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Define live range splitting instructions.

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The copy/spill/fill instructions will be used by the register allocator
for splitting live ranges. The copy instruction is also useful when
rewriting values:

If a primary value is rewritten as a secondary result, a copy
instruction can be used instead:

  a = foo x
=>
  t, vx1 = call ...
  a = copy vx1

Since a primary value must be the first value of an instruction, this
doesn't work:

  a = foo x
=>
  t, a = call ...
This commit is contained in:
Jakob Stoklund Olesen
2016-10-20 15:34:16 -07:00
parent bb0bb1e91c
commit 84172ddf98
2 changed files with 59 additions and 1 deletions
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26
docs/langref.rst
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@@ -633,11 +633,35 @@ A few instructions have variants that take immediate operands (e.g.,
:inst:`band` / :inst:`band_imm`), but in general an instruction is required to
load a constant into an SSA value.
.. autoinst:: select
Constant materialization
------------------------
.. autoinst:: iconst
.. autoinst:: f32const
.. autoinst:: f64const
.. autoinst:: vconst
.. autoinst:: select
Live range splitting
--------------------
Cretonne's register allocator assigns each SSA value to a register or a spill
slot on the stack for its entire live range. Since the live range of an SSA
value can be quite large, it is sometimes beneficial to split the live range
into smaller parts.
A live range is split by creating new SSA values that are copies or the
original value or each other. The copies are created by inserting :inst:`copy`,
:inst:`spill`, or :inst:`fill` instructions, depending on whether the values
are assigned to registers or stack slots.
This approach permits SSA form to be preserved throughout the register
allocation pass and beyond.
.. autoinst:: copy
.. autoinst:: spill
.. autoinst:: fill
Vector operations
-----------------