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90 lines
3.6 KiB
OpenEdge ABL
90 lines
3.6 KiB
OpenEdge ABL
[CodeGen::Labels::] Eliminate Redundant Labels.
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To remove labels which are defined but never jumped to.
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@h Pipeline stage.
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I7 tends to produce a lot of labels when compiling complicated text
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substitutions, but many (around 2000 in a typical run) are never branched to,
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either by a jump invocation or by assembly language.
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These spurious labels cause no real problem except untidiness, but removing
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them provides a simple example of how peephole optimisation can be performed
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on the Inter functions in a repository.
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=
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void CodeGen::Labels::create_pipeline_stage(void) {
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CodeGen::Stage::new(I"eliminate-redundant-labels", CodeGen::Labels::run_pipeline_stage, NO_STAGE_ARG, FALSE);
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}
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int CodeGen::Labels::run_pipeline_stage(pipeline_step *step) {
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Inter::Tree::traverse(step->repository, CodeGen::Labels::visitor, NULL, NULL, 0);
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return TRUE;
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}
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void CodeGen::Labels::visitor(inter_tree *I, inter_tree_node *P, void *state) {
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if (P->W.data[ID_IFLD] == PACKAGE_IST) {
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inter_package *pack = Inter::Package::defined_by_frame(P);
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if (Inter::Packages::is_codelike(pack)) @<Perform peephole optimisation on this block@>;
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}
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}
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@h Peephole optimisation.
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We can now forget about the larger package structure, and just look through
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the peephole at a single function in Inter. It has its own symbols table, for
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local variables and label names, and also has a tree of code to define what
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it does.
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@<Perform peephole optimisation on this block@> =
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inter_symbols_table *local_symbols = Inter::Packages::scope(pack);
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@<Mark all the labels for this function as being unused@>;
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@<Look through the function for mentions of labels, marking those as used@>;
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@<Remove the label declarations for any that are still marked unused@>;
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@ The symbol flag |USED_MARK_BIT| is free for us to use, but its value for
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any given symbol is undefined when we begin. We'll clear it for all labels.
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@<Mark all the labels for this function as being unused@> =
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LOOP_OVER_SYMBOLS_TABLE(S, local_symbols)
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if (Inter::Symbols::is_label(S))
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Inter::Symbols::clear_flag(S, USED_MARK_BIT);
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@<Look through the function for mentions of labels, marking those as used@> =
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inter_tree_node *D = Inter::Packages::definition(pack);
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CodeGen::Labels::traverse_code_tree(D);
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@ Anything not marked used must be unused, so we can get rid of it. We do this
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by striking its definition; the definition of a label symbol is the line
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which shows where it belongs in the function (written |.Example| in Inter
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syntax). Striking this does two things: it removes the definition line; and
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it renders the symbol undefined. It still lives on in the function's symbols
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table, though, and (since we have made sure there are no references to it from
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anywhere) we may as well remove it.
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@<Remove the label declarations for any that are still marked unused@> =
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LOOP_OVER_SYMBOLS_TABLE(S, local_symbols)
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if (Inter::Symbols::is_label(S))
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if (Inter::Symbols::get_flag(S, USED_MARK_BIT) == FALSE) {
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Inter::Symbols::strike_definition(S);
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Inter::Symbols::remove_from_table(S);
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}
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@ The following visits every line of code in the function, in the same order
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it would be written out in a listing.
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=
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void CodeGen::Labels::traverse_code_tree(inter_tree_node *P) {
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LOOP_THROUGH_INTER_CHILDREN(F, P) {
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@<Examine a line of code in the function@>;
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CodeGen::Labels::traverse_code_tree(F);
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}
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}
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@ If a label is used, there will be line reading |lab Example| or similar.
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We look for such lines.
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@<Examine a line of code in the function@> =
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if (F->W.data[ID_IFLD] == LAB_IST) {
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inter_symbol *lab = Inter::Lab::label_symbol(F);
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Inter::Symbols::set_flag(lab, USED_MARK_BIT);
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}
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