From 344f9c2f844b20a8ba206595c28429a196383e23 Mon Sep 17 00:00:00 2001
From: Tavian Barnes <tavianator@tavianator.com>
Date: Wed, 20 Mar 2019 18:29:16 -0400
Subject: opt: Replace -a -false/-o -true with -not when possible

---
 opt.c                            | 17 +++++++++++++++++
 tests.sh                         |  7 +++++++
 tests/test_and_false_or_true.out |  1 +
 3 files changed, 25 insertions(+)
 create mode 100644 tests/test_and_false_or_true.out

diff --git a/opt.c b/opt.c
index dd98ca0..fd2dc3c 100644
--- a/opt.c
+++ b/opt.c
@@ -50,6 +50,7 @@
 
 static char *fake_and_arg = "-a";
 static char *fake_or_arg = "-o";
+static char *fake_not_arg = "!";
 
 /**
  * A contrained integer range.
@@ -433,6 +434,14 @@ static struct expr *optimize_and_expr(const struct opt_state *state, struct expr
 		} else if (lhs->always_false) {
 			debug_opt(state, "-O1: short-circuit: %e <==> %e\n", expr, lhs);
 			return extract_child_expr(expr, &expr->lhs);
+		} else if (lhs->always_true && rhs == &expr_false) {
+			debug_opt(state, "-O1: strength reduction: %e <==> ", expr);
+			struct expr *ret = extract_child_expr(expr, &expr->lhs);
+			ret = negate_expr(ret, &fake_not_arg);
+			if (ret) {
+				debug_opt(state, "%e\n", ret);
+			}
+			return ret;
 		} else if (optlevel >= 2 && lhs->pure && rhs == &expr_false) {
 			debug_opt(state, "-O2: purity: %e <==> %e\n", expr, rhs);
 			return extract_child_expr(expr, &expr->rhs);
@@ -493,6 +502,14 @@ static struct expr *optimize_or_expr(const struct opt_state *state, struct expr
 		} else if (rhs == &expr_false) {
 			debug_opt(state, "-O1: disjunctive syllogism: %e <==> %e\n", expr, lhs);
 			return extract_child_expr(expr, &expr->lhs);
+		} else if (lhs->always_false && rhs == &expr_true) {
+			debug_opt(state, "-O1: strength reduction: %e <==> ", expr);
+			struct expr *ret = extract_child_expr(expr, &expr->lhs);
+			ret = negate_expr(ret, &fake_not_arg);
+			if (ret) {
+				debug_opt(state, "%e\n", ret);
+			}
+			return ret;
 		} else if (optlevel >= 2 && lhs->pure && rhs == &expr_true) {
 			debug_opt(state, "-O2: purity: %e <==> %e\n", expr, rhs);
 			return extract_child_expr(expr, &expr->rhs);
diff --git a/tests.sh b/tests.sh
index 2df3448..a219812 100755
--- a/tests.sh
+++ b/tests.sh
@@ -609,6 +609,7 @@ gnu_tests=(
     test_and_purity
     test_not_reachability
     test_comma_reachability
+    test_and_false_or_true
 )
 
 bfs_tests=(
@@ -2003,6 +2004,12 @@ function test_de_morgan_or() {
     bfs_diff basic \( \! -name 'foo' -o \! -type f \)
 }
 
+function test_and_false_or_true() {
+    # Test (-a lhs(always_true) false) <==> (! lhs),
+    # (-a lhs(always_false) true) <==> (! lhs)
+    bfs_diff basic -prune -false -o -true
+}
+
 function test_data_flow_depth() {
     bfs_diff basic -depth +1 -depth -4
 }
diff --git a/tests/test_and_false_or_true.out b/tests/test_and_false_or_true.out
new file mode 100644
index 0000000..15a13db
--- /dev/null
+++ b/tests/test_and_false_or_true.out
@@ -0,0 +1 @@
+basic
-- 
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