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

I'm not familiar with the data structures involved in the passes after dfa,
so I'm not sure how to safely improve on this (removing NOPs).

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.

Author: TysonAndre

ext/opcache/Optimizer/dfa_pass.c

@@ -273,6 +273,24 @@ static void zend_ssa_remove_nops(zend_op_array *op_array, zend_ssa *ssa, zend_op
 			}
 		}
 
+		/* Clean up FE_RESET_R and FE_FREE for foreach over empty arrays (TODO: Remove left over no-op) */
+		for (i = 0; i < target; i++) {
+			zend_op *cur_op = &op_array->opcodes[i];
+			zend_op *next_op;
+			zend_ssa_op *ssa_op;
+			if ((cur_op->opcode != ZEND_FE_RESET_R && cur_op->opcode != ZEND_FE_RESET_RW) || cur_op->op1_type != IS_CONST) {
+				continue;
+			}
+			next_op = cur_op + 1;
+			if (next_op->opcode != ZEND_FE_FREE || next_op->op1_type != IS_VAR || cur_op->result_type != IS_VAR || cur_op->result.var != next_op->op1.var) {
+				continue;
+			}
+			ssa_op = &ssa->ops[i];
+			zend_ssa_remove_result_def(ssa, ssa_op);
+			zend_ssa_remove_instr(ssa, next_op, ssa_op + 1);
+			zend_ssa_remove_instr(ssa, cur_op, ssa_op);
+		}
+
 		op_array->last = target;
 	}
 	free_alloca(shiftlist, use_heap);

ext/opcache/tests/opt/dfa_001.phpt

@@ -0,0 +1,72 @@
+--TEST--
+DFA 001: 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) {
+        }
+    }
+}
+// TODO: Clean up remaining ZEND_NOOP opcodes from previous passes in a final pass?
+Loop::test();
+Loop::test2();
+Loop::test3();
+--EXPECTF--
+$_main: ; (lines=7, args=0, vars=0, tmps=0)
+    ; (after optimizer)
+    ; %sdfa_001.php:1-24
+L0 (21):    INIT_STATIC_METHOD_CALL 0 string("Loop") string("test")
+L1 (21):    DO_UCALL
+L2 (22):    INIT_STATIC_METHOD_CALL 0 string("Loop") string("test2")
+L3 (22):    DO_UCALL
+L4 (23):    INIT_STATIC_METHOD_CALL 0 string("Loop") string("test3")
+L5 (23):    DO_UCALL
+L6 (24):    RETURN int(1)
+
+Loop::test: ; (lines=5, args=0, vars=0, tmps=0)
+    ; (after optimizer)
+    ; %sdfa_001.php:4-10
+L0 (5):     ECHO string("Start
+")
+L1 (7):     NOP
+L2 (7):     NOP
+L3 (9):     ECHO string("Done
+")
+L4 (10):    RETURN null
+
+Loop::test2: ; (lines=3, args=0, vars=0, tmps=0)
+    ; (after optimizer)
+    ; %sdfa_001.php:11-14
+L0 (12):    NOP
+L1 (12):    NOP
+L2 (14):    RETURN null
+
+Loop::test3: ; (lines=3, args=0, vars=0, tmps=0)
+    ; (after optimizer)
+    ; %sdfa_001.php:15-18
+L0 (16):    NOP
+L1 (16):    NOP
+L2 (18):    RETURN null
+Start
+Done