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Allow the MLDS backend to generate indexing switches (switches implemented
more efficiently than just a if-then-else chain) for strings even if the target
language does not support gotos.
Previously, we use always used gotos to break out of search loops
after we found a match:
do {
if (we have a match) {
... handle the match ...
goto end
} else {
... handle nonmatches ...
}
} while (loop should continue);
maybe some code to handle the failure of the search
end:
Now, if the "maybe some code" is empty, we prefer to use break statements
if the target language supports this:
do {
if (we have a match) {
... handle the match ...
break;
} else {
... handle nonmatches ...
}
} while (loop should continue)
If we cannot use either gotos or break statements, we instead use
a boolean variable named "stop_loop":
stop_loop = 0;
do {
if (we have a match) {
... handle the match ...
stop_loop = 1;
} else {
... handle nonmatches ...
}
} while (stop_loop == 0 && loop should continue)
if (stop_loop == 0) {
maybe some code to handle the failure of the search
}
We omit the final if statement if the then-part would be empty.
The break method generates the smallest code, followed by the goto code.
I don't have information on speed, since we don't have a benchmark that
runs long enough, and the compiler itself does not spend any significant
amount of time on string switches. Probably the break method is also the
fastest, simply because it leaves the code looking most like normal C code.
(Some optimizations are harder to apply to code containing gotos, and some
optimizer writers do not bother.)
For C, we now normally prefer to generate code using the second method
(breaks), if we can, though normally "maybe some code" is not empty,
in which case we use the first method (goto).
However, if the value of the --experiment option is set to "use_stop_loop",
we always use the third method, and if it is set to "use_end_label", we always
use the first, even when we could use the second. This allow us to test all
three approaches using the C back end.
With backends that support neither gotos nor break, we always use the third
method (stop_loop).
With backends that don't support gotos but do support breaks, we also always
use the third method. This is because trying to use the second method would
require us to commit to not creating the stop_loop variable BEFORE we know
that the "maybe some code to handle the failure of the search" is empty,
and if it isn't empty, then we don't have the goto method to fall back on.
compiler/ml_string_switch.m:
Make the change described above. Where possible, make the required
change not to the original code, but to a version in which common
parts have been factored out. (Previously, the duplicated code was
small; now, it would be big.)
compiler/ml_target_util.m:
A new module containing existing functions that test various properties
of the target language. Keeping some of those functions in their
original modules would have introduced a circular dependency.
compiler/ml_switch_gen.m:
Enable the new functionality by removing the tests that previously
prevented the compiler from using indexing switches on strings
if the target language did not support gotos.
Remove the code moved to ml_target_util.m.
compiler/ml_optimize.m:
compiler/ml_unify_gen.m:
Remove the code moved to ml_target_util.m.
compiler/ml_backend.m:
compiler/notes/compiler_design.m:
Add the new module.
compiler/ml_proc_gen.m:
Delete a predicate that hasn't been used for a long time.
tools/makebatch:
Fix an old pair of typos.