Avoid invalid gcc 14.3 warning about array bounds in mbedtls_xor

The combination of the multi-byte loop with the single byte loop
confuses GCC 14.3's array bounds checker. When the loop size is
constant, check to see if it is a multiple of the multi-byte size and
bail early. As this will be evaluated at compile time, there should be
no run-time cost.

This change uses the __builtin_constant_p compile-time operation. To
check if that is supported, the change uses the existing
MBEDTLS_HAS_BUILTIN macro. That macro was defined later in
library/common.h than is needed for this change, so it was moved up to
join some other macros that looked similar.

Upstream PR: Mbed-TLS/mbedtls#10318

Signed-off-by: Keith Packard <keithp@keithp.com>

Author: keith-packard

library/common.h

@@ -99,6 +99,13 @@ extern void (*mbedtls_test_hook_test_fail)(const char *test, int line, const cha
  * fall back to the unsafe implementation. */
 #define ARRAY_LENGTH(array) ARRAY_LENGTH_UNSAFE(array)
 #endif
+
+#if defined(__has_builtin)
+#define MBEDTLS_HAS_BUILTIN(x) __has_builtin(x)
+#else
+#define MBEDTLS_HAS_BUILTIN(x) 0
+#endif
+
 /** Allow library to access its structs' private members.
  *
  * Although structs defined in header files are publicly available,
@@ -208,6 +215,11 @@ static inline void mbedtls_xor(unsigned char *r,
         return;
     }
 #endif
+#if defined(MBEDTLS_COMPILER_IS_GCC) && MBEDTLS_HAS_BUILTIN(__builtin_constant_p)
+    if (__builtin_constant_p(n) && n % 16 == 0) {
+        return;
+    }
+#endif
 #elif defined(MBEDTLS_ARCH_IS_X64) || defined(MBEDTLS_ARCH_IS_ARM64)
     /* This codepath probably only makes sense on architectures with 64-bit registers */
     for (; (i + 8) <= n; i += 8) {
@@ -219,6 +231,11 @@ static inline void mbedtls_xor(unsigned char *r,
         return;
     }
 #endif
+#if defined(MBEDTLS_COMPILER_IS_GCC) && MBEDTLS_HAS_BUILTIN(__builtin_constant_p)
+    if (__builtin_constant_p(n) && n % 8 == 0) {
+        return;
+    }
+#endif
 #else
     for (; (i + 4) <= n; i += 4) {
         uint32_t x = mbedtls_get_unaligned_uint32(a + i) ^ mbedtls_get_unaligned_uint32(b + i);
@@ -229,6 +246,11 @@ static inline void mbedtls_xor(unsigned char *r,
         return;
     }
 #endif
+#if defined(MBEDTLS_COMPILER_IS_GCC) && MBEDTLS_HAS_BUILTIN(__builtin_constant_p)
+    if (__builtin_constant_p(n) && n % 4 == 0) {
+        return;
+    }
+#endif
 #endif
 #endif
     for (; i < n; i++) {
@@ -367,12 +389,6 @@ static inline void mbedtls_xor_no_simd(unsigned char *r,
     struct ISO_C_does_not_allow_extra_semicolon_outside_of_a_function
 #endif
 
-#if defined(__has_builtin)
-#define MBEDTLS_HAS_BUILTIN(x) __has_builtin(x)
-#else
-#define MBEDTLS_HAS_BUILTIN(x) 0
-#endif
-
 /* Define compiler branch hints */
 #if MBEDTLS_HAS_BUILTIN(__builtin_expect)
 #define MBEDTLS_LIKELY(x)       __builtin_expect(!!(x), 1)