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Implement multi-arm switches for the MLDS backend. When possible, the code of a
switch arm that has more than one cons_id is included in the output only once,
though of course with a condition that causes it to be executed for any of its
matching cons_ids. However, in the case of a tag switch in which a switch arm
has cons_ids with different primary tags, at least one of which has a
secondary tag that requires a switch on *it*, we duplicate the MLDS code of the
switch arm (we generate one copy for each such primary tag).
The diff yields a speedup of 0.4% on speedtest and a 0.5% reduction in code
size, but the main reason for it is so that programmers don't have any
incentive anymore to prefer an if-then-else chain to a switch for code
that is logically a switch with a default case that applies to many cons_ids.
compiler/handle_options.m:
Keep disabling multi-arm switches for non-C MLDS backends, but stop
disabling it for the C MLDS backend.
compiler/hlds_goal.m:
Add a case number to the tagged_case type. This is to allow us to
create maps that effectively function as maps from pieces of code
(the code generated for the goal in the tagged case) to other things
(various forms of the switch conditions in which that code applies)
without making the code itself the key in the map; we can use the
code's associated case number as the key instead.
compiler/mlds.m:
Change the representation of the match conditions of a switch arm
from just a simple list of conditions to a first condition and a list
of other conditions. This enforces the invariant that the switch arm
must always apply in at least one condition.
compiler/ml_string_switch.m:
compiler/ml_switch_gen.m:
compiler/ml_tag_switch.m:
Implement the above.
In ml_switch_gen.m, change the structure of the predicate that decides
which code generation scheme to employ from an if-then-else chain
to being basically a switch. This structure, which is modelled on the
one used in the LLDS code generator, should be significantly clearer.
As part of this change of structure, sort the cases by the cost of the
tag tests only if the chosen code generation wants the cases sorted in
this way.
compiler/switch_util.m:
Add facilities for grouping together primary tags that both have
exactly the same code, so that we don't have to duplicate its code.
This is possible only when neither primary tag is shared by more
than one cons_id. This grouping benefits even the LLDS backend.
This involved separating out the types and predicates that are intended
for use in compilation schemes in which more than one switch value can
share the same piece of code (such as switches or if-then-else chains
in C) from those intended for use in compilation in which this is not
possible (such as lookup tables).
Replace several uses of pairs with named types and function symbols.
Delete a predicate that isn't needed anymore.
Add a predicate to support the fix to switch_gen.m.
compiler/switch_gen.m:
Fix an old oversight. When I added multi-arm switches for the LLDS
backend, I did not update the test for whether the switch is big enough
for each kind of nontrivial indexing to count cons_ids rather than
switch arms (the benefit of smart indexing is proportional to the
former). This diff fixes that.
compiler/ml_unify_gen.m:
Change the interface of some predicates to make them more useful
for generating code for switches.
compiler/tag_switch.m:
Conform to the changes in switch_util.m.
compiler/ml_simplify_switch.m:
Conform to the changes above.
Rename some predicates to better reflect their purpose.
compiler/dense_switch.m:
compiler/lookup_switch.m:
compiler/ml_elim_nested.m:
compiler/ml_optimize.m:
compiler/ml_tailcall.m:
compiler/ml_util.m:
compiler/mlds_to_c.m:
compiler/mlds_to_gcc.m:
compiler/mlds_to_il.m:
compiler/mlds_to_java.m:
compiler/switch_case.m:
Conform to the changes above.
tests/hard_coded/multi_arm_switch_2.{m,exp}:
tests/hard_coded/string_switch.{m,exp}:
New test cases to exercise the new functionality.
tests/hard_coded/Mmakefile:
Enable the new tests.
14 lines
158 B
Plaintext
14 lines
158 B
Plaintext
f1 -> 1
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f2 -> 1
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f3 -> 2
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f4(104) -> 2
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f5(105) -> 2
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f6(106) -> 3
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f7(107) -> 3
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f8(108) -> 3
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f9(109) -> 3
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f10(110) -> 3
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f11(111) -> 3
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f12(112) -> 4
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f13(113) -> 4
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