Fix bug #78271

When cleaning nops in the dfa pass, we were always keeping the
smart branch inhibiting nop that occurs directly before the jump
instruction. However, as we skip unreachable blocks entirely, it
may happen that we need to keep a nop that occurs further back,
prior to the unreachable blocks. Account for that case now.

We should really do something about the smart branch situation,
this is very fragile...

3 files changed, 59 insertions, 10 deletions

diff --git a/NEWS b/NEWS
index b0e3d5f2a0..f69a04a919 100644
--- a/NEWS
+++ b/NEWS
@@ -24,6 +24,7 @@ PHP                                                                        NEWS
   . Fixed bug #78189 (file cache strips last character of uname hash). (cmb)
   . Fixed bug #78202 (Opcache stats for cache hits are capped at 32bit NUM).
     (cmb)
+  . Fixed bug #78271 (Invalid result of if-else). (Nikita)
 
 - PCRE:
   . Fixed bug #78197 (PCRE2 version check in configure fails for "##.##-xxx"
diff --git a/Zend/tests/bug78271.phpt b/Zend/tests/bug78271.phpt
new file mode 100644
index 0000000000..fbbe016d75
--- /dev/null
+++ b/Zend/tests/bug78271.phpt
@@ -0,0 +1,27 @@
+--TEST--
+Bug #78271: Invalid result of if-else
+--FILE--
+<?
+function test($a, $b){
+    if ($a==10) {
+        $w="x";
+    } else {
+        $w="y";
+    }
+
+    if ($b) {
+        $d1="none";
+        $d2="block";
+    } else {
+        $d1="block";
+        $d2="none";
+    }
+
+    echo $d2.$b."\n";
+
+}
+
+test(1, 1);
+?>
+--EXPECT--
+block1
diff --git a/ext/opcache/Optimizer/dfa_pass.c b/ext/opcache/Optimizer/dfa_pass.c
index 8ea96d4bb9..55b9597850 100644
--- a/ext/opcache/Optimizer/dfa_pass.c
+++ b/ext/opcache/Optimizer/dfa_pass.c
@@ -125,10 +125,37 @@ int zend_dfa_analyze_op_array(zend_op_array *op_array, zend_optimizer_ctx *ctx,
 	return SUCCESS;
 }
 
+static zend_bool is_smart_branch_inhibiting_nop(
+		zend_op_array *op_array, uint32_t target, uint32_t current,
+		zend_basic_block *b, zend_basic_block *blocks_end)
+{
+	uint32_t next;
+	/* Target points one past the last non-nop instruction. Make sure there is one. */
+	if (target == 0) {
+		return 0;
+	}
+
+	/* Find the next instruction, skipping unreachable or empty blocks. */
+	next = current + 1;
+	if (next >= b->start + b->len) {
+		do {
+			b++;
+			if (b == blocks_end) {
+				return 0;
+			}
+		} while (!(b->flags & ZEND_BB_REACHABLE) || b->len == 0);
+		next = b->start;
+	}
+
+	return (op_array->opcodes[next].opcode == ZEND_JMPZ ||
+		 op_array->opcodes[next].opcode == ZEND_JMPNZ) &&
+		zend_is_smart_branch(op_array->opcodes + target - 1);
+}
+
 static void zend_ssa_remove_nops(zend_op_array *op_array, zend_ssa *ssa, zend_optimizer_ctx *ctx)
 {
 	zend_basic_block *blocks = ssa->cfg.blocks;
-	zend_basic_block *end = blocks + ssa->cfg.blocks_count;
+	zend_basic_block *blocks_end = blocks + ssa->cfg.blocks_count;
 	zend_basic_block *b;
 	zend_func_info *func_info;
 	int j;
@@ -152,7 +179,7 @@ static void zend_ssa_remove_nops(zend_op_array *op_array, zend_ssa *ssa, zend_op
 		}
 	}
 
-	for (b = blocks; b < end; b++) {
+	for (b = blocks; b < blocks_end; b++) {
 		if (b->flags & (ZEND_BB_REACHABLE|ZEND_BB_UNREACHABLE_FREE)) {
 			uint32_t end;
 
@@ -174,13 +201,7 @@ static void zend_ssa_remove_nops(zend_op_array *op_array, zend_ssa *ssa, zend_op
 				while (i < end) {
 					shiftlist[i] = i - target;
 					if (EXPECTED(op_array->opcodes[i].opcode != ZEND_NOP) ||
-					   /* Keep NOP to support ZEND_VM_SMART_BRANCH. Using "target-1" instead of
-					    * "i-1" here to check the last non-NOP instruction. */
-					   (target > 0 &&
-					    i + 1 < op_array->last &&
-					    (op_array->opcodes[i+1].opcode == ZEND_JMPZ ||
-					     op_array->opcodes[i+1].opcode == ZEND_JMPNZ) &&
-					    zend_is_smart_branch(op_array->opcodes + target - 1))) {
+						is_smart_branch_inhibiting_nop(op_array, target, i, b, blocks_end)) {
 						if (i != target) {
 							op_array->opcodes[target] = op_array->opcodes[i];
 							ssa->ops[target] = ssa->ops[i];
@@ -240,7 +261,7 @@ static void zend_ssa_remove_nops(zend_op_array *op_array, zend_ssa *ssa, zend_op
 		}
 
 		/* update branch targets */
-		for (b = blocks; b < end; b++) {
+		for (b = blocks; b < blocks_end; b++) {
 			if ((b->flags & ZEND_BB_REACHABLE) && b->len != 0) {
 				zend_op *opline = op_array->opcodes + b->start + b->len - 1;
 				zend_optimizer_shift_jump(op_array, opline, shiftlist);