/*
- * Copyright (C) 2012, 2013, 2014
+ * Copyright (C) 2012, 2013, 2014, 2015
* Dale Weiler
*
* Permission is hereby granted, free of charge, to any person obtaining a copy of
*/
#include "gmqcc.h"
#include <limits.h>
-#include <string.h>
#if defined(_MSC_VER)
# define HASH_ROTL32(X, Y) _rotl((X), (Y))
return hash_murmur_result(hash, carry, length);
}
-size_t hash(const char *key) {
-#if 0
- const char *s = key;
- const char *a = s;
+/*
+ * The following hash function implements it's own strlen to avoid using libc's
+ * which isn't always slow but isn't always fastest either.
+ *
+ * Some things to note about this strlen that are otherwise confusing to grasp
+ * at first is that it does intentionally depend on undefined behavior.
+ *
+ * The first step to the strlen is to ensure alignment before checking words,
+ * without this step we risk crossing a page boundry with the word check and
+ * that would cause a crash.
+ *
+ * The second step to the strlen contains intentional undefined behavior. When
+ * accessing a word of any size, the first byte of that word is accessible if
+ * and only if the whole word is accessible because words are aligned. This is
+ * indicated by the fact that size / alignment always divides the page size.
+ * One could argue that an architecture exists where size_t and alignment are
+ * different, if that were the case, the alignment will always assume to be the
+ * size of the type (size_t). So it's always safe in that regard.
+ *
+ * In other words, an aligned 2^n load cannot cross a page boundry unless
+ * n > log2(PAGE_SIZE). There are no known architectures which support such
+ * a wide load larger than PAGE_SIZE.
+ *
+ * Valgrind and address sanatizer may choke on this because they're strictly
+ * trying to find bugs, it's a false positive to assume this is a bug when it's
+ * intentional. To prevent these false positives, both things need to be taught
+ * about the intentional behavior; for address sanatizer this can be done with
+ * a compiler attribute, effectively preventing the function from being
+ * instrumented. Valgrind requires a little more work as there is no way to
+ * downright prevent a function from being instrumented, instead we can mark
+ * + sizeof(size_t) bytes ahead of each byte we're reading as we calculate
+ * the length of the string, then we can make that additional + sizeof(size_t)
+ * on the end undefined after the length has been calculated.
+ *
+ * If the compiler doesn't have the attribute to disable address sanatizer
+ * instrumentation we fall back to using libc's strlen instead. This isn't the
+ * best solution. On windows we can assume this method always because neither
+ * address sanatizer or valgrind exist.
+ */
+
+/* Some compilers expose this */
+#if defined(__has_feature)
+# if __has_feature(address_sanitizer)
+# define ASAN_DISABLE __attribute__((no_sanitize_address))
+# define HAS_ASAN_DISABLE
+# endif
+#endif
+
+/* If they don't try to find by version the attriubte was introduces */
+#if defined(__GNUC__) && __GNUC__ >= 4
+# define ASAN_DISABLE __attribute__((no_sanitize_address))
+# define HAS_ASAN_DISABLE
+#elif defined(__clang__) && __clang_major__ >= 3
+# define ASAN_DISABLE __attribute__((no_sanatize_address))
+# define HAS_ASAN_DISABLE
+/* On windows asan doesn't exist */
+#elif defined(_WIN32)
+# define ASAN_DISABLE /* nothing */
+# define HAS_ASAN_DISABLE
+#endif
+
+#ifndef HAS_ASAN_DISABLE
+# include <string.h>
+#endif
+
+#ifndef NVALGRIND
+# include <valgrind/valgrind.h>
+# include <valgrind/memcheck.h>
+#else
+# define VALGRIND_MAKE_MEM_DEFINED(PTR, REDZONE_SIZE)
+# define VALGRIND_MAKE_MEM_NOACCESS(PTR, REDZONE_SIZE)
+#endif
+
+#ifdef HAS_ASAN_DISABLE
+#define STRLEN_ALIGN (sizeof(size_t))
+#define STRLEN_ONES ((size_t)-1/UCHAR_MAX)
+#define STRLEN_HIGHS (STRLEN_ONES * (UCHAR_MAX/2+1))
+#define STRLEN_HASZERO(X) (((X)-STRLEN_ONES) & ~(X) & STRLEN_HIGHS)
+
+static ASAN_DISABLE size_t hash_strlen(const char *key) {
+ const char *s = key;
+ const char *a = s;
const size_t *w;
- for (; (uintptr_t)s % sizeof(size_t); s++)
- if (!*s)
- return hash_murmur((const void *)key, s-a);
+ for (; (uintptr_t)s % STRLEN_ALIGN; s++) {
+ if (*s)
+ continue;
+ return s-a;
+ }
+
+ VALGRIND_MAKE_MEM_DEFINED(s, STRLEN_ALIGN);
+ for (w = (const size_t *)s; !STRLEN_HASZERO(*w); w++) {
+ /* Make the next word legal to access */
+ VALGRIND_MAKE_MEM_DEFINED(w + STRLEN_ALIGN, STRLEN_ALIGN);
+ }
- for (w = (const size_t*)s; !((*w-(size_t)-1/UCHAR_MAX) & ~*w & ((size_t)-1/UCHAR_MAX) * (UCHAR_MAX / 2 + 1)); w++);
for (s = (const char *)w; *s; s++);
- return hash_murmur((const void *)key, s-a);
+ /* It's not legal to access this area anymore */
+ VALGRIND_MAKE_MEM_NOACCESS(s + 1, STRLEN_ALIGN);
+ return s-a;
+}
#else
- return hash_murmur((const void *)key, strlen(key));
-#endif
+static GMQCC_INLINE size_t hash_strlen(const char *key) {
+ return strlen(key);
}
+#endif
-#undef HASH_ROTL32
-#undef HASH_MURMUR_MASK1
-#undef HASH_MURMUR_MASK2
-#undef HASH_MURMUR_SEED
-#undef HASH_MURMUR_SAFEREAD
-#undef HASH_MURMUR_BLOCK
-#undef HASH_MURMUR_BYTES
-#undef HASH_MURMUR_TAIL
+size_t hash(const char *key) {
+ return hash_murmur((const void *)key, hash_strlen(key));
+}
projects / xonotic / gmqcc.git / blobdiff