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Diffstat (limited to 'opt.c')
| -rw-r--r-- | opt.c | 25 |
1 files changed, 25 insertions, 0 deletions
@@ -14,6 +14,31 @@ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. * ****************************************************************************/ +/** + * The expression optimizer. Different optimization levels are supported: + * + * -O1: basic logical simplifications, like folding (-true -and -foo) to -foo. + * + * -O2: dead code elimination and data flow analysis. struct opt_facts is used + * to record data flow facts that are true at various points of evaluation. + * Specifically, struct opt_facts records the facts that must be true before an + * expression is evaluated (state->facts), and those that must be true after the + * expression is evaluated, given that it returns true (state->facts_when_true) + * or false (state->facts_when_true). Additionally, state->facts_when_impure + * records the possible data flow facts before any expressions with side effects + * are evaluated. + * + * -O3: expression re-ordering to reduce expected cost. In an expression like + * (-foo -and -bar), if both -foo and -bar are pure (no side effects), they can + * be re-ordered to (-bar -and -foo). This is profitable if the expected cost + * is lower for the re-ordered expression, for example if -foo is very slow or + * -bar is likely to return false. + * + * -O4/-Ofast: aggressive optimizations that may affect correctness in corner + * cases. The main effect is to use facts_when_impure to determine if any side- + * effects are reachable at all, and skipping the traversal if not. + */ + #include "cmdline.h" #include "color.h" #include "eval.h" |