commit f85038b7f3e9a0bd7d2bfbed96cc966863aeea57
author Antonio Sanchez <cantonios@google.com> Wed Feb 03 08:18:28 2021 -0800
committer Antonio Sanchez <cantonios@google.com> Wed Feb 03 09:01:48 2021 -0800
tree a890999030a9b7b22f0091ba5185b1a58d06d550
parent 56c8b14d875ae42a52d0da52916fac1e29305ca7

Fix excessive GEBP register spilling for 32-bit NEON.

Clang does a poor job of optimizing the GEBP microkernel on 32-bit ARM,
leading to excessive 16-byte register spills, slowing down basic f32
matrix multiplication by approx 50%.

By specializing `gebp_traits`, we can eliminate the register spills.
Volatile inline ASM both acts as a barrier to prevent reordering and
enforces strict register use. In a simple f32 matrix multiply example,
this modification reduces 16-byte spills from 109 instances to zero,
leading to a 1.5x speed increase (search for `16-byte Spill` in the
assembly in https://godbolt.org/z/chsPbE).

This is a replacement of !379.  See there for further discussion.

Also moved `gebp_traits` specializations for NEON to
`Eigen/src/Core/arch/NEON/GeneralBlockPanelKernel.h` to be alongside
other NEON-specific code.

Fixes #2138.