Preserve floating point state when calling randomx_calculate_hash

src/instructions_portable.cpp

@@ -157,6 +157,21 @@ void rx_set_rounding_mode(uint32_t mode) {
 	}
 }
 
+uint32_t rx_get_rounding_mode() {
+	switch (fegetround()) {
+	case FE_DOWNWARD:
+		return RoundDown;
+	case FE_UPWARD:
+		return RoundUp;
+	case FE_TOWARDZERO:
+		return RoundToZero;
+	case FE_TONEAREST:
+		return RoundToNearest;
+	default:
+		UNREACHABLE;
+	}
+}
+
 #endif
 
 #ifdef RANDOMX_USE_X87

src/intrin_portable.h

@@ -173,6 +173,10 @@ FORCE_INLINE void rx_set_rounding_mode(uint32_t mode) {
 	_mm_setcsr(rx_mxcsr_default | (mode << 13));
 }
 
+FORCE_INLINE uint32_t rx_get_rounding_mode() {
+	return (_mm_getcsr() >> 13) & 3;
+}
+
 #elif defined(__PPC64__) && defined(__ALTIVEC__) && defined(__VSX__) //sadly only POWER7 and newer will be able to use SIMD acceleration. Earlier processors cant use doubles or 64 bit integers with SIMD
 #include <cstdint>
 #include <stdexcept>
@@ -736,6 +740,8 @@ void rx_reset_float_state();
 
 void rx_set_rounding_mode(uint32_t mode);
 
+uint32_t rx_get_rounding_mode();
+
 #endif
 
 double loadDoublePortable(const void* addr);

src/randomx.cpp

@@ -36,6 +36,7 @@ OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 #include "cpu.hpp"
 #include <cassert>
 #include <limits>
+#include <cfenv>
 
 extern "C" {
 
@@ -349,6 +350,8 @@ extern "C" {
 		assert(machine != nullptr);
 		assert(inputSize == 0 || input != nullptr);
 		assert(output != nullptr);
+		fenv_t fpstate;
+		fegetenv(&fpstate);
 		alignas(16) uint64_t tempHash[8];
 		int blakeResult = blake2b(tempHash, sizeof(tempHash), input, inputSize, nullptr, 0);
 		assert(blakeResult == 0);
@@ -361,6 +364,7 @@ extern "C" {
 		}
 		machine->run(&tempHash);
 		machine->getFinalResult(output, RANDOMX_HASH_SIZE);
+		fesetenv(&fpstate);
 	}
 
 	void randomx_calculate_hash_first(randomx_vm* machine, const void* input, size_t inputSize) {

src/randomx.h

@@ -246,6 +246,8 @@ RANDOMX_EXPORT void randomx_calculate_hash(randomx_vm *machine, const void *inpu
  *                              and begin the calculation of the next hash.
  * randomx_calculate_hash_last  will output the hash value of the previous input.
  *
+ * WARNING: These functions may alter the floating point rounding mode of the calling thread.
+ *
  * @param machine is a pointer to a randomx_vm structure. Must not be NULL.
  * @param input is a pointer to memory to be hashed. Must not be NULL.
  * @param inputSize is the number of bytes to be hashed.

src/tests/tests.cpp

@@ -1051,6 +1051,10 @@ int main() {
 		assert(cacheMemory[33554431] == 0x1f47f056d05cd99b);
 	});
 
+	if (cache != nullptr)
+		randomx_release_cache(cache);
+	cache = randomx_alloc_cache(RANDOMX_FLAG_DEFAULT);
+
 	runTest("Hash batch test", RANDOMX_HAVE_COMPILER && stringsEqual(RANDOMX_ARGON_SALT, "RandomX\x03"), []() {
 		char hash1[RANDOMX_HASH_SIZE];
 		char hash2[RANDOMX_HASH_SIZE];
@@ -1070,6 +1074,14 @@ int main() {
 		assert(equalsHex(hash3, "c36d4ed4191e617309867ed66a443be4075014e2b061bcdaf9ce7b721d2b77a8"));
 	});
 
+	runTest("Preserve rounding mode", RANDOMX_FREQ_CFROUND > 0, []() {
+		rx_set_rounding_mode(RoundToNearest);
+		char hash[RANDOMX_HASH_SIZE];
+		calcStringHash("test key 000", "Lorem ipsum dolor sit amet", &hash);
+		assert(equalsHex(hash, "300a0adb47603dedb42228ccb2b211104f4da45af709cd7547cd049e9489c969"));
+		assert(rx_get_rounding_mode() == RoundToNearest);
+	});
+
 	randomx_destroy_vm(vm);
 	vm = nullptr;