Source / Include Folder (#88)

Moved Source Files Into `src` Folder

1 files changed, 1088 insertions, 0 deletions

diff --git a/src/opt.c b/src/opt.c
new file mode 100644
index 0000000..f8c0ba3
--- /dev/null
+++ b/src/opt.c
@@ -0,0 +1,1088 @@
+/****************************************************************************
+ * bfs                                                                      *
+ * Copyright (C) 2017-2022 Tavian Barnes <tavianator@tavianator.com>        *
+ *                                                                          *
+ * Permission to use, copy, modify, and/or distribute this software for any *
+ * purpose with or without fee is hereby granted.                           *
+ *                                                                          *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES *
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF         *
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR  *
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES   *
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN    *
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF  *
+ * 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 "opt.h"
+#include "color.h"
+#include "ctx.h"
+#include "diag.h"
+#include "eval.h"
+#include "expr.h"
+#include "pwcache.h"
+#include "util.h"
+#include <assert.h>
+#include <limits.h>
+#include <stdarg.h>
+#include <stdbool.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <string.h>
+#include <unistd.h>
+
+static char *fake_and_arg = "-a";
+static char *fake_or_arg = "-o";
+static char *fake_not_arg = "!";
+
+/**
+ * A contrained integer range.
+ */
+struct range {
+	/** The (inclusive) minimum value. */
+	long long min;
+	/** The (inclusive) maximum value. */
+	long long max;
+};
+
+/** Compute the minimum of two values. */
+static long long min_value(long long a, long long b) {
+	if (a < b) {
+		return a;
+	} else {
+		return b;
+	}
+}
+
+/** Compute the maximum of two values. */
+static long long max_value(long long a, long long b) {
+	if (a > b) {
+		return a;
+	} else {
+		return b;
+	}
+}
+
+/** Constrain the minimum of a range. */
+static void constrain_min(struct range *range, long long value) {
+	range->min = max_value(range->min, value);
+}
+
+/** Contrain the maximum of a range. */
+static void constrain_max(struct range *range, long long value) {
+	range->max = min_value(range->max, value);
+}
+
+/** Remove a single value from a range. */
+static void range_remove(struct range *range, long long value) {
+	if (range->min == value) {
+		if (range->min == LLONG_MAX) {
+			range->max = LLONG_MIN;
+		} else {
+			++range->min;
+		}
+	}
+
+	if (range->max == value) {
+		if (range->max == LLONG_MIN) {
+			range->min = LLONG_MAX;
+		} else {
+			--range->max;
+		}
+	}
+}
+
+/** Compute the union of two ranges. */
+static void range_union(struct range *result, const struct range *lhs, const struct range *rhs) {
+	result->min = min_value(lhs->min, rhs->min);
+	result->max = max_value(lhs->max, rhs->max);
+}
+
+/** Check if a range contains no values. */
+static bool range_is_impossible(const struct range *range) {
+	return range->min > range->max;
+}
+
+/** Set a range to contain no values. */
+static void set_range_impossible(struct range *range) {
+	range->min = LLONG_MAX;
+	range->max = LLONG_MIN;
+}
+
+/**
+ * Types of ranges we track.
+ */
+enum range_type {
+	/** Search tree depth. */
+	DEPTH_RANGE,
+	/** Group ID. */
+	GID_RANGE,
+	/** Inode number.  */
+	INUM_RANGE,
+	/** Hard link count. */
+	LINKS_RANGE,
+	/** File size. */
+	SIZE_RANGE,
+	/** User ID. */
+	UID_RANGE,
+	/** The number of range_types. */
+	RANGE_TYPES,
+};
+
+/**
+ * A possibly-known value of a predicate.
+ */
+enum known_pred {
+	/** The state is impossible to reach. */
+	PRED_IMPOSSIBLE = -2,
+	/** The value of the predicate is not known. */
+	PRED_UNKNOWN = -1,
+	/** The predicate is known to be false. */
+	PRED_FALSE = false,
+	/** The predicate is known to be true. */
+	PRED_TRUE = true,
+};
+
+/** Make a predicate known. */
+static void constrain_pred(enum known_pred *pred, bool value) {
+	if (*pred == PRED_UNKNOWN) {
+		*pred = value;
+	} else if (*pred == !value) {
+		*pred = PRED_IMPOSSIBLE;
+	}
+}
+
+/** Compute the union of two known predicates. */
+static enum known_pred pred_union(enum known_pred lhs, enum known_pred rhs) {
+	if (lhs == PRED_IMPOSSIBLE) {
+		return rhs;
+	} else if (rhs == PRED_IMPOSSIBLE) {
+		return lhs;
+	} else if (lhs == rhs) {
+		return lhs;
+	} else {
+		return PRED_UNKNOWN;
+	}
+}
+
+/**
+ * Types of predicates we track.
+ */
+enum pred_type {
+	/** -readable */
+	READABLE_PRED,
+	/** -writable */
+	WRITABLE_PRED,
+	/** -executable */
+	EXECUTABLE_PRED,
+	/** -acl */
+	ACL_PRED,
+	/** -capable */
+	CAPABLE_PRED,
+	/** -empty */
+	EMPTY_PRED,
+	/** -hidden */
+	HIDDEN_PRED,
+	/** -nogroup */
+	NOGROUP_PRED,
+	/** -nouser */
+	NOUSER_PRED,
+	/** -sparse */
+	SPARSE_PRED,
+	/** -xattr */
+	XATTR_PRED,
+	/** The number of pred_types. */
+	PRED_TYPES,
+};
+
+/**
+ * Data flow facts about an evaluation point.
+ */
+struct opt_facts {
+	/** The value ranges we track. */
+	struct range ranges[RANGE_TYPES];
+
+	/** The predicates we track. */
+	enum known_pred preds[PRED_TYPES];
+
+	/** Bitmask of possible file types. */
+	unsigned int types;
+	/** Bitmask of possible link target types. */
+	unsigned int xtypes;
+};
+
+/** Initialize some data flow facts. */
+static void facts_init(struct opt_facts *facts) {
+	for (int i = 0; i < RANGE_TYPES; ++i) {
+		struct range *range = &facts->ranges[i];
+		range->min = 0; // All ranges we currently track are non-negative
+		range->max = LLONG_MAX;
+	}
+
+	for (int i = 0; i < PRED_TYPES; ++i) {
+		facts->preds[i] = PRED_UNKNOWN;
+	}
+
+	facts->types = ~0;
+	facts->xtypes = ~0;
+}
+
+/** Compute the union of two fact sets. */
+static void facts_union(struct opt_facts *result, const struct opt_facts *lhs, const struct opt_facts *rhs) {
+	for (int i = 0; i < RANGE_TYPES; ++i) {
+		range_union(&result->ranges[i], &lhs->ranges[i], &rhs->ranges[i]);
+	}
+
+	for (int i = 0; i < PRED_TYPES; ++i) {
+		result->preds[i] = pred_union(lhs->preds[i], rhs->preds[i]);
+	}
+
+	result->types = lhs->types | rhs->types;
+	result->xtypes = lhs->xtypes | rhs->xtypes;
+}
+
+/** Determine whether a fact set is impossible. */
+static bool facts_are_impossible(const struct opt_facts *facts) {
+	for (int i = 0; i < RANGE_TYPES; ++i) {
+		if (range_is_impossible(&facts->ranges[i])) {
+			return true;
+		}
+	}
+
+	for (int i = 0; i < PRED_TYPES; ++i) {
+		if (facts->preds[i] == PRED_IMPOSSIBLE) {
+			return true;
+		}
+	}
+
+	if (!facts->types || !facts->xtypes) {
+		return true;
+	}
+
+	return false;
+}
+
+/** Set some facts to be impossible. */
+static void set_facts_impossible(struct opt_facts *facts) {
+	for (int i = 0; i < RANGE_TYPES; ++i) {
+		set_range_impossible(&facts->ranges[i]);
+	}
+
+	for (int i = 0; i < PRED_TYPES; ++i) {
+		facts->preds[i] = PRED_IMPOSSIBLE;
+	}
+
+	facts->types = 0;
+	facts->xtypes = 0;
+}
+
+/**
+ * Optimizer state.
+ */
+struct opt_state {
+	/** The context we're optimizing. */
+	const struct bfs_ctx *ctx;
+
+	/** Data flow facts before this expression is evaluated. */
+	struct opt_facts facts;
+	/** Data flow facts after this expression returns true. */
+	struct opt_facts facts_when_true;
+	/** Data flow facts after this expression returns false. */
+	struct opt_facts facts_when_false;
+	/** Data flow facts before any side-effecting expressions are evaluated. */
+	struct opt_facts *facts_when_impure;
+};
+
+/** Log an optimization. */
+BFS_FORMATTER(3, 4)
+static bool opt_debug(const struct opt_state *state, int level, const char *format, ...) {
+	assert(state->ctx->optlevel >= level);
+
+	if (bfs_debug(state->ctx, DEBUG_OPT, "${cyn}-O%d${rs}: ", level)) {
+		va_list args;
+		va_start(args, format);
+		cvfprintf(state->ctx->cerr, format, args);
+		va_end(args);
+		return true;
+	} else {
+		return false;
+	}
+}
+
+/** Warn about an expression. */
+BFS_FORMATTER(3, 4)
+static void opt_warning(const struct opt_state *state, const struct bfs_expr *expr, const char *format, ...) {
+	if (bfs_expr_warning(state->ctx, expr)) {
+		va_list args;
+		va_start(args, format);
+		bfs_warning(state->ctx, format, args);
+		va_end(args);
+	}
+}
+
+/** Extract a child expression, freeing the outer expression. */
+static struct bfs_expr *extract_child_expr(struct bfs_expr *expr, struct bfs_expr **child) {
+	struct bfs_expr *ret = *child;
+	*child = NULL;
+	bfs_expr_free(expr);
+	return ret;
+}
+
+/**
+ * Negate an expression.
+ */
+static struct bfs_expr *negate_expr(struct bfs_expr *rhs, char **argv) {
+	if (rhs->eval_fn == eval_not) {
+		return extract_child_expr(rhs, &rhs->rhs);
+	}
+
+	struct bfs_expr *expr = bfs_expr_new(eval_not, 1, argv);
+	if (!expr) {
+		bfs_expr_free(rhs);
+		return NULL;
+	}
+
+	if (argv == &fake_not_arg) {
+		expr->synthetic = true;
+	}
+
+	expr->lhs = NULL;
+	expr->rhs = rhs;
+	return expr;
+}
+
+static struct bfs_expr *optimize_not_expr(const struct opt_state *state, struct bfs_expr *expr);
+static struct bfs_expr *optimize_and_expr(const struct opt_state *state, struct bfs_expr *expr);
+static struct bfs_expr *optimize_or_expr(const struct opt_state *state, struct bfs_expr *expr);
+
+/**
+ * Apply De Morgan's laws.
+ */
+static struct bfs_expr *de_morgan(const struct opt_state *state, struct bfs_expr *expr, char **argv) {
+	bool debug = opt_debug(state, 1, "De Morgan's laws: %pe ", expr);
+
+	struct bfs_expr *parent = negate_expr(expr, argv);
+	if (!parent) {
+		return NULL;
+	}
+
+	bool has_parent = true;
+	if (parent->eval_fn != eval_not) {
+		expr = parent;
+		has_parent = false;
+	}
+
+	assert(expr->eval_fn == eval_and || expr->eval_fn == eval_or);
+	if (expr->eval_fn == eval_and) {
+		expr->eval_fn = eval_or;
+		expr->argv = &fake_or_arg;
+	} else {
+		expr->eval_fn = eval_and;
+		expr->argv = &fake_and_arg;
+	}
+	expr->synthetic = true;
+
+	expr->lhs = negate_expr(expr->lhs, argv);
+	expr->rhs = negate_expr(expr->rhs, argv);
+	if (!expr->lhs || !expr->rhs) {
+		bfs_expr_free(parent);
+		return NULL;
+	}
+
+	if (debug) {
+		cfprintf(state->ctx->cerr, "<==> %pe\n", parent);
+	}
+
+	if (expr->lhs->eval_fn == eval_not) {
+		expr->lhs = optimize_not_expr(state, expr->lhs);
+	}
+	if (expr->rhs->eval_fn == eval_not) {
+		expr->rhs = optimize_not_expr(state, expr->rhs);
+	}
+	if (!expr->lhs || !expr->rhs) {
+		bfs_expr_free(parent);
+		return NULL;
+	}
+
+	if (expr->eval_fn == eval_and) {
+		expr = optimize_and_expr(state, expr);
+	} else {
+		expr = optimize_or_expr(state, expr);
+	}
+	if (has_parent) {
+		parent->rhs = expr;
+	} else {
+		parent = expr;
+	}
+	if (!expr) {
+		bfs_expr_free(parent);
+		return NULL;
+	}
+
+	if (has_parent) {
+		parent = optimize_not_expr(state, parent);
+	}
+	return parent;
+}
+
+/** Optimize an expression recursively. */
+static struct bfs_expr *optimize_expr_recursive(struct opt_state *state, struct bfs_expr *expr);
+
+/**
+ * Optimize a negation.
+ */
+static struct bfs_expr *optimize_not_expr(const struct opt_state *state, struct bfs_expr *expr) {
+	assert(expr->eval_fn == eval_not);
+
+	struct bfs_expr *rhs = expr->rhs;
+
+	int optlevel = state->ctx->optlevel;
+	if (optlevel >= 1) {
+		if (rhs == &bfs_true) {
+			opt_debug(state, 1, "constant propagation: %pe <==> %pe\n", expr, &bfs_false);
+			bfs_expr_free(expr);
+			return &bfs_false;
+		} else if (rhs == &bfs_false) {
+			opt_debug(state, 1, "constant propagation: %pe <==> %pe\n", expr, &bfs_true);
+			bfs_expr_free(expr);
+			return &bfs_true;
+		} else if (rhs->eval_fn == eval_not) {
+			opt_debug(state, 1, "double negation: %pe <==> %pe\n", expr, rhs->rhs);
+			return extract_child_expr(expr, &rhs->rhs);
+		} else if (bfs_expr_never_returns(rhs)) {
+			opt_debug(state, 1, "reachability: %pe <==> %pe\n", expr, rhs);
+			return extract_child_expr(expr, &expr->rhs);
+		} else if ((rhs->eval_fn == eval_and || rhs->eval_fn == eval_or)
+			   && (rhs->lhs->eval_fn == eval_not || rhs->rhs->eval_fn == eval_not)) {
+			return de_morgan(state, expr, expr->argv);
+		}
+	}
+
+	expr->pure = rhs->pure;
+	expr->always_true = rhs->always_false;
+	expr->always_false = rhs->always_true;
+	expr->cost = rhs->cost;
+	expr->probability = 1.0 - rhs->probability;
+
+	return expr;
+}
+
+/** Optimize a negation recursively. */
+static struct bfs_expr *optimize_not_expr_recursive(struct opt_state *state, struct bfs_expr *expr) {
+	struct opt_state rhs_state = *state;
+	expr->rhs = optimize_expr_recursive(&rhs_state, expr->rhs);
+	if (!expr->rhs) {
+		goto fail;
+	}
+
+	state->facts_when_true = rhs_state.facts_when_false;
+	state->facts_when_false = rhs_state.facts_when_true;
+
+	return optimize_not_expr(state, expr);
+
+fail:
+	bfs_expr_free(expr);
+	return NULL;
+}
+
+/** Optimize a conjunction. */
+static struct bfs_expr *optimize_and_expr(const struct opt_state *state, struct bfs_expr *expr) {
+	assert(expr->eval_fn == eval_and);
+
+	struct bfs_expr *lhs = expr->lhs;
+	struct bfs_expr *rhs = expr->rhs;
+
+	const struct bfs_ctx *ctx = state->ctx;
+	int optlevel = ctx->optlevel;
+	if (optlevel >= 1) {
+		if (lhs == &bfs_true) {
+			opt_debug(state, 1, "conjunction elimination: %pe <==> %pe\n", expr, rhs);
+			return extract_child_expr(expr, &expr->rhs);
+		} else if (rhs == &bfs_true) {
+			opt_debug(state, 1, "conjunction elimination: %pe <==> %pe\n", expr, lhs);
+			return extract_child_expr(expr, &expr->lhs);
+		} else if (lhs->always_false) {
+			opt_debug(state, 1, "short-circuit: %pe <==> %pe\n", expr, lhs);
+			opt_warning(state, expr->rhs, "This expression is unreachable.\n\n");
+			return extract_child_expr(expr, &expr->lhs);
+		} else if (lhs->always_true && rhs == &bfs_false) {
+			bool debug = opt_debug(state, 1, "strength reduction: %pe <==> ", expr);
+			struct bfs_expr *ret = extract_child_expr(expr, &expr->lhs);
+			ret = negate_expr(ret, &fake_not_arg);
+			if (debug && ret) {
+				cfprintf(ctx->cerr, "%pe\n", ret);
+			}
+			return ret;
+		} else if (optlevel >= 2 && lhs->pure && rhs == &bfs_false) {
+			opt_debug(state, 2, "purity: %pe <==> %pe\n", expr, rhs);
+			opt_warning(state, expr->lhs, "The result of this expression is ignored.\n\n");
+			return extract_child_expr(expr, &expr->rhs);
+		} else if (lhs->eval_fn == eval_not && rhs->eval_fn == eval_not) {
+			return de_morgan(state, expr, expr->lhs->argv);
+		}
+	}
+
+	expr->pure = lhs->pure && rhs->pure;
+	expr->always_true = lhs->always_true && rhs->always_true;
+	expr->always_false = lhs->always_false || rhs->always_false;
+	expr->cost = lhs->cost + lhs->probability*rhs->cost;
+	expr->probability = lhs->probability*rhs->probability;
+
+	return expr;
+}
+
+/** Optimize a conjunction recursively. */
+static struct bfs_expr *optimize_and_expr_recursive(struct opt_state *state, struct bfs_expr *expr) {
+	struct opt_state lhs_state = *state;
+	expr->lhs = optimize_expr_recursive(&lhs_state, expr->lhs);
+	if (!expr->lhs) {
+		goto fail;
+	}
+
+	struct opt_state rhs_state = *state;
+	rhs_state.facts = lhs_state.facts_when_true;
+	expr->rhs = optimize_expr_recursive(&rhs_state, expr->rhs);
+	if (!expr->rhs) {
+		goto fail;
+	}
+
+	state->facts_when_true = rhs_state.facts_when_true;
+	facts_union(&state->facts_when_false, &lhs_state.facts_when_false, &rhs_state.facts_when_false);
+
+	return optimize_and_expr(state, expr);
+
+fail:
+	bfs_expr_free(expr);
+	return NULL;
+}
+
+/** Optimize a disjunction. */
+static struct bfs_expr *optimize_or_expr(const struct opt_state *state, struct bfs_expr *expr) {
+	assert(expr->eval_fn == eval_or);
+
+	struct bfs_expr *lhs = expr->lhs;
+	struct bfs_expr *rhs = expr->rhs;
+
+	const struct bfs_ctx *ctx = state->ctx;
+	int optlevel = ctx->optlevel;
+	if (optlevel >= 1) {
+		if (lhs->always_true) {
+			opt_debug(state, 1, "short-circuit: %pe <==> %pe\n", expr, lhs);
+			opt_warning(state, expr->rhs, "This expression is unreachable.\n\n");
+			return extract_child_expr(expr, &expr->lhs);
+		} else if (lhs == &bfs_false) {
+			opt_debug(state, 1, "disjunctive syllogism: %pe <==> %pe\n", expr, rhs);
+			return extract_child_expr(expr, &expr->rhs);
+		} else if (rhs == &bfs_false) {
+			opt_debug(state, 1, "disjunctive syllogism: %pe <==> %pe\n", expr, lhs);
+			return extract_child_expr(expr, &expr->lhs);
+		} else if (lhs->always_false && rhs == &bfs_true) {
+			bool debug = opt_debug(state, 1, "strength reduction: %pe <==> ", expr);
+			struct bfs_expr *ret = extract_child_expr(expr, &expr->lhs);
+			ret = negate_expr(ret, &fake_not_arg);
+			if (debug && ret) {
+				cfprintf(ctx->cerr, "%pe\n", ret);
+			}
+			return ret;
+		} else if (optlevel >= 2 && lhs->pure && rhs == &bfs_true) {
+			opt_debug(state, 2, "purity: %pe <==> %pe\n", expr, rhs);
+			opt_warning(state, expr->lhs, "The result of this expression is ignored.\n\n");
+			return extract_child_expr(expr, &expr->rhs);
+		} else if (lhs->eval_fn == eval_not && rhs->eval_fn == eval_not) {
+			return de_morgan(state, expr, expr->lhs->argv);
+		}
+	}
+
+	expr->pure = lhs->pure && rhs->pure;
+	expr->always_true = lhs->always_true || rhs->always_true;
+	expr->always_false = lhs->always_false && rhs->always_false;
+	expr->cost = lhs->cost + (1 - lhs->probability)*rhs->cost;
+	expr->probability = lhs->probability + rhs->probability - lhs->probability*rhs->probability;
+
+	return expr;
+}
+
+/** Optimize a disjunction recursively. */
+static struct bfs_expr *optimize_or_expr_recursive(struct opt_state *state, struct bfs_expr *expr) {
+	struct opt_state lhs_state = *state;
+	expr->lhs = optimize_expr_recursive(&lhs_state, expr->lhs);
+	if (!expr->lhs) {
+		goto fail;
+	}
+
+	struct opt_state rhs_state = *state;
+	rhs_state.facts = lhs_state.facts_when_false;
+	expr->rhs = optimize_expr_recursive(&rhs_state, expr->rhs);
+	if (!expr->rhs) {
+		goto fail;
+	}
+
+	facts_union(&state->facts_when_true, &lhs_state.facts_when_true, &rhs_state.facts_when_true);
+	state->facts_when_false = rhs_state.facts_when_false;
+
+	return optimize_or_expr(state, expr);
+
+fail:
+	bfs_expr_free(expr);
+	return NULL;
+}
+
+/** Optimize an expression in an ignored-result context. */
+static struct bfs_expr *ignore_result(const struct opt_state *state, struct bfs_expr *expr) {
+	int optlevel = state->ctx->optlevel;
+
+	if (optlevel >= 1) {
+		while (true) {
+			if (expr->eval_fn == eval_not) {
+				opt_debug(state, 1, "ignored result: %pe --> %pe\n", expr, expr->rhs);
+				opt_warning(state, expr, "The result of this expression is ignored.\n\n");
+				expr = extract_child_expr(expr, &expr->rhs);
+			} else if (optlevel >= 2
+			           && (expr->eval_fn == eval_and || expr->eval_fn == eval_or || expr->eval_fn == eval_comma)
+			           && expr->rhs->pure) {
+				opt_debug(state, 2, "ignored result: %pe --> %pe\n", expr, expr->lhs);
+				opt_warning(state, expr->rhs, "The result of this expression is ignored.\n\n");
+				expr = extract_child_expr(expr, &expr->lhs);
+			} else {
+				break;
+			}
+		}
+
+		if (optlevel >= 2 && expr->pure && expr != &bfs_false) {
+			opt_debug(state, 2, "ignored result: %pe --> %pe\n", expr, &bfs_false);
+			opt_warning(state, expr, "The result of this expression is ignored.\n\n");
+			bfs_expr_free(expr);
+			expr = &bfs_false;
+		}
+	}
+
+	return expr;
+}
+
+/** Optimize a comma expression. */
+static struct bfs_expr *optimize_comma_expr(const struct opt_state *state, struct bfs_expr *expr) {
+	assert(expr->eval_fn == eval_comma);
+
+	struct bfs_expr *lhs = expr->lhs;
+	struct bfs_expr *rhs = expr->rhs;
+
+	int optlevel = state->ctx->optlevel;
+	if (optlevel >= 1) {
+		lhs = expr->lhs = ignore_result(state, lhs);
+
+		if (bfs_expr_never_returns(lhs)) {
+			opt_debug(state, 1, "reachability: %pe <==> %pe\n", expr, lhs);
+			opt_warning(state, expr->rhs, "This expression is unreachable.\n\n");
+			return extract_child_expr(expr, &expr->lhs);
+		} else if ((lhs->always_true && rhs == &bfs_true)
+			   || (lhs->always_false && rhs == &bfs_false)) {
+			opt_debug(state, 1, "redundancy elimination: %pe <==> %pe\n", expr, lhs);
+			return extract_child_expr(expr, &expr->lhs);
+		} else if (optlevel >= 2 && lhs->pure) {
+			opt_debug(state, 2, "purity: %pe <==> %pe\n", expr, rhs);
+			opt_warning(state, expr->lhs, "The result of this expression is ignored.\n\n");
+			return extract_child_expr(expr, &expr->rhs);
+		}
+	}
+
+	expr->pure = lhs->pure && rhs->pure;
+	expr->always_true = bfs_expr_never_returns(lhs) || rhs->always_true;
+	expr->always_false = bfs_expr_never_returns(lhs) || rhs->always_false;
+	expr->cost = lhs->cost + rhs->cost;
+	expr->probability = rhs->probability;
+
+	return expr;
+}
+
+/** Optimize a comma expression recursively. */
+static struct bfs_expr *optimize_comma_expr_recursive(struct opt_state *state, struct bfs_expr *expr) {
+	struct opt_state lhs_state = *state;
+	expr->lhs = optimize_expr_recursive(&lhs_state, expr->lhs);
+	if (!expr->lhs) {
+		goto fail;
+	}
+
+	struct opt_state rhs_state = *state;
+	facts_union(&rhs_state.facts, &lhs_state.facts_when_true, &lhs_state.facts_when_false);
+	expr->rhs = optimize_expr_recursive(&rhs_state, expr->rhs);
+	if (!expr->rhs) {
+		goto fail;
+	}
+
+	return optimize_comma_expr(state, expr);
+
+fail:
+	bfs_expr_free(expr);
+	return NULL;
+}
+
+/** Infer data flow facts about a predicate. */
+static void infer_pred_facts(struct opt_state *state, enum pred_type pred) {
+	constrain_pred(&state->facts_when_true.preds[pred], true);
+	constrain_pred(&state->facts_when_false.preds[pred], false);
+}
+
+/** Infer data flow facts about an -{execut,read,writ}able expression. */
+static void infer_access_facts(struct opt_state *state, const struct bfs_expr *expr) {
+	if (expr->num & R_OK) {
+		infer_pred_facts(state, READABLE_PRED);
+	}
+	if (expr->num & W_OK) {
+		infer_pred_facts(state, WRITABLE_PRED);
+	}
+	if (expr->num & X_OK) {
+		infer_pred_facts(state, EXECUTABLE_PRED);
+	}
+}
+
+/** Infer data flow facts about an icmp-style ([+-]N) expression. */
+static void infer_icmp_facts(struct opt_state *state, const struct bfs_expr *expr, enum range_type type) {
+	struct range *range_when_true = &state->facts_when_true.ranges[type];
+	struct range *range_when_false = &state->facts_when_false.ranges[type];
+	long long value = expr->num;
+
+	switch (expr->int_cmp) {
+	case BFS_INT_EQUAL:
+		constrain_min(range_when_true, value);
+		constrain_max(range_when_true, value);
+		range_remove(range_when_false, value);
+		break;
+
+	case BFS_INT_LESS:
+		constrain_min(range_when_false, value);
+		constrain_max(range_when_true, value);
+		range_remove(range_when_true, value);
+		break;
+
+	case BFS_INT_GREATER:
+		constrain_max(range_when_false, value);
+		constrain_min(range_when_true, value);
+		range_remove(range_when_true, value);
+		break;
+	}
+}
+
+/** Infer data flow facts about a -gid expression. */
+static void infer_gid_facts(struct opt_state *state, const struct bfs_expr *expr) {
+	infer_icmp_facts(state, expr, GID_RANGE);
+
+	const struct bfs_groups *groups = bfs_ctx_groups(state->ctx);
+	struct range *range = &state->facts_when_true.ranges[GID_RANGE];
+	if (groups && range->min == range->max) {
+		gid_t gid = range->min;
+		bool nogroup = !bfs_getgrgid(groups, gid);
+		constrain_pred(&state->facts_when_true.preds[NOGROUP_PRED], nogroup);
+	}
+}
+
+/** Infer data flow facts about a -uid expression. */
+static void infer_uid_facts(struct opt_state *state, const struct bfs_expr *expr) {
+	infer_icmp_facts(state, expr, UID_RANGE);
+
+	const struct bfs_users *users = bfs_ctx_users(state->ctx);
+	struct range *range = &state->facts_when_true.ranges[UID_RANGE];
+	if (users && range->min == range->max) {
+		uid_t uid = range->min;
+		bool nouser = !bfs_getpwuid(users, uid);
+		constrain_pred(&state->facts_when_true.preds[NOUSER_PRED], nouser);
+	}
+}
+
+/** Infer data flow facts about a -samefile expression. */
+static void infer_samefile_facts(struct opt_state *state, const struct bfs_expr *expr) {
+	struct range *range_when_true = &state->facts_when_true.ranges[INUM_RANGE];
+	constrain_min(range_when_true, expr->ino);
+	constrain_max(range_when_true, expr->ino);
+}
+
+/** Infer data flow facts about a -type expression. */
+static void infer_type_facts(struct opt_state *state, const struct bfs_expr *expr) {
+	state->facts_when_true.types &= expr->num;
+	state->facts_when_false.types &= ~expr->num;
+}
+
+/** Infer data flow facts about an -xtype expression. */
+static void infer_xtype_facts(struct opt_state *state, const struct bfs_expr *expr) {
+	state->facts_when_true.xtypes &= expr->num;
+	state->facts_when_false.xtypes &= ~expr->num;
+}
+
+static struct bfs_expr *optimize_expr_recursive(struct opt_state *state, struct bfs_expr *expr) {
+	int optlevel = state->ctx->optlevel;
+
+	state->facts_when_true = state->facts;
+	state->facts_when_false = state->facts;
+
+	if (optlevel >= 2 && facts_are_impossible(&state->facts)) {
+		opt_debug(state, 2, "reachability: %pe --> %pe\n", expr, &bfs_false);
+		opt_warning(state, expr, "This expression is unreachable.\n\n");
+		bfs_expr_free(expr);
+		expr = &bfs_false;
+		goto done;
+	}
+
+	if (!bfs_expr_has_children(expr) && !expr->pure) {
+		facts_union(state->facts_when_impure, state->facts_when_impure, &state->facts);
+	}
+
+	if (expr->eval_fn == eval_access) {
+		infer_access_facts(state, expr);
+	} else if (expr->eval_fn == eval_acl) {
+		infer_pred_facts(state, ACL_PRED);
+	} else if (expr->eval_fn == eval_capable) {
+		infer_pred_facts(state, CAPABLE_PRED);
+	} else if (expr->eval_fn == eval_depth) {
+		infer_icmp_facts(state, expr, DEPTH_RANGE);
+	} else if (expr->eval_fn == eval_empty) {
+		infer_pred_facts(state, EMPTY_PRED);
+	} else if (expr->eval_fn == eval_gid) {
+		infer_gid_facts(state, expr);
+	} else if (expr->eval_fn == eval_hidden) {
+		infer_pred_facts(state, HIDDEN_PRED);
+	} else if (expr->eval_fn == eval_inum) {
+		infer_icmp_facts(state, expr, INUM_RANGE);
+	} else if (expr->eval_fn == eval_links) {
+		infer_icmp_facts(state, expr, LINKS_RANGE);
+	} else if (expr->eval_fn == eval_nogroup) {
+		infer_pred_facts(state, NOGROUP_PRED);
+	} else if (expr->eval_fn == eval_nouser) {
+		infer_pred_facts(state, NOUSER_PRED);
+	} else if (expr->eval_fn == eval_samefile) {
+		infer_samefile_facts(state, expr);
+	} else if (expr->eval_fn == eval_size) {
+		infer_icmp_facts(state, expr, SIZE_RANGE);
+	} else if (expr->eval_fn == eval_sparse) {
+		infer_pred_facts(state, SPARSE_PRED);
+	} else if (expr->eval_fn == eval_type) {
+		infer_type_facts(state, expr);
+	} else if (expr->eval_fn == eval_uid) {
+		infer_uid_facts(state, expr);
+	} else if (expr->eval_fn == eval_xattr) {
+		infer_pred_facts(state, XATTR_PRED);
+	} else if (expr->eval_fn == eval_xtype) {
+		infer_xtype_facts(state, expr);
+	} else if (expr->eval_fn == eval_not) {
+		expr = optimize_not_expr_recursive(state, expr);
+	} else if (expr->eval_fn == eval_and) {
+		expr = optimize_and_expr_recursive(state, expr);
+	} else if (expr->eval_fn == eval_or) {
+		expr = optimize_or_expr_recursive(state, expr);
+	} else if (expr->eval_fn == eval_comma) {
+		expr = optimize_comma_expr_recursive(state, expr);
+	}
+
+	if (!expr) {
+		goto done;
+	}
+
+	if (bfs_expr_has_children(expr)) {
+		struct bfs_expr *lhs = expr->lhs;
+		struct bfs_expr *rhs = expr->rhs;
+		if (rhs) {
+			expr->persistent_fds = rhs->persistent_fds;
+			expr->ephemeral_fds = rhs->ephemeral_fds;
+		}
+		if (lhs) {
+			expr->persistent_fds += lhs->persistent_fds;
+			if (lhs->ephemeral_fds > expr->ephemeral_fds) {
+				expr->ephemeral_fds = lhs->ephemeral_fds;
+			}
+		}
+	}
+
+	if (expr->always_true) {
+		set_facts_impossible(&state->facts_when_false);
+	}
+	if (expr->always_false) {
+		set_facts_impossible(&state->facts_when_true);
+	}
+
+	if (optlevel < 2 || expr == &bfs_true || expr == &bfs_false) {
+		goto done;
+	}
+
+	if (facts_are_impossible(&state->facts_when_true)) {
+		if (expr->pure) {
+			opt_debug(state, 2, "data flow: %pe --> %pe\n", expr, &bfs_false);
+			opt_warning(state, expr, "This expression is always false.\n\n");
+			bfs_expr_free(expr);
+			expr = &bfs_false;
+		} else {
+			expr->always_false = true;
+			expr->probability = 0.0;
+		}
+	} else if (facts_are_impossible(&state->facts_when_false)) {
+		if (expr->pure) {
+			opt_debug(state, 2, "data flow: %pe --> %pe\n", expr, &bfs_true);
+			opt_warning(state, expr, "This expression is always true.\n\n");
+			bfs_expr_free(expr);
+			expr = &bfs_true;
+		} else {
+			expr->always_true = true;
+			expr->probability = 1.0;
+		}
+	}
+
+done:
+	return expr;
+}
+
+/** Swap the children of a binary expression if it would reduce the cost. */
+static bool reorder_expr(const struct opt_state *state, struct bfs_expr *expr, float swapped_cost) {
+	if (swapped_cost < expr->cost) {
+		bool debug = opt_debug(state, 3, "cost: %pe <==> ", expr);
+		struct bfs_expr *lhs = expr->lhs;
+		expr->lhs = expr->rhs;
+		expr->rhs = lhs;
+		if (debug) {
+			cfprintf(state->ctx->cerr, "%pe (~${ylw}%g${rs} --> ~${ylw}%g${rs})\n", expr, expr->cost, swapped_cost);
+		}
+		expr->cost = swapped_cost;
+		return true;
+	} else {
+		return false;
+	}
+}
+
+/**
+ * Recursively reorder sub-expressions to reduce the overall cost.
+ *
+ * @param expr
+ *         The expression to optimize.
+ * @return
+ *         Whether any subexpression was reordered.
+ */
+static bool reorder_expr_recursive(const struct opt_state *state, struct bfs_expr *expr) {
+	if (!bfs_expr_has_children(expr)) {
+		return false;
+	}
+
+	struct bfs_expr *lhs = expr->lhs;
+	struct bfs_expr *rhs = expr->rhs;
+
+	bool ret = false;
+	if (lhs) {
+		ret |= reorder_expr_recursive(state, lhs);
+	}
+	if (rhs) {
+		ret |= reorder_expr_recursive(state, rhs);
+	}
+
+	if (expr->eval_fn == eval_and || expr->eval_fn == eval_or) {
+		if (lhs->pure && rhs->pure) {
+			float rhs_prob = expr->eval_fn == eval_and ? rhs->probability : 1.0 - rhs->probability;
+			float swapped_cost = rhs->cost + rhs_prob*lhs->cost;
+			ret |= reorder_expr(state, expr, swapped_cost);
+		}
+	}
+
+	return ret;
+}
+
+/**
+ * Optimize a top-level expression.
+ */
+static struct bfs_expr *optimize_expr(struct opt_state *state, struct bfs_expr *expr) {
+	struct opt_facts saved_impure = *state->facts_when_impure;
+
+	expr = optimize_expr_recursive(state, expr);
+	if (!expr) {
+		return NULL;
+	}
+
+	if (state->ctx->optlevel >= 3 && reorder_expr_recursive(state, expr)) {
+		// Re-do optimizations to account for the new ordering
+		*state->facts_when_impure = saved_impure;
+		expr = optimize_expr_recursive(state, expr);
+		if (!expr) {
+			return NULL;
+		}
+	}
+
+	return expr;
+}
+
+int bfs_optimize(struct bfs_ctx *ctx) {
+	bfs_ctx_dump(ctx, DEBUG_OPT);
+
+	struct opt_facts facts_when_impure;
+	set_facts_impossible(&facts_when_impure);
+
+	struct opt_state state = {
+		.ctx = ctx,
+		.facts_when_impure = &facts_when_impure,
+	};
+	facts_init(&state.facts);
+
+	ctx->exclude = optimize_expr(&state, ctx->exclude);
+	if (!ctx->exclude) {
+		return -1;
+	}
+
+	// Only non-excluded files are evaluated
+	state.facts = state.facts_when_false;
+
+	struct range *depth = &state.facts.ranges[DEPTH_RANGE];
+	constrain_min(depth, ctx->mindepth);
+	constrain_max(depth, ctx->maxdepth);
+
+	ctx->expr = optimize_expr(&state, ctx->expr);
+	if (!ctx->expr) {
+		return -1;
+	}
+
+	ctx->expr = ignore_result(&state, ctx->expr);
+
+	if (facts_are_impossible(&facts_when_impure)) {
+		bfs_warning(ctx, "This command won't do anything.\n\n");
+	}
+
+	const struct range *depth_when_impure = &facts_when_impure.ranges[DEPTH_RANGE];
+	long long mindepth = depth_when_impure->min;
+	long long maxdepth = depth_when_impure->max;
+
+	int optlevel = ctx->optlevel;
+
+	if (optlevel >= 2 && mindepth > ctx->mindepth) {
+		if (mindepth > INT_MAX) {
+			mindepth = INT_MAX;
+		}
+		ctx->mindepth = mindepth;
+		opt_debug(&state, 2, "data flow: mindepth --> %d\n", ctx->mindepth);
+	}
+
+	if (optlevel >= 4 && maxdepth < ctx->maxdepth) {
+		if (maxdepth < INT_MIN) {
+			maxdepth = INT_MIN;
+		}
+		ctx->maxdepth = maxdepth;
+		opt_debug(&state, 4, "data flow: maxdepth --> %d\n", ctx->maxdepth);
+	}
+
+	return 0;
+}