[CODEGEN] Enable inline llvm asm code

[tqchen]

Ref #1276

Usage

We can have a pragma attribute pragma_import_llvm, which can be used in either ir builder or schedule

Example

def test_llvm_import():
    # extern "C" is necessary to get the correct signature
    # we can put inline asm here
    cc_code = """
    extern "C" float my_add(float x, float y) {
      return x + y;
    }
    """
    n = 10
    A = tvm.placeholder((n,), name='A')
    B = tvm.compute((n,), lambda *i:
                    tvm.call_pure_extern("float32", "my_add", A(*i), 1.0),
                    name='B')
    def check_llvm(use_file):
        if not tvm.module.enabled("llvm"):
            return
        if not clang.find_clang(required=False):
            print("skip because clang is not available")
            return
        temp = util.tempdir()
        ll_path = temp.relpath("temp.ll")
        # Create LLVM ir from c source code
        ll_code = clang.create_llvm(cc_code, output=ll_path)
        s = tvm.create_schedule(B.op)
        if use_file:
            s[B].pragma(s[B].op.axis[0], "import_llvm", ll_path)
        else:
            s[B].pragma(s[B].op.axis[0], "import_llvm", ll_code)
        # BUILD and invoke the kernel.
        f = tvm.build(s, [A, B], "llvm")
        ctx = tvm.cpu(0)
        # launch the kernel.
        a = tvm.nd.array(np.random.uniform(size=n).astype(A.dtype), ctx)
        b = tvm.nd.array(np.random.uniform(size=n).astype(B.dtype), ctx)
        f(a, b)
        np.testing.assert_allclose(
            b.asnumpy(), a.asnumpy() + 1.0)
    check_llvm(use_file=True)

[tqchen]

@masahi @yzhliu @eqy @ajtulloch please review

[masahi]

Interesting. I'll try using PTX's shuffle instructions and AMDGCN's DPP instructions. I have a good use case for them.

[aditya4d1]

@masahi dpp instructions are cross lane ops. For this test, you may not need dpp instructions

[masahi]

@adityaatluri what do you mean by "for this test"? I found that MIOpen uses dpp instructions for doing winograd input/filter/output transform. I can imagine using 4 threads cooperatively to compute 4x4 tile trasnform, but I haven't figured out how exactly they do it.

[ajtulloch]

This looks great @tqchen. One thing I'd add (which was surprising) was that generating the .ll/.bc with O3 is quite important, since LLVM generates a lot of crap before/after the inline asm block, and this doesn't get entirely stripped when it's inlined into the larger function. So I think adding options=["-O3"] or similar would be useful here, I can send a patch.

[masahi]

@ajtulloch I think you can pass any compiler flags you like to create_llvm() function above.

[tqchen]

I think it is good to options=["-O3"] by default if user do not provide any option flags

[aditya4d1]

I don't know if MIOpen uses DPP, but DPP are cross lane ops and there are no programming language constructs to express them.
If you are interested in how DPP can be used for GEMM, this blog can help you:
https://adityaatluri.github.io/2018/04/29/decreasing-lds-memory-requests.html