tree 1d83181e28d32972dd8c4d43bc22cd338a4bef64
parent 6418b33f21d3541f7a9678b29dacd53fcf58ab4f
author Andrew Hunter <ahh@google.com> 1444165990 -0700
committer Keir Mierle <mierle@gmail.com> 1449536947 +0000

Align Jet matrices where supported

We currently don't align the infinitesimal part of a Jet to a 16-byte
boundary (and thus force Eigen to avoid using SSE ops)--as a member of a
larger struct, we couldn't guarantee Jets would be allocated on
appropriately-aligned boundaries.  However, C++11 adds better support
for requesting alignment: we can use it to guarantee the members will be
properly aligned, and tell Eigen to vectorize.

There is a significant gotcha here: the standard gives wide latitude to
implementations as to which alignments they choose to support.  If we
ask for 16 and the system only supports 8, we may have misaligned
Jets. So we test (using alignof(std::max_align_t)) that the current
system supports 16-byte aligned values; if not, we fall back to the
current solution.

Two other small notes:
- This is obviously gated on C++11 support, and
  thus we put the logic in port.h and export some useful #defines.

- GCC 4.8.x has a
  bug (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56019) that has
  max_align_t in the wrong namespace.  This will not be a problem with a
  modern GCC, but add a small workaround since many systems still ship 4.8.

This results (on a x86 workstation) in a 60% speedup in Jacobian
evaluation on bin/simple_bundle_adjuster problem-16-22106-pre.txt.

Change-Id: I169b637a1e2a106956b536c41d6a514a266e7cc0