> > > > error is found. Sound about right?
> > > 
> > 
> 
> > > All of this is great.
> > 
> 
> > > The part that is not clear to me is why isn't it to have (what
> > > does
> > > it buy us over, or why is it better for us compared to) a
> > > possible
> > > a
> > > C/C++ reference implementation of the filter, and hoisting (and
> > > refactor) all the logic to feed the tests and validate the output
> > > *out* of the filters. A filter would be just the mathematical
> > > function performed and nothing more (separation of concerns, more
> > > robust framework, easier debugging when things-go-wrong, etc.).
> > 
> 
> > I believe the answer is that Halide generates vectorized code in a
> > way that is not generated by llvm when starting from C/C++.
> 
> I don't really see how *this* addresses my point. This is justifying
> why your bitcode is interesting and why we are having this
> conversation at all :)
> It does not say why we can't have a scalar naive C/C++ impl along
> with the bitcode for filter.

> > Having a C/C++ scalar reference would involve quite a bit of effort
> > for all tests. The primary reason Halide is being used is that you
> > don't need to write a lot of C/C++ code to get different
> > optimizations for the same code (e.g. vectorized vs scalar is a one
> > line difference).
> 
> Yes, this is what is nice with Halide: "write once, codegen multiple
> variant". But it does not mean you can't write a c++ reference for
> every Halide filter (not for every codegen variant of a filter!)

> It's been 2 years since my last experiments with Halide, but my
> memories were that there was a C backend?
> I had in mind for each test to have (possibly in a separate directory
> for each test):
> - the halide source for the filter.
> - the c/c++ (maybe generated?) for the filter.
> - the bitcode generated for the filter (potentially multiple variant
> depending on the CPU support and/or the schedule).

> Then some common code/infrastructure to interact with the individual
> filters, loading them, executing the variants for a filter, and
> checking results.

> If the reference c/c++ can't be generated by halide (or obtained
> somehow), and we can't do better than the current tests
> infrastructure you have, then I'm worried about the cost/benefit for
> this test-suite.

I think that a C/C++ version would be nice to have, but not necessary. IR generated by non-Clang frontends and/or IR going through alternate optimization pipelines tend to hit bugs that are much harder to hit with Clang alone. It would help to have a description of what each test does, but including, for example, the Halide source code for each test will hopefully be enough of a guide. 

> > So we can get really fast test coverage for possible codegen bugs
> > by
> > comparing that different layout optimizations in Halide give the
> > same result.
> 
> > I think having each filter tested separately should give a good
> > separation of concerns and easy debugging for each particular test.
> 
> This is great for halide validation, we are all agreeing with this I
> think. The question is where is the tradeoff for the LLVM project.
> I'm trying to make sure that the extra coverage doesn't come with a
> burden to debug and triage issues when something will break: i.e.
> the tests need to be very friendly to interact with.
I don't think that any non-trivial tests are truly "friendly" to interact with. Tracking down self-hosting bugs is not friendly, and those aren't anywhere near the worst ;) -- These tests with their simple driver seem like good input that bugpoint can reduce (assuming the tests runtimes are not too long), and that's friendlier than most of the other multisource tests. 

-Hal 

> This is the motivation for my comments so far.
> Other people in the community may have a different
> opinion/appreciation of the situation, this just represents my
> current thoughts.

> Hope it makes sense.

> --
> Mehdi

> > > > > Also, just looking quickly at one IR I'm surprised by things
> > > > > like:
> > > > 
> > > 
> > 
> 

> > > > > "assert succeeded165": ; preds = %"assert succeeded146"
> > > > 
> > > 
> > 
> 
> > > > > %buf_host181 = getelementptr inbounds %struct.buffer_t,
> > > > > %struct.buffer_t* %error_op_pcmpeqq_272.buffer, i64 0, i32 1
> > > > 
> > > 
> > 
> 
> > > > > %23 = bitcast i8** %buf_host181 to double**
> > > > 
> > > 
> > 
> 
> > > > > %error_op_pcmpeqq_272.host226227232 = load double*, double**
> > > > > %23,
> > > > > align 8
> > > > 
> > > 
> > 
> 
> > > > > %24 = icmp eq %struct.buffer_t* %error_op_pcmpeqq_272.buffer,
> > > > > null
> > > > 
> > > 
> > 
> 
> > > > > br i1 %24, label %"assert failed183", label %"assert
> > > > > succeeded184",
> > > > > !prof !4
> > > > 
> > > 
> > 
> 

> > > > > Here you have as check for nullptr at %24, but you already
> > > > > loaded
> > > > > %error_op_pcmpeqq_272.host226227232 from this pointer just
> > > > > before!
> > > > 
> > > 
> > 
> 
> > > > It's checking that the host value loaded from buffer_t is not
> > > > null.
> > > > I
> > > > don't see what's wrong with this. What am I missing?
> > > 
> > 
> 
> > > I may be misreading it, my impression when skimming through the
> > > code
> > > was that it seems equivalent to:
> > 
> 

> > > foo(buffer_t *out) {
> > 
> 
> > > auto value = out->host;
> > 
> 
> > > if (!out) {
> > 
> 
> > > error("nullptr");
> > 
> 
> > > }
> > 
> 
> > > }
> > 
> 

> > > In case I haven't been clear: I think this work is valuable for
> > > the
> > > project, and thank you for putting some effort into it :)
> > 
> 

> > > --
> > 
> 
> > > Mehdi
> > 
> 

> > > > > > > A separate discussion is on reading metadata (mcpu and
> > > > > > > mattr)
> > > > > > > in
> > > > > > > llc.
> > > > > > > I added a script to work around that for now.
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > The generic way of doing it in llvm is (I think) to use
> > > > > function
> > > > > attributes:
> > > > 
> > > 
> > 
> 

> > > > > attributes #0 = { "target-cpu"="x86-64"
> > > > > "target-features"="+avx2"
> > > > > }
> > > > 
> > > 
> > 
> 

> > > > > You shouldn't need it on the command line I think?
> > > > 
> > > 
> > 
> 
> > > > Yes, I believe so too. Currently these are set in mcpu and
> > > > mattr
> > > > by
> > > > Halide and not read in by llc, hence the need for feeding them
> > > > as
> > > > parameters. It's a separate issue that we'll need to go into in
> > > > depth, but I don't want it to interfere with getting feedback
> > > > on
> > > > how
> > > > to best publish these tests.
> > > 
> > 
> 

> > > > > --
> > > > 
> > > 
> > 
> 
> > > > > Mehdi
> > > > 
> > > 
> > 
> 

> > > > > > > Looking forward to your feedback!
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 

> > > > > > > Thanks,
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 

> > > > > > > Alina
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 

> > > > > > > On Fri, Feb 19, 2016 at 6:50 AM, Kristof Beyls <
> > > > > > > kristof.beyls at arm.com > wrote:
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 

> > > > > > > > On 18/02/2016 19:12, Alina Sbirlea via llvm-dev wrote:
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 

> > > > > > > > > > I have more questions for Alina. What kind of tests
> > > > > > > > > > do
> > > > > > > > > > you
> > > > > > > > > > have:
> > > > > > > > > 
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 

> > > > > > > > > > - "the compiler takes the bitcode and generates
> > > > > > > > > > code
> > > > > > > > > > without
> > > > > > > > > > crashing"
> > > > > > > > > 
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > > > - "the compiled test runs without crashing"
> > > > > > > > > 
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > > > - "the compiled test will produce an output that be
> > > > > > > > > > checked
> > > > > > > > > > against
> > > > > > > > > > a
> > > > > > > > > > reference"
> > > > > > > > > 
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > > > - "the compiled test is meaningful as a benchmarks"
> > > > > > > > > 
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 

> > > > > > > > > We have all 4 kinds of tests in Halide. The bitcode
> > > > > > > > > files
> > > > > > > > > for
> > > > > > > > > the
> > > > > > > > > first category is already available and I'm working
> > > > > > > > > on
> > > > > > > > > building
> > > > > > > > > the
> > > > > > > > > ones for the next 3. We'd like to include all
> > > > > > > > > incrementally.
> > > > > > > > 
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 

> > > > > > > > It seems to me that the first category ("the compiler
> > > > > > > > takes
> > > > > > > > the
> > > > > > > > bitcode and generates code without crashing") are tests
> > > > > > > > that
> > > > > > > > should
> > > > > > > > be added to the "make check-all" tests in the LLVM
> > > > > > > > subproject,
> > > > > > > > rather than the test-suite subproject?
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > > Or if these tests currently don't crash the compiler
> > > > > > > > anymore,
> > > > > > > > the
> > > > > > > > bugs must have been fixed, and there should already be
> > > > > > > > equivalent
> > > > > > > > tests?
> > > > > > > 
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 

> > > > > > > _______________________________________________
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > LLVM Developers mailing list
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > llvm-dev at lists.llvm.org
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> > > > > > 
> > > > > 
> > > > 
> > > 
> > 
> 

> > > > > > _______________________________________________
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > LLVM Developers mailing list
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > llvm-dev at lists.llvm.org
> > > > > 
> > > > 
> > > 
> > 
> 
> > > > > > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> > > > > 
> > > > 
> > > 
> > 
> 
> _______________________________________________
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> llvm-dev at lists.llvm.org
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-- 

Hal Finkel 
Assistant Computational Scientist 
Leadership Computing Facility 
Argonne National Laboratory 
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