Avoid throw expression leaks

Mark "throw" used in expression context with a flag, and don't
treat it as a BB terminator in that case. This prevents us from
optimizing away the following opcodes as unreachable, which may
result in live ranges being dropped incorrectly.

Close GH-5450.

Author: nikic

Zend/tests/throw/leaks.phpt

@@ -0,0 +1,31 @@
+--TEST--
+throw expression should not leak temporaries
+--FILE--
+<?php
+
+try {
+    new stdClass(throw new Exception);
+} catch (Exception $e) {
+    echo "Caught\n";
+}
+
+try {
+    $a = [];
+    ($a + [1]) + throw new Exception;
+} catch (Exception $e) {
+    echo "Caught\n";
+}
+
+try {
+    @throw new Exception;
+} catch (Exception $e) {
+    echo "Caught\n";
+}
+var_dump(error_reporting());
+
+?>
+--EXPECT--
+Caught
+Caught
+Caught
+int(32767)

Zend/zend_compile.c

@@ -4548,10 +4548,13 @@ void zend_compile_throw(znode *result, zend_ast *ast) /* {{{ */
 	znode expr_node;
 	zend_compile_expr(&expr_node, expr_ast);
 
-	zend_emit_op(NULL, ZEND_THROW, &expr_node, NULL);
-
-	result->op_type = IS_CONST;
-	ZVAL_BOOL(&result->u.constant, 1);
+	zend_op *opline = zend_emit_op(NULL, ZEND_THROW, &expr_node, NULL);
+	if (result) {
+		/* Mark this as an "expression throw" for opcache. */
+		opline->extended_value = ZEND_THROW_IS_EXPR;
+		result->op_type = IS_CONST;
+		ZVAL_BOOL(&result->u.constant, 1);
+	}
 }
 /* }}} */
 
@@ -8791,6 +8794,9 @@ void zend_compile_stmt(zend_ast *ast) /* {{{ */
 		case ZEND_AST_HALT_COMPILER:
 			zend_compile_halt_compiler(ast);
 			break;
+		case ZEND_AST_THROW:
+			zend_compile_throw(NULL, ast);
+			break;
 		default:
 		{
 			znode result;

Zend/zend_compile.h

@@ -927,6 +927,8 @@ ZEND_API zend_string *zend_type_to_string(zend_type type);
 #define ZEND_SEND_BY_REF     1u
 #define ZEND_SEND_PREFER_REF 2u
 
+#define ZEND_THROW_IS_EXPR 1u
+
 /* The send mode and is_variadic flag are stored as part of zend_type */
 #define _ZEND_SEND_MODE_SHIFT _ZEND_TYPE_EXTRA_FLAGS_SHIFT
 #define _ZEND_IS_VARIADIC_BIT (1 << (_ZEND_TYPE_EXTRA_FLAGS_SHIFT + 2))

ext/opcache/Optimizer/zend_cfg.c

@@ -296,11 +296,17 @@ int zend_build_cfg(zend_arena **arena, const zend_op_array *op_array, uint32_t b
 			case ZEND_RETURN_BY_REF:
 			case ZEND_GENERATOR_RETURN:
 			case ZEND_EXIT:
-			case ZEND_THROW:
 				if (i + 1 < op_array->last) {
 					BB_START(i + 1);
 				}
 				break;
+			case ZEND_THROW:
+				/* Don't treat THROW as terminator if it's used in expression context,
+				 * as we may lose live ranges when eliminating unreachable code. */
+				if (opline->extended_value != ZEND_THROW_IS_EXPR && i + 1 < op_array->last) {
+					BB_START(i + 1);
+				}
+				break;
 			case ZEND_INCLUDE_OR_EVAL:
 				flags |= ZEND_FUNC_INDIRECT_VAR_ACCESS;
 			case ZEND_GENERATOR_CREATE: