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| author | Tavian Barnes <tavianator@tavianator.com> | 2016-06-09 18:37:58 -0400 |
|---|---|---|
| committer | Tavian Barnes <tavianator@tavianator.com> | 2016-06-09 18:37:58 -0400 |
| commit | 88cc26afec80dae40d8e08de0e6db6c308e64f79 (patch) | |
| tree | c05aef57bb22278227bb475492e2696b1a19193c /RELEASES.md | |
| parent | cad5dc5cc50a43203971939fd2806bfbf9a5dac9 (diff) | |
| download | bfs-88cc26afec80dae40d8e08de0e6db6c308e64f79.tar.xz | |
Re-work optimization levels.
-O3 is the new default, for the future cost-based optimizer. -O4
enables the surprising/aggressive optimizations that used to be under
-O3. -Ofast is a synonym for -O4.
Diffstat (limited to 'RELEASES.md')
| -rw-r--r-- | RELEASES.md | 52 |
1 files changed, 38 insertions, 14 deletions
diff --git a/RELEASES.md b/RELEASES.md index 2b55ff9..3d75210 100644 --- a/RELEASES.md +++ b/RELEASES.md @@ -9,21 +9,45 @@ 62/76 GNU find features supported. +- Rework optimization levels + - `-O1` (logical simplification): + - Constant propagation (`! -false <==> -true`, `! -true <==> -false`) + - Double negation (`! ! X <==> X`) + - Conjunction elimination: + - `-true -a X <==> X` + - `X -a -true <==> X` + - Disjunctive syllogism: + - `-false -o X <==> X` + - `X -o -false <==> X` + - Short-circuiting: + - `-false -a X <==> -false` + - `-true -o X <==> -true` + - De Morgan's laws (**new**): + - `( ! X -a ! Y ) <==> ! ( X -o Y )` + - `( ! X -o ! Y ) <==> ! ( X -a Y )` + - `! ( X -a ! Y ) <==> ( ! X -o Y )` + - `! ( X -o ! Y ) <==> ( ! X -a Y )` + - `! ( ! X -a Y ) <==> ( X -o ! Y )` + - `! ( ! X -o Y ) <==> ( X -a ! Y )` + - Unused result (**new**): `! X , Y <==> X , Y` + - `-O2` (purity): + - (These optimizations take the purity of predicates into account, allowing side-effect-free tests like `-name` or `-type` to be moved or removed) + - `PURE -a -false <==> -false` + - `PURE -o -true <==> -true` + - `PURE , X <==> X` + - Top-level unused result (**new**): `X (-a|-o|,) PURE <==> X` + - `-O3` (cost-based, **default**): + - Re-order tests to reduce the expected cost (TODO) + - `-O4` (aggressive): + - (These are very aggressive optimizations that may have surprising effects on warning/error messages and runtime, but still should not affect the resulting output) + - Change top-level expressions with no actions to `-false` (**new**): + - For example, `bfs -O4 -true -o -print` becomes `-false`, because `-print` is unreachable + - Skip the entire traversal if the top-level expression is `-false` + - `bfs -O4 -false` (or anything that optimizes to `-false`) will exit immediately + - This may cause messages about non-existent files, symbolic link cycles, etc. to be skipped + - `-Ofast`: + - Always the highest level, currently the same as `-O4` - Color files with multiple hard links correctly -- At `-O3`, replace command lines with no action with `-false` - - For example, `bfs -true -o -print` becomes `-false`, because `-print` is not reachable -- Move optimizations that rely on the purity of a predicate to `-O2` (the new default) -- Optimize using De Morgan's laws: - - `( ! X -a ! Y ) <==> ! ( X -o Y )` - - `( ! X -o ! Y ) <==> ! ( X -a Y )` - - `! ( X -a ! Y ) <==> ( ! X -o Y )` - - `! ( X -o ! Y ) <==> ( ! X -a Y )` - - `! ( ! X -a Y ) <==> ( X -o ! Y )` - - `! ( ! X -o Y ) <==> ( X -a ! Y )` -- At `-O2`, remove top-level pure expressions - - For example, `bfs -print , -false` becomes `-print`, because the top-level return value is ignored -- At `-O2`, remove negations from the left-hand side of the comma operator - - For example, `-not -print , -print` becomes `-print , -print`, because the return value of the LHS is ignored - Treat `-`, `)`, and `,` as paths when required to by POSIX - `)` and `,` are only supported before the expression begins - Implement `-D opt` |