opcache: preserve loop-carried cv values across trace guards

ext/opcache/jit/zend_jit_ir.c

@@ -17476,6 +17476,20 @@ static bool zend_jit_opline_supports_reg(const zend_op_array *op_array, zend_ssa
 
 static bool zend_jit_var_supports_reg(zend_ssa *ssa, int var)
 {
+	/* A transient assignment to a live loop-carried CV needs an explicit,
+	 * control-dependent store. Its spill must not be hoisted before the
+	 * loop guards, where the incoming value is still observable. */
+	if (ssa->vars[var].definition >= 0
+	 && !ssa->vars[var].phi_use_chain
+	 && ssa->ops[ssa->vars[var].definition].op1_def == var) {
+		int use = ssa->ops[ssa->vars[var].definition].op1_use;
+
+		if (use >= 0 && !ssa->vars[use].no_val
+		 && ssa->vars[use].definition_phi
+		 && (ssa->cfg.blocks[ssa->vars[use].definition_phi->block].flags & ZEND_BB_LOOP_HEADER)) {
+			return 0;
+		}
+	}
 	if (ssa->vars[var].no_val) {
 		/* we don't need the value */
 		return 0;

ext/opcache/tests/jit/gh24683_b.phpt

@@ -0,0 +1,80 @@
+--TEST--
+GH-24683 (B): tracing JIT must preserve overwritten runtime CV values
+--EXTENSIONS--
+opcache
+--SKIPIF--
+<?php if (PHP_INT_SIZE != 8) die("skip 64-bit only"); ?>
+--INI--
+opcache.enable=1
+opcache.enable_cli=1
+opcache.jit=tracing
+opcache.jit_buffer_size=64M
+opcache.jit_hot_loop=61
+opcache.jit_hot_func=127
+opcache.jit_hot_return=8
+opcache.jit_hot_side_exit=8
+opcache.file_update_protection=0
+--FILE--
+<?php
+// php -d opcache.enable_cli=1 -d opcache.jit=tracing -d opcache.jit_buffer_size=64M repro.php
+//
+// Case 1 stores to $ram and moves on to case 2. Case 2 sets $x to a value it
+// only knows at run time (0x800E0000 | $zero, where $zero is 0), uses it as an
+// address, and overwrites $x with the byte loaded from there (4). Once the loop
+// trace is compiled, $x sometimes comes back as 0x800E0000 instead.
+
+final class Bus
+{
+    public function read8(int $address): int { return 0; }
+}
+
+$fn = static function (Bus $bus, array &$r, array &$ram, array &$sp, int $pc): int {
+    $value = $r[4];
+    $base = $r[15];
+    $zero = $r[0];
+    while (true) {
+        switch ($pc) {
+            case 1:
+                $a = ($base + 2) & 0xFFFFFFFF;
+                {
+                    $p = $a & 0x1FFFFFFF;
+                    $inRam = $a < 0xC0000000 && $p < 0x00800000;
+                    if ($inRam) { $k = ($p & 0x1FFFFF) >> 2; $ram[$k] = ($ram[$k] & ~(65535 << (($p & 3) << 3))) | (($value & 65535) << (($p & 3) << 3)); }
+                }
+                $pc = 2;
+                continue 2;
+            case 2:
+                $x = 0x800E0000 | $zero;
+                $a = ($x + 0xFFFFEDED) & 0xFFFFFFFF;
+                $p = $a & 0x1FFFFFFF;
+                $byte = ((($inRam ? ($ram[($p & 0x1FFFFF) >> 2] >> (($p & 3) << 3)) & 0xFF : ($inSp ? ($sp[($p - 0x1F800000) >> 2] >> (($p & 3) << 3)) & 0xFF : $bus->read8($a))) ^ 0x80) - 0x80) & 0xFFFFFFFF;
+                $x = $byte;
+                $pc = 3;
+                continue 2;
+            default:
+                goto leave;
+        }
+    }
+    leave:
+    $r[12] = $x;
+    return $pc;
+};
+
+$bus = new Bus();
+$ram = array_fill(0, 0x80000, 0);
+$ram[0xDEDED >> 2] = 0x0003047F; // the byte at 0xDEDED is 4
+$sp = [];
+$wrong = 0;
+for ($k = 0; $k < 300; $k++) {
+    $r = array_fill(0, 32, 0);
+    $r[4] = 2;
+    $fn($bus, $r, $ram, $sp, 1);
+    if ($r[12] !== 4) {
+        if ($wrong++ === 0) {
+            printf("call %d: \$x is 0x%X, expected 4\n", $k, $r[12]);
+        }
+    }
+}
+echo "wrong: $wrong\n";
+--EXPECT--
+wrong: 0

ext/opcache/tests/jit/gh24683_b_loop.phpt

@@ -0,0 +1,47 @@
+--TEST--
+GH-24683 (B): loop guards must observe the incoming CV value
+--EXTENSIONS--
+opcache
+--SKIPIF--
+<?php if (PHP_INT_SIZE != 8) die("skip 64-bit only"); ?>
+--INI--
+opcache.enable=1
+opcache.enable_cli=1
+opcache.jit=tracing
+opcache.jit_buffer_size=64M
+opcache.jit_hot_loop=61
+opcache.jit_hot_func=127
+opcache.jit_hot_return=8
+opcache.jit_hot_side_exit=8
+opcache.file_update_protection=0
+--FILE--
+<?php
+function test(array $r, int $pc) {
+    $zero = $r[0];
+    $x = $r[2];
+    while (true) {
+        switch ($pc) {
+            case 2:
+                $x = 0x800E0000 | $zero;
+                $a = ($x + 0xFFFFEDED) & 0xFFFFFFFF;
+                $p = $a & 0x1FFFFFFF;
+                $byte = ((($r[1] >> (($p & 3) << 3)) & 0xFF) ^ 0x80) - 0x80;
+                $x = $byte;
+                $pc = 3;
+                continue 2;
+            default: goto leave;
+        }
+    }
+    leave: return $x;
+}
+$wrong = 0;
+for ($i = 0; $i < 1000; $i++) {
+    $r = [0, 0x0003047F, 4];
+    $pc = $i < 500 ? 2 : 3;
+    if (test($r, $pc) !== 4) {
+        $wrong++;
+    }
+}
+echo "wrong: $wrong\n";
+--EXPECT--
+wrong: 0