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

Mon Feb 29 13:50:08 PST 2016

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?

>
> 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?

>
> 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?
>>
>
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