diff options
| -rw-r--r-- | src/passes/TypeSSA.cpp | 10 | ||||
| -rw-r--r-- | src/support/hash.h | 22 |
2 files changed, 19 insertions, 13 deletions
diff --git a/src/passes/TypeSSA.cpp b/src/passes/TypeSSA.cpp index 8ba6c10c4..cda78474b 100644 --- a/src/passes/TypeSSA.cpp +++ b/src/passes/TypeSSA.cpp @@ -92,8 +92,12 @@ std::vector<HeapType> ensureTypesAreInNewRecGroup(RecGroup recGroup, // "random" extra item in the rec group that is so outlandish it will // surely (?) never collide with anything. We must loop while doing so, // until we find a hash that does not collide. - auto hashSize = num + 10; - size_t random = num; + // + // Note that we use uint64_t here, and deterministic_hash_combine below, to + // ensure our output is fully deterministic - the types we add here are + // observable in the output. + uint64_t hashSize = num + 10; + uint64_t random = num; while (1) { // Make a builder and add a slot for the hash. TypeBuilder builder(num + 1); @@ -106,7 +110,7 @@ std::vector<HeapType> ensureTypesAreInNewRecGroup(RecGroup recGroup, for (Index i = 0; i < hashSize; i++) { // TODO: a denser encoding? auto type = (random & 1) ? Type::i32 : Type::f64; - hash_combine(random, hashSize + i); + deterministic_hash_combine(random, hashSize + i); hashStruct.fields.push_back(Field(type, Mutable)); } builder[num] = hashStruct; diff --git a/src/support/hash.h b/src/support/hash.h index b99902616..ba1001d66 100644 --- a/src/support/hash.h +++ b/src/support/hash.h @@ -28,29 +28,31 @@ template<typename T> inline std::size_t hash(const T& value) { } // Combines two digests into the first digest. Use instead of `rehash` if -// `otherDigest` is another digest and not a `size_t` value. This is also useful -// when you want deterministic behavior across systems, as this method does not -// call std::hash, so it does not depend on the behavior of the local machine's -// C++ standard library implementation. +// `otherDigest` is another digest. This is also deterministic, aside from +// possible differences in size_t (see deterministic_hash_combine, below). inline void hash_combine(std::size_t& digest, const std::size_t otherDigest) { // see: boost/container_hash/hash.hpp // The constant is the N-bits reciprocal of the golden ratio: // phi = (1 + sqrt(5)) / 2 -#if SIZE_MAX == UINT64_MAX // trunc(2^64 / phi) = 0x9e3779b97f4a7c15 digest ^= otherDigest + 0x9e3779b97f4a7c15 + (digest << 12) + (digest >> 4); -#else - // trunc(2^32 / phi) = 0x9e3779b9 - digest ^= otherDigest + 0x9e3779b9 + (digest << 6) + (digest >> 2); -#endif } // Hashes `value` and combines the resulting digest into the existing digest. -// Use instead of `hash_combine` if `value` is not another digest. +// Use instead of `hash_combine` if `value` is not another digest (i.e., it +// needs to be hashed first). template<typename T> inline void rehash(std::size_t& digest, const T& value) { hash_combine(digest, hash(value)); } +// Similar to hash_combine, but guaranteed to produce the exact same results on +// all machines, even ones where size_t differs. This is essentially identical +// to hash_combine, but the types are all uint64_t. +inline void deterministic_hash_combine(uint64_t& digest, + const uint64_t otherDigest) { + digest ^= otherDigest + 0x9e3779b97f4a7c15 + (digest << 12) + (digest >> 4); +} + } // namespace wasm namespace std { |