improve collapsible value detection (#1638)

- #1634 bars variables with cross-scope references in between to collapse
- but if assigned value is side-effect-free, no states can be modified, so it is safe to move

1 files changed, 3 insertions, 1 deletions

diff --git a/lib/compress.js b/lib/compress.js
index cfa8f230..e75d7c96 100644
--- a/lib/compress.js
+++ b/lib/compress.js
@@ -592,9 +592,10 @@ merge(Compressor.prototype, {
                     // Restrict var replacement to constants if side effects encountered.
                     if (side_effects_encountered |= lvalues_encountered) continue;
 
+                    var value_has_side_effects = var_decl.value.has_side_effects(compressor);
                     // Non-constant single use vars can only be replaced in same scope.
                     if (ref.scope !== self) {
-                        side_effects_encountered |= var_decl.value.has_side_effects(compressor);
+                        side_effects_encountered |= value_has_side_effects;
                         continue;
                     }
 
@@ -620,6 +621,7 @@ merge(Compressor.prototype, {
                                 || (parent instanceof AST_If          && node !== parent.condition)
                                 || (parent instanceof AST_Conditional && node !== parent.condition)
                                 || (node instanceof AST_SymbolRef
+                                    && value_has_side_effects
                                     && !are_references_in_scope(node.definition(), self))
                                 || (parent instanceof AST_Binary
                                     && (parent.operator == "&&" || parent.operator == "||")