Replace beam_dead with beam_ssa_dead #1955
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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 question.
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:
{test,is_eq_exact,{f,42},[{x,0},{atom,value}]}.
{move,{atom,value},{x,0}}.
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:
{test_is_eq_exact,{f,42},[{x,0},{x,0}]}.
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:
{select_val,Reg,{f,Fail},{list,[{atom,true},Lbl,{atom,false},Lbl]}}.
can be replaced with an is_boolean test:
{test,is_boolean,{f,Fail},[Reg]}.
{jump,{f,Lbl}}.
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:
10:
_1 = phi {literal value1, label 8}, {Var, label 9}
br 11
11:
_2 = phi {_1, label 10}, {literal false, label 3}
The phi nodes can be coalesced like this:
11:
_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.
jhogberg
approved these changes
Sep 17, 2018
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This pull request introduces the new
beam_ssa_deadpass and removes thebeam_deadandbeam_splitcompiler passes. There are also new and improved optimizations inbeam_ssa_optandbeam_ssa_type. A few of the optimizations inbeam_deadhave been moved tobeam_jump,beam_a, andbeam_peep.In general, the new optimizations improves the code in many more places than
beam_deaddoes. In a few cases,beam_deadcould find optimizations that the new optimization passes fail to find.For more details, see the individual commit messages.