Operator fusion breaks '+' operator on GPU with CachedOp

[vafl]

Description

In some cases broadcast_add and + give different results. This issue seems to be new in 1.6.

To Reproduce

The following network gives the wrong outputs when hybridizing and running on a GPU.

import mxnet as mx
import numpy as np


class MyModel(mx.gluon.HybridBlock):
    def __init__(self) -> None:
        super().__init__()
        with self.name_scope():
            self.cs = self.params.get_constant("cs", np.arange(10))

    def hybrid_forward(self, F, dummy, cs):
        u = F.broadcast_add(cs, dummy.zeros_like())

        # r = F.broadcast_add(
        #     F.slice_axis(u, axis=-1, begin=0, end=-1),
        #     F.slice_axis(u, axis=-1, begin=1, end=None)
        # ) / 2.0

        r = (
            F.slice_axis(u, axis=-1, begin=0, end=-1) +
            F.slice_axis(u, axis=-1, begin=1, end=None)
        ) / 2.0

        return r


def main():
    ctx = mx.Context('gpu')
    # ctx = mx.Context('cpu')

    model = MyModel()
    model.collect_params().initialize(mx.init.Xavier(), ctx=ctx)
    print("hybridizing");  model.hybridize()

    dummy = mx.nd.array(
        [
            np.ones(10),
            np.ones(10),
            np.ones(10),
        ],
        ctx=ctx
    )
    with mx.autograd.record():
        out = model(dummy)
        print(out.asnumpy())


if __name__ == '__main__':
    main()

It returns:

[[0.  1.  2.  3.  4.  5.  6.  7.  8. ]
 [9.  0.  1.  2.  3.  4.  5.  6.  7. ]
 [8.  9.  0.  0.5 1.  1.5 2.  2.5 3. ]]

The correct result is:

[[0.5 1.5 2.5 3.5 4.5 5.5 6.5 7.5 8.5]
 [0.5 1.5 2.5 3.5 4.5 5.5 6.5 7.5 8.5]
 [0.5 1.5 2.5 3.5 4.5 5.5 6.5 7.5 8.5]]

Any of the following changes will give the correct result:

• switch to cpu
• don't hybridize
• switch to broadcast_add instead of + (see code that is commented out)

This did not happen on mxnet 1.5.

I think it is related to the get_constant and broadcasting that happens before in the network.

Environment

----------Python Info----------
Version      : 3.6.5
Compiler     : GCC 7.2.0
Build        : ('default', 'Apr 29 2018 16:14:56')
Arch         : ('64bit', '')
------------Pip Info-----------
Version      : 19.3.1
Directory    : /home/ubuntu/anaconda3/envs/amazonei_mxnet_p36/lib/python3.6/site-packages/pip
----------MXNet Info-----------
Version      : 1.6.0
Directory    : /home/ubuntu/anaconda3/envs/amazonei_mxnet_p36/lib/python3.6/site-packages/mxnet
Num GPUs     : 1
Commit Hash   : 6eec9da55c5096079355d1f1a5fa58dcf35d6752
----------System Info----------
Platform     : Linux-4.4.0-1104-aws-x86_64-with-debian-stretch-sid
system       : Linux
node         : ip-172-31-28-101
release      : 4.4.0-1104-aws
version      : #115-Ubuntu SMP Mon Mar 2 06:35:35 UTC 2020
----------Hardware Info----------
machine      : x86_64
processor    : x86_64
Architecture:          x86_64
CPU op-mode(s):        32-bit, 64-bit
Byte Order:            Little Endian
CPU(s):                8
On-line CPU(s) list:   0-7
Thread(s) per core:    2
Core(s) per socket:    4
Socket(s):             1
NUMA node(s):          1
Vendor ID:             GenuineIntel
CPU family:            6
Model:                 79
Model name:            Intel(R) Xeon(R) CPU E5-2686 v4 @ 2.30GHz
Stepping:              1
CPU MHz:               2213.210
CPU max MHz:           3000.0000
CPU min MHz:           1200.0000
BogoMIPS:              4600.12
Hypervisor vendor:     Xen
Virtualization type:   full
L1d cache:             32K
L1i cache:             32K
L2 cache:              256K
L3 cache:              46080K
NUMA node0 CPU(s):     0-7
Flags:                 fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx pdpe1gb rdtscp lm constant_tsc rep_good nopl xtopology nonstop_tsc aperfmperf pni pclmulqdq ssse3 fma cx16 pcid sse4_1 sse4_2 x2apic movbe popcnt tsc_deadline_timer aes xsave avx f16c rdrand hypervisor lahf_lm abm 3dnowprefetch invpcid_single kaiser fsgsbase bmi1 hle avx2 smep bmi2 erms invpcid rtm rdseed adx xsaveopt
----------Network Test----------
Setting timeout: 10
Timing for MXNet: https://github.com/apache/incubator-mxnet, DNS: 0.0022 sec, LOAD: 0.6087 sec.
Timing for GluonNLP GitHub: https://github.com/dmlc/gluon-nlp, DNS: 0.0006 sec, LOAD: 0.5831 sec.
Timing for GluonNLP: http://gluon-nlp.mxnet.io, DNS: 0.1805 sec, LOAD: 0.4238 sec.
Timing for D2L: http://d2l.ai, DNS: 0.0274 sec, LOAD: 0.0487 sec.
Timing for D2L (zh-cn): http://zh.d2l.ai, DNS: 0.0148 sec, LOAD: 0.1029 sec.
Timing for FashionMNIST: https://repo.mxnet.io/gluon/dataset/fashion-mnist/train-labels-idx1-ubyte.gz, DNS: 0.1235 sec, LOAD: 0.1370 sec.
Timing for PYPI: https://pypi.python.org/pypi/pip, DNS: 0.0178 sec, LOAD: 0.3628 sec.
Timing for Conda: https://repo.continuum.io/pkgs/free/, DNS: 0.0015 sec, LOAD: 0.0679 sec.

[leezu]

I can reproduce this with mxnet-cu101==1.6 on a G4 instancen. This appears to be a bug with operator fusion. If you export MXNET_USE_FUSION=0 the program works as expect.

% MXNET_USE_FUSION=0 python3 test.py                                           24s ~ ip-172-31-32-170
hybridizing
[[0.5 1.5 2.5 3.5 4.5 5.5 6.5 7.5 8.5]
 [0.5 1.5 2.5 3.5 4.5 5.5 6.5 7.5 8.5]
 [0.5 1.5 2.5 3.5 4.5 5.5 6.5 7.5 8.5]]
% MXNET_USE_FUSION=1 python3 test.py                                                                                                                                                  7s ~ ip-172-31-32-170
hybridizing
[[0.  1.  2.  3.  4.  5.  6.  7.  8. ]
 [9.  0.  1.  2.  3.  4.  5.  6.  7. ]
 [8.  9.  0.  0.5 1.  1.5 2.  2.5 3. ]]

CC @ptrendx

[ptrendx]

I will look into this - I'm not sure if this is + though that is problematic. It looks more like a problem with handling of slice_axis. I will investigate.

[ptrendx]

Ok, I see where the problem comes from - there is a bug in handling negative values for parameters in the code generator - in your example if you change axis to 1 and end to 9 you will get the right answer. I will fix that and submit PR shortly.