tree ccdc6b88fa7c5b45a3ade7389dacea64c889d5fe
parent e0d9afa80ee55711d25fae87e50d11685d1ad6d7
author Jim Stichnoth <stichnot@chromium.org> 1438816902 -0700
committer Jim Stichnoth <stichnot@chromium.org> 1438816902 -0700

Subzero: Slight improvement to phi lowering.

When doing post phi lowering register allocation, the original code limited register allocation to pre-colored or infinite-weight variables with a non-empty live range within the new edge-split nodes.  This limitation ends up missing some opportunities.  Specifically, when a temporary is introduced to break a dependency cycle, e.g.:
  // a = phi(b)
  // b = phi(a)
  t = a
  a = b
  b = t
then t was always stack-allocated, even if a physical register was available.

In the new design, the RangeMask bitvector specifies which variables should have their live ranges tracked and computed.  For normal liveness analysis, all variables are tracked.  For post phi lowering liveness analysis, all variables created from phi lowering, plus all pre-colored variables, plus all infinite-weight variables, are tracked.

The result is slightly better code quality, and sometimes the frame size is 1 or 2 words smaller.

The hope was to narrow the 10% translation-time degradation in pnacl-llc.pexe compared to the old, hackish way of phi lowering, but that goal still proves to be elusive.

BUG= none
R=jvoung@chromium.org

Review URL: https://codereview.chromium.org/1271923002.