Optimize various switch statements

lib/compress/index.js

@@ -53,7 +53,6 @@ import {
     AST_Boolean,
     AST_Break,
     AST_Call,
-    AST_Case,
     AST_Catch,
     AST_Chain,
     AST_Class,
@@ -109,6 +108,7 @@ import {
     AST_String,
     AST_Sub,
     AST_Switch,
+    AST_SwitchBranch,
     AST_Symbol,
     AST_SymbolBlockDeclaration,
     AST_SymbolCatch,
@@ -1592,58 +1592,251 @@ def_optimize(AST_Switch, function(self, compressor) {
                 }
             }
         }
-        if (aborts(branch)) {
-            var prev = body[body.length - 1];
-            if (aborts(prev) && prev.body.length == branch.body.length
-                && make_node(AST_BlockStatement, prev, prev).equivalent_to(make_node(AST_BlockStatement, branch, branch))) {
-                prev.body = [];
-            }
-        }
         body.push(branch);
     }
     while (i < len) eliminate_branch(self.body[i++], body[body.length - 1]);
+    self.body = body;
+
+    for (let i = 0; i < body.length; i++) {
+        let branch = body[i];
+        if (branch.body.length === 0) continue;
+        if (!aborts(branch)) continue;
+
+        for (let j = i + 1; j < body.length; i++, j++) {
+            let next = body[j];
+            if (next.body.length === 0) continue;
+            if (
+                branches_equivalent(next, branch, false)
+                || (j === body.length - 1 && branches_equivalent(next, branch, true))
+            ) {
+                branch.body = [];
+                branch = next;
+                continue;
+            }
+            break;
+        }
+    }
+
+    let default_or_exact = default_branch || exact_match;
+    default_branch = null;
+    exact_match = null;
+
+    // Prune any empty branches at the end of the switch statement.
+    {
+        let i = body.length - 1;
+        for (; i >= 0; i--) {
+            let bbody = body[i].body;
+            if (is_break(bbody[bbody.length - 1], compressor)) bbody.pop();
+            if (!is_inert_body(body[i])) break;
+        }
+        // i now points to the index of a branch that contains a body. By incrementing, it's
+        // pointing to the first branch that's empty.
+        i++;
+        if (!default_or_exact || body.indexOf(default_or_exact) >= i) {
+            // The default behavior is to do nothing. We can take advantage of that to
+            // remove all case expressions that are side-effect free that also do
+            // nothing, since they'll default to doing nothing. But we can't remove any
+            // case expressions before one that would side-effect, since they may cause
+            // the side-effect to be skipped.
+            for (let j = body.length - 1; j >= i; j--) {
+                let branch = body[j];
+                if (branch === default_or_exact) {
+                    default_or_exact = null;
+                    body.pop();
+                } else if (!branch.expression.has_side_effects(compressor)) {
+                    body.pop();
+                } else {
+                    break;
+                }
+            }
+        }
+    }
+
+
+    // Prune side-effect free branches that fall into default.
+    if (default_or_exact) {
+        let default_index = body.indexOf(default_or_exact);
+        let default_body_index = default_index;
+        for (; default_body_index < body.length - 1; default_body_index++) {
+            if (!is_inert_body(body[default_body_index])) break;
+        }
+        let side_effect_index = body.length - 1;
+        for (; side_effect_index >= 0; side_effect_index--) {
+            let branch = body[side_effect_index];
+            if (branch === default_or_exact) continue;
+            if (branch.expression.has_side_effects(compressor)) break;
+        }
+        // If the default behavior comes after any side-effect case expressions,
+        // then we can fold all side-effect free cases into the default branch.
+        // If the side-effect case is after the default, then any side-effect
+        // free cases could prevent the side-effect from occurring.
+        if (default_body_index > side_effect_index) {
+            let prev_body_index = default_index - 1;
+            for (; prev_body_index >= 0; prev_body_index--) {
+                if (!is_inert_body(body[prev_body_index])) break;
+            }
+            let before = Math.max(side_effect_index, prev_body_index) + 1;
+            let after = default_index;
+            if (side_effect_index > default_index) {
+                // If the default falls into the same body as a side-effect
+                // case, then we need preserve that case and only prune the
+                // cases after it.
+                after = side_effect_index;
+                body[side_effect_index].body = body[default_body_index].body;
+            } else {
+                // The default will be the last branch.
+                default_or_exact.body = body[default_body_index].body;
+            }
+
+            // Prune everything after the default (or last side-effect case)
+            // until the next case with a body.
+            body.splice(after + 1, default_body_index - after);
+            // Prune everything before the default that falls into it.
+            body.splice(before, default_index - before);
+        }
+    }
+
+    // See if we can remove the switch entirely if all cases (the default) fall into the same case body.
+    DEFAULT: if (default_or_exact) {
+        let i = body.findIndex(branch => !is_inert_body(branch));
+        let caseBody;
+        // `i` is equal to one of the following:
+        // - `-1`, there is no body in the switch statement.
+        // - `body.length - 1`, all cases fall into the same body.
+        // - anything else, there are multiple bodies in the switch.
+        if (i === body.length - 1) {
+            // All cases fall into the case body.
+            let branch = body[i];
+            if (has_nested_break(self)) break DEFAULT;
+
+            // This is the last case body, and we've already pruned any breaks, so it's
+            // safe to hoist.
+            caseBody = make_node(AST_BlockStatement, branch, {
+                body: branch.body
+            });
+            branch.body = [];
+        } else if (i !== -1) {
+            // If there are multiple bodies, then we cannot optimize anything.
+            break DEFAULT;
+        }
+
+        let sideEffect = body.find(branch => {
+            return (
+                branch !== default_or_exact
+                && branch.expression.has_side_effects(compressor)
+            );
+        });
+        // If no cases cause a side-effect, we can eliminate the switch entirely.
+        if (!sideEffect) {
+            return make_node(AST_BlockStatement, self, {
+                body: decl.concat(
+                    statement(self.expression),
+                    default_or_exact.expression ? statement(default_or_exact.expression) : [],
+                    caseBody || []
+                )
+            }).optimize(compressor);
+        }
+
+        // If we're this far, either there was no body or all cases fell into the same body.
+        // If there was no body, then we don't need a default branch (because the default is
+        // do nothing). If there was a body, we'll extract it to after the switch, so the
+        // switch's new default is to do nothing and we can still prune it.
+        const default_index = body.indexOf(default_or_exact);
+        body.splice(default_index, 1);
+        default_or_exact = null;
+
+        if (caseBody) {
+            // Recurse into switch statement one more time so that we can append the case body
+            // outside of the switch. This recursion will only happen once since we've pruned
+            // the default case.
+            return make_node(AST_BlockStatement, self, {
+                body: decl.concat(self, caseBody)
+            }).optimize(compressor);
+        }
+        // If we fall here, there is a default branch somewhere, there are no case bodies,
+        // and there's a side-effect somewhere. Just let the below paths take care of it.
+    }
+
     if (body.length > 0) {
         body[0].body = decl.concat(body[0].body);
     }
-    self.body = body;
-    while (branch = body[body.length - 1]) {
-        var stat = branch.body[branch.body.length - 1];
-        if (stat instanceof AST_Break && compressor.loopcontrol_target(stat) === self)
-            branch.body.pop();
-        if (branch.body.length || branch instanceof AST_Case
-            && (default_branch || branch.expression.has_side_effects(compressor))) break;
-        if (body.pop() === default_branch) default_branch = null;
-    }
+
     if (body.length == 0) {
         return make_node(AST_BlockStatement, self, {
-            body: decl.concat(make_node(AST_SimpleStatement, self.expression, {
-                body: self.expression
-            }))
+            body: decl.concat(statement(self.expression))
         }).optimize(compressor);
     }
-    if (body.length == 1 && (body[0] === exact_match || body[0] === default_branch)) {
-        var has_break = false;
-        var tw = new TreeWalker(function(node) {
-            if (has_break
-                || node instanceof AST_Lambda
-                || node instanceof AST_SimpleStatement) return true;
-            if (node instanceof AST_Break && tw.loopcontrol_target(node) === self)
-                has_break = true;
-        });
-        self.walk(tw);
-        if (!has_break) {
-            var statements = body[0].body.slice();
-            var exp = body[0].expression;
-            if (exp) statements.unshift(make_node(AST_SimpleStatement, exp, {
-                body: exp
-            }));
-            statements.unshift(make_node(AST_SimpleStatement, self.expression, {
-                body:self.expression
-            }));
-            return make_node(AST_BlockStatement, self, {
-                body: statements
+    if (body.length == 1 && !has_nested_break(self)) {
+        // This is the last case body, and we've already pruned any breaks, so it's
+        // safe to hoist.
+        let branch = body[0];
+        return make_node(AST_If, self, {
+            condition: make_node(AST_Binary, self, {
+                operator: "===",
+                left: self.expression,
+                right: branch.expression,
+            }),
+            body: make_node(AST_BlockStatement, branch, {
+                body: branch.body
+            }),
+            alternative: null
+        }).optimize(compressor);
+    }
+    if (body.length === 2 && default_or_exact && !has_nested_break(self)) {
+        let branch = body[0] === default_or_exact ? body[1] : body[0];
+        let exact_exp = default_or_exact.expression && statement(default_or_exact.expression);
+        if (aborts(body[0])) {
+            // Only the first branch body could have a break (at the last statement)
+            let first = body[0];
+            if (is_break(first.body[first.body.length - 1], compressor)) {
+                first.body.pop();
+            }
+            return make_node(AST_If, self, {
+                condition: make_node(AST_Binary, self, {
+                    operator: "===",
+                    left: self.expression,
+                    right: branch.expression,
+                }),
+                body: make_node(AST_BlockStatement, branch, {
+                    body: branch.body
+                }),
+                alternative: make_node(AST_BlockStatement, default_or_exact, {
+                    body: [].concat(
+                        exact_exp || [],
+                        default_or_exact.body
+                    )
+                })
             }).optimize(compressor);
         }
+        let operator = "===";
+        let consequent = make_node(AST_BlockStatement, branch, {
+            body: branch.body,
+        });
+        let always = make_node(AST_BlockStatement, default_or_exact, {
+            body: [].concat(
+                exact_exp || [],
+                default_or_exact.body
+            )
+        });
+        if (body[0] === default_or_exact) {
+            operator = "!==";
+            let tmp = always;
+            always = consequent;
+            consequent = tmp;
+        }
+        return make_node(AST_BlockStatement, self, {
+            body: [
+                make_node(AST_If, self, {
+                    condition: make_node(AST_Binary, self, {
+                        operator: operator,
+                        left: self.expression,
+                        right: branch.expression,
+                    }),
+                    body: consequent,
+                    alternative: null
+                })
+            ].concat(always)
+        }).optimize(compressor);
     }
     return self;
 
@@ -1654,6 +1847,50 @@ def_optimize(AST_Switch, function(self, compressor) {
             trim_unreachable_code(compressor, branch, decl);
         }
     }
+    function branches_equivalent(branch, prev, insertBreak) {
+        let bbody = branch.body;
+        let pbody = prev.body;
+        if (insertBreak) {
+            bbody = bbody.concat(make_node(AST_Break));
+        }
+        if (bbody.length !== pbody.length) return false;
+        let bblock = make_node(AST_BlockStatement, branch, { body: bbody });
+        let pblock = make_node(AST_BlockStatement, prev, { body: pbody });
+        return bblock.equivalent_to(pblock);
+    }
+    function statement(expression) {
+        return make_node(AST_SimpleStatement, expression, {
+            body: expression
+        });
+    }
+    function has_nested_break(root) {
+        let has_break = false;
+        let tw = new TreeWalker(node => {
+            if (has_break) return true;
+            if (node instanceof AST_Lambda) return true;
+            if (node instanceof AST_SimpleStatement) return true;
+            if (!is_break(node, tw)) return;
+            let parent = tw.parent();
+            if (
+                parent instanceof AST_SwitchBranch
+                && parent.body[parent.body.length - 1] === node
+            ) {
+                return;
+            }
+            has_break = true;
+        });
+        root.walk(tw);
+        return has_break;
+    }
+    function is_break(node, stack) {
+        return node instanceof AST_Break
+            && stack.loopcontrol_target(node) === self;
+    }
+    function is_inert_body(branch) {
+        return !aborts(branch) && !make_node(AST_BlockStatement, branch, {
+            body: branch.body
+        }).has_side_effects(compressor)
+    }
 });
 
 def_optimize(AST_Try, function(self, compressor) {

test/compress/functions.js

@@ -1328,9 +1328,7 @@ issue_2620_4: {
     expect: {
         var c = "FAIL";
         !function() {
-            switch (NaN) {
-              case void (c = "PASS"):
-            }
+            if (NaN === void (c = "PASS"));
         }();
         console.log(c);
     }

test/compress/issue-1750.js

@@ -16,10 +16,7 @@ case_1: {
     }
     expect: {
         var a = 0, b = 1;
-        switch (true) {
-          case a || true:
-            b = 2;
-        }
+        if (true === (a || true)) b = 2;
         console.log(a, b);
     }
     expect_stdout: "0 2"
@@ -44,10 +41,7 @@ case_2: {
     }
     expect: {
         var a = 0, b = 1;
-        switch (0) {
-          case a:
-            a = 3;
-        }
+        if (0 === a) a = 3;
         console.log(a, b);
     }
     expect_stdout: "3 1"