shared/util.h: Use generic for size_t arithmetic

This improves support for rare systems like s390 in 31 bit mode, where
the size of size_t is equals uint32_t, yet types still differ.

Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>

Author: stoeckmann

shared/util.h

@@ -118,15 +118,21 @@ static inline bool uadd64_overflow(uint64_t a, uint64_t b, uint64_t *res)
 #endif
 }
 
+static inline bool simple_uaddsz_overflow(size_t a, size_t b, size_t *res)
+{
+	*res = a + b;
+	return SIZE_MAX - a < b;
+}
+
+#define optimized_uaddsz_overflow(a, b, res) \
+	_Generic((res),                      \
+		uint32_t *: uadd32_overflow, \
+		uint64_t *: uadd64_overflow, \
+		default: simple_uaddsz_overflow)(a, b, res)
+
 static inline bool uaddsz_overflow(size_t a, size_t b, size_t *res)
 {
-#if __SIZEOF_SIZE_T__ == 8
-	return uadd64_overflow(a, b, res);
-#elif __SIZEOF_SIZE_T__ == 4
-	return uadd32_overflow(a, b, res);
-#else
-#error "Unknown sizeof(size_t)"
-#endif
+	return optimized_uaddsz_overflow(a, b, res);
 }
 
 static inline bool umul32_overflow(uint32_t a, uint32_t b, uint32_t *res)
@@ -162,15 +168,21 @@ static inline bool umulll_overflow(unsigned long long a, unsigned long long b,
 #endif
 }
 
+static inline bool simple_umulsz_overflow(size_t a, size_t b, size_t *res)
+{
+	*res = a * b;
+	return SIZE_MAX / a < b;
+}
+
+#define optimized_umulsz_overflow(a, b, res) \
+	_Generic((res),                      \
+		uint32_t *: umul32_overflow, \
+		uint64_t *: umul64_overflow, \
+		default: simple_umulsz_overflow)(a, b, res)
+
 static inline bool umulsz_overflow(size_t a, size_t b, size_t *res)
 {
-#if __SIZEOF_SIZE_T__ == 8
-	return umul64_overflow(a, b, res);
-#elif __SIZEOF_SIZE_T__ == 4
-	return umul32_overflow(a, b, res);
-#else
-#error "Unknown sizeof(size_t)"
-#endif
+	return optimized_umulsz_overflow(a, b, res);
 }
 
 #define TAKE_PTR(x)                \