[PATCH] Rewrite arm_neon.h generation from scratch

James Molloy james.molloy at arm.com
Wed Jun 4 03:39:44 PDT 2014

Hi all,

[Obvious stakeholders put on CC, feel free to add more]

NeonEmitter.cpp, the arm_neon.td tablegen backend, really needs improving. I
to change the behaviour slightly for big endian, but quickly realised that
this may
well be the hack that broke the camel's back. I tried to incrementally
refactor it
but to be honest it's beyond repair. The attached patch reimplements it from
Luckily the testing coverage on this stuff is absolutely massive, both with
regression tests and the "emperor" random test case generator.

The main change is that previously, in arm_neon.td a bunch of "Operation"s
defined with special names. NeonEmitter.cpp knew about these Operations and
would emit code based on a huge switch. Actually this doesn't make much
sense -
the type information was held as strings, so type checking was impossible.
TableGen's DAG type actually suits this sort of code generation very well
(surprising that...)

So now every operation is defined in terms of TableGen DAGs. There are a
of operators to use, including "op" (a generic unary or binary operator),
(to call other intrinsics) and "shuffle" (take a guess...). One of the main
advantages of this apart from making it more obvious what is going on, is
we have proper type inference. This has two obvious advantages:

  1) TableGen can error on bad intrinsic definitions easier, instead of just
     generating wrong code.
  2) Calls to other intrinsics are typechecked too. So
     we no longer need to work out whether the thing we call needs to be the
     version or the D-lane version - TableGen knows that itself!

Here's an example: before:

  case OpAbdl: {
    std::string abd = MangleName("vabd", typestr, ClassS) + "(__a, __b)";
    if (typestr[0] != 'U') {
      // vabd results are always unsigned and must be zero-extended.
      std::string utype = "U" + typestr.str();
      s += "(" + TypeString(proto[0], typestr) + ")";
      abd = "(" + TypeString('d', utype) + ")" + abd;
      s += Extend(utype, abd) + ";";
    } else {
      s += Extend(typestr, abd) + ";";


  def OP_ABDL     : Op<(cast "R", (call "vmovl", (cast $p0, "U",
                                                       (call "vabd", $p0,

As an example of what happens if you do something wrong now, here's what
if you make $p0 unsigned before the call to "vabd" - that is, $p0 -> (cast
"U", $p0):

arm_neon.td:574:1: error: No compatible intrinsic found - looking up
intrinsic 'vabd(uint8x8_t, int8x8_t)'
Available overloads:
  - float64x2_t vabdq_v(float64x2_t, float64x2_t)
  - float64x1_t vabd_v(float64x1_t, float64x1_t)
  - float64_t vabdd_f64(float64_t, float64_t)
  - float32_t vabds_f32(float32_t, float32_t)
... snip ...

This makes it seriously easy to work out what you've done wrong in fairly

As part of this I've massively beefed up the documentation in arm_neon.td

Things still to do / on the radar:
  - Testcase generation. This was implemented in the previous version and
not in
    the new one, because
    - Autogenerated tests are not being run. The testcase in test/ differs
      the autogenerated version.
    - There were a whole slew of special cases in the testcase generation
that just
      felt (and looked) like hacks.
    If someone really feels strongly about this, I can try and reimplement
it too.
  - Big endian. That's coming soon and should be a very small diff on top of
this one.



 include/clang/Basic/arm_neon.td   |  653 +++--
 test/CodeGen/arm64_vcvtfp.c       |    2 +-
 test/Sema/arm-neon-types.c        |    2 +-
 test/Sema/arm64-neon-args.c       |    2 +-
 utils/TableGen/NeonEmitter.cpp    | 4833
 utils/TableGen/TableGenBackends.h |    3 +
 6 files changed, 2303 insertions(+), 3192 deletions(-)
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