Clean up remaining opcodes for foreach([] as $x)

Previously, two useless FE_RESET_R and FE_FREE would be left over whether the empty array
was from a literal, a variable, or a class constant.

This doesn't pick up the RESET_RW case due to a weakness in our "may throw"
modeling. (for foreach by reference).

Co-Authored-By: Nikita Popov <nikita.ppv@gmail.com>
using https://gist.github.com/nikic/58d367ad605e10299f5433d2d83a0b5b

Closes GH-4949

Author: TysonAndre

ext/opcache/Optimizer/dce.c

@@ -82,6 +82,7 @@ static inline zend_bool may_have_side_effects(
 		case ZEND_IS_NOT_IDENTICAL:
 		case ZEND_QM_ASSIGN:
 		case ZEND_FREE:
+		case ZEND_FE_FREE:
 		case ZEND_TYPE_CHECK:
 		case ZEND_DEFINED:
 		case ZEND_ADD:
@@ -246,6 +247,11 @@ static inline zend_bool may_have_side_effects(
 			return 0;
 		case ZEND_CHECK_VAR:
 			return (OP1_INFO() & MAY_BE_UNDEF) != 0;
+		case ZEND_FE_RESET_R:
+		case ZEND_FE_RESET_RW:
+			/* Model as not having side-effects -- let the side-effect be introduced by
+			 * FE_FETCH if the array is not known to be non-empty. */
+			return (OP1_INFO() & MAY_BE_ANY) != MAY_BE_ARRAY;
 		default:
 			/* For everything we didn't handle, assume a side-effect */
 			return 1;
@@ -373,6 +379,21 @@ static zend_bool try_remove_var_def(context *ctx, int free_var, int use_chain, z
 	return 0;
 }
 
+static inline zend_bool is_free_of_live_var(context *ctx, zend_op *opline, zend_ssa_op *ssa_op) {
+	switch (opline->opcode) {
+		case ZEND_FREE:
+			/* It is always safe to remove FREEs of non-refcounted values, even if they are live. */
+			if (!(ctx->ssa->var_info[ssa_op->op1_use].type & (MAY_BE_STRING|MAY_BE_ARRAY|MAY_BE_OBJECT|MAY_BE_RESOURCE|MAY_BE_REF))) {
+				return 0;
+			}
+			/* break missing intentionally */
+		case ZEND_FE_FREE:
+			return !is_var_dead(ctx, ssa_op->op1_use);
+		default:
+			return 0;
+	}
+}
+
 /* Returns whether the instruction has been DCEd */
 static zend_bool dce_instr(context *ctx, zend_op *opline, zend_ssa_op *ssa_op) {
 	zend_ssa *ssa = ctx->ssa;
@@ -384,9 +405,7 @@ static zend_bool dce_instr(context *ctx, zend_op *opline, zend_ssa_op *ssa_op) {
 	}
 
 	/* We mark FREEs as dead, but they're only really dead if the destroyed var is dead */
-	if (opline->opcode == ZEND_FREE
-			&& (ssa->var_info[ssa_op->op1_use].type & (MAY_BE_STRING|MAY_BE_ARRAY|MAY_BE_OBJECT|MAY_BE_RESOURCE|MAY_BE_REF))
-			&& !is_var_dead(ctx, ssa_op->op1_use)) {
+	if (is_free_of_live_var(ctx, opline, ssa_op)) {
 		return 0;
 	}
 

ext/opcache/Optimizer/zend_inference.c

@@ -4391,6 +4391,7 @@ int zend_may_throw_ex(const zend_op *opline, const zend_ssa_op *ssa_op, const ze
 		case ZEND_BEGIN_SILENCE:
 		case ZEND_END_SILENCE:
 		case ZEND_FREE:
+		case ZEND_FE_FREE:
 		case ZEND_SEPARATE:
 		case ZEND_TYPE_CHECK:
 		case ZEND_DEFINED:

ext/opcache/tests/opt/dce_009.phpt

@@ -0,0 +1,71 @@
+--TEST--
+DCE 009: Foreach over empty array is a no-op
+--INI--
+opcache.enable=1
+opcache.enable_cli=1
+opcache.optimization_level=-1
+opcache.opt_debug_level=0x20000
+opcache.preload=
+--SKIPIF--
+<?php require_once('skipif.inc'); ?>
+--FILE--
+<?php
+class Loop {
+    const VALUES = [];
+    public static function test() {
+        echo "Start\n";
+        $y = [];
+        foreach ($y as $x) {
+        }
+        echo "Done\n";
+    }
+    public static function test2() {
+        foreach (self::VALUES as $x) {
+        }
+    }
+    public static function test3() {
+        foreach ([] as $k => &$v) {
+        }
+    }
+}
+Loop::test();
+Loop::test2();
+Loop::test3();
+--EXPECTF--
+$_main:
+     ; (lines=7, args=0, vars=0, tmps=0)
+     ; (after optimizer)
+     ; %sdce_009.php:1-23
+0000 INIT_STATIC_METHOD_CALL 0 string("Loop") string("test")
+0001 DO_UCALL
+0002 INIT_STATIC_METHOD_CALL 0 string("Loop") string("test2")
+0003 DO_UCALL
+0004 INIT_STATIC_METHOD_CALL 0 string("Loop") string("test3")
+0005 DO_UCALL
+0006 RETURN int(1)
+
+Loop::test:
+     ; (lines=3, args=0, vars=0, tmps=0)
+     ; (after optimizer)
+     ; %sdce_009.php:4-10
+0000 ECHO string("Start
+")
+0001 ECHO string("Done
+")
+0002 RETURN null
+
+Loop::test2:
+     ; (lines=1, args=0, vars=0, tmps=0)
+     ; (after optimizer)
+     ; %sdce_009.php:11-14
+0000 RETURN null
+
+Loop::test3:
+     ; (lines=3, args=0, vars=0, tmps=1)
+     ; (after optimizer)
+     ; %sdce_009.php:15-18
+0000 V0 = FE_RESET_RW array(...) 0001
+0001 FE_FREE V0
+0002 RETURN null
+Start
+Done