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Replace beam_dead with beam_ssa_dead #1955

merged 30 commits into from Sep 19, 2018


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@bjorng bjorng commented Sep 17, 2018

This pull request introduces the new beam_ssa_dead pass and removes the beam_dead and beam_split compiler passes. There are also new and improved optimizations in beam_ssa_opt and beam_ssa_type. A few of the optimizations in beam_dead have been moved to beam_jump, beam_a, and beam_peep.

In general, the new optimizations improves the code in many more places than beam_dead does. In a few cases, beam_dead could find optimizations that the new optimization passes fail to find.

For more details, see the individual commit messages.

When creating a phi node for the common exit block of a receive,
the code failed to take into account that there could be more
than one predecessor to the exit block for each remove_message.

Rename exit_predessor/3 to exit_predessors/3 and make it return a
list of the predecessors.
When optimizations get more powerful, beam_validator
must keep up.
Add normalize/1 to simplify optimizations.
Since beam_ssa:linearize/1 may remove blocks that are unreachable,
adjust phi nodes to make sure that they don't refer to discarded
blocks or to blocks that no longer branch to the phi node in
Add trim_unreachable/1 to remove unreachable blocks
and adjust phi nodes.
It is faster to use cerl_sets instead of gb_sets to keep track of
seen blocks.
The 'move' instruction can be eliminated in code such as:


Move that optimization from beam_dead to beam_a. The optimization
will be simpler because the 'move' instruction has not yet
been moved into a block. Getting rid of 'move' earlier will
also save work for later passes.

Also move the optimization that eliminates instructions such
as from beam_dead to beam_a:

This functionality will soon be needed.
Those optimizations are unsafe if beam_dead has been run before.
A select_val instruction that test whether a register is a boolean
like this:


can be replaced with an is_boolean test:


This optimization is currently done in beam_dead. However, if done in
the beam_peep, it can catch more opportunities to do the optimization,
because after having run beam_jump, labels that were different have
been coalesced.
Add more instructions to the list of functions that can be safely
removed if their values are not used. This is necessary for
correctness when doing more aggressive optimizations. Without this
change, the 'succeeded' instruction could be optimized away leaving
just the instruction followed by an unconditional branch, which the
beam_ssa_codegen does not know how to handle. Here is an example:

    _3 = bs_start_match _1
    br label 13

By adding bs_start_match to the list, the bs_start_match instruction
will be removed too. (If the result of bs_start_match is actually
used, the succeeded instruction would not be removed.)

While we are it, rename the misnamed function is_pure/1 to
no_side_effect/1 and move it to beam_ssa. is_pure/1 is a bad name
because bif:get has no side effect, but is not pure.
Nested cases can led to code such as this:

       _1 = phi {literal value1, label 8}, {Var, label 9}
      br 11

      _2 = phi {_1, label 10}, {literal false, label 3}

The phi nodes can be coalesced like this:

      _2 = phi {literal value1, label 8}, {Var, label 9},
               {literal false, label 3}

Coalescing can help other optimizations, and can in some cases reduce
register shuffling (if the phi variables for two phi nodes happens to
be allocated to different registers).
When the argument for a #b_switch{} comes from a phi node
with only literal values, the switch list could be pruned to
only contain the possible values. It could also be possible
to eliminate the failure label.

Also simplify a switch with a single value list or switch that can be
replaced with an is_boolean test.
This optimization working on the SSA format will replace
the similar optimization in beam_dead. See the comment
for an explanation of what the new optimization does.
Phi nodes with only literals are fairly common, so it's worthwhile
to optimize this case.
Not doing CSE for tuple_size/1 seems to generate slightly better
code in most cases.
The floating point optimization relies on heavily on the block
order in the lineararized representation. A new optimization
could easily break the optimization, for example so that no
`fcheckerror` instructions were emitted.

Rewrite the optimization to avoid dependencies on the linear
block order.
Remove the following clause from the `fun` clauses in arith_op_types/2
because it cannot possibly match:

    (_, any) -> number;

Here is why it cannot match:

The second argument is the accumulator for lists:foldl/3.
Its initial value is `unknown`.

None of the clauses will update the accumulator to `any`, including
the clause that matches `any` in the first argument -- it will set the
accumulator to `number`.

Thus, the accumulator (second argument) can never be `any` and
the clause can never match. QED.
In beam_ssa_type, do substitutions similar to what ssa_opt_misc does
to get rid of variables that evaluate to constant values.  That
somewhat simplifies the code of beam_ssa_type, and could improve
performance of the compiler since instructions and variables are
eliminated, reducing the amount of work for later passes.
Omitting `kill` instructions before BIFs that throw exceptions
will reduce the code size. This optimization supersedes the same
optimizations in beam_dead.
Add beam_ssa_dead to perform the main optimizations done
by beam_dead:

* Shortcut branches that jump to another block with a branch.
If it can be seen that the second branch will always branch
to a specific block, replace the target of the first branch.

* Combined nested sequences of '=:=' tests and switch
instructions operating on the same variable to a single switch.

Diffing the compiler output, it seems that beam_ssa_dead finds
many more opportunities for optimizations than beam_dead,
although it does not find all opportunities that beam_dead does.
In total, beam_ssa_dead is such improvement over beam_dead that
there is no reason to keep beam_dead as well as beam_ssa_dead.

Note that beam_ssa_dead does not attempt to optimize away
redundant bs_context_binary instructions, because that instruction
will be superseded by new instructions in the near future.
Most of the optimizations in beam_dead have been superseded
by the optimizations in beam_ssa_dead.

The forward/1 pass of beam_dead has been moved to beam_jump.

The beam_split pass splits blocks that contain instructions with
non-zero labels. Because there are no optimizations left that optimize
instructions within blocks, beam_block never needs to put such
instructions into blocks in the first place.  beam_split also moved
'move' instructions out block to help beam_dead. That is no longer
necessary since beam_dead no longer exists.
@bjorng bjorng added team:VM Assigned to OTP team VM enhancement labels Sep 17, 2018
@bjorng bjorng self-assigned this Sep 17, 2018
lib/compiler/src/beam_ssa_opt.erl Show resolved Hide resolved
@bjorng bjorng merged commit 70cb897 into erlang:master Sep 19, 2018
@bjorng bjorng deleted the bjorn/compiler/beam_ssa_dead branch September 19, 2018 07:46
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