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Random rounding mode #15

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2b63496
Add macro to define __RAPTOR_MPFR_DEFAULT_ROUNDING_MODE to use random…
MinliLiao Jun 18, 2026
33217b6
Limit rounding modes to only round up and round down
MinliLiao Jun 19, 2026
61c136b
Move random number generator inside ifdef to avoid overhead when not …
MinliLiao Jun 19, 2026
5fa3338
Initial draft of stochastic rounding
MinliLiao Jun 22, 2026
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13 changes: 13 additions & 0 deletions runtime/include/private/raptor/Common.h
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Original file line number Diff line number Diff line change
Expand Up @@ -4,6 +4,7 @@
#include <atomic>
#include <cstdlib>
#include <cstring>
#include <random>
#include <mpfr.h>

#define MAX_MPFR_OPERANDS 3
Expand All @@ -12,7 +13,19 @@
#define __RAPTOR_MPFR_DECL_ATTRIBUTES extern "C"
#define __RAPTOR_MPFR_ORIGINAL_ATTRIBUTES extern "C" __attribute__((weak))

#ifdef __RAPTOR_RANDOM_ROUNDING_MODE
#define __RAPTOR_MPFR_DEFAULT_ROUNDING_MODE __raptor_fprt_get_rand_rounding_mode()
inline thread_local std::mt19937 __raptor_fprt_rnd_gen{std::random_device{}()};
static inline mpfr_rnd_t __raptor_fprt_get_rand_rounding_mode() {
// Assuming 0=RNDN, 1=RNDZ, 2=RNDU, 3=RNDD, 4=RNDA
// may need to change if mpfr.h changes
return mpfr_rnd_t(
std::uniform_int_distribution<>(MPFR_RNDU, MPFR_RNDD)
(__raptor_fprt_rnd_gen));
}
#else
#define __RAPTOR_MPFR_DEFAULT_ROUNDING_MODE GMP_RNDN
#endif
#define __RAPTOR_MPFR_MALLOC_FAILURE_EXIT_STATUS 114

extern std::atomic<long long> shadow_err_counter;
Expand Down
123 changes: 123 additions & 0 deletions runtime/ir/Mpfr.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -82,6 +82,89 @@
} while (0)
#endif

#ifdef __RAPTOR_STOCHASTIC_ROUNDING_MODE
#define __RATPTOR_USE_STOCHASTIC_ROUNDING true

// Structure with internal high precision mpfr_t used to set target mpfr_t
// value with stochastic rounding
struct __raptor_stochastic_rnd_double {
// Precision used for the internal high precision mpfr_t
static constexpr mpfr_prec_t high_prec = 53;
// Rounding mode used for the internal high precision mpfr_t
static constexpr mpfr_rnd_t high_prec_rnd =
__RAPTOR_MPFR_DEFAULT_ROUNDING_MODE;

// Internal mpfr_t of high precision used to calculate rounding direction
mpfr_t op_high_prec;
// Random value generator used to decide rounding direction
std::mt19937 rnd_gen{std::random_device{}()};

// Constructor that also initializes op_high_prec with the high precision
__raptor_stochastic_rnd_double() { mpfr_init2(op_high_prec, high_prec); }

// Given x and it's rounded up/down values, determine if x should round up
bool is_round_up(double x, double x_up, double x_down) {
return std::uniform_real_distribution<>(0.0, x_up - x_down)(rnd_gen)
< (x - x_down);
}

// Set op_rnd from internal op_high_prec with stochastic rounding
virtual void round(mpfr_t op_rnd) {
double x = mpfr_get_d(op_high_prec, high_prec_rnd);
mpfr_set(op_rnd, op_high_prec, MPFR_RNDU);
double x_up = mpfr_get_d(op_rnd, high_prec_rnd);
mpfr_set(op_rnd, op_high_prec, MPFR_RNDD);
double x_down = mpfr_get_d(op_rnd, high_prec_rnd);
if (is_round_up(x, x_up, x_down)) {
mpfr_set(op_rnd, op_high_prec, MPFR_RNDU);
}
}
};

using __raptor_stochastic_rnd_t = __raptor_stochastic_rnd_double;

// Thread local variable for calculating stochastic rounding
inline thread_local __raptor_stochastic_rnd_t __raptor_stochastic_rnd;

// Force macro expansion and concatenate to get mpfr function names
#define __RAPTOR_CONCAT_MPFR(prefix, macro) mpfr_##prefix##macro

// Set MPFR_VAR from VAL (of type using to mpfr_set_##MPFR_SET) with stochastic
// rounding
#define __RAPTOR_STOCHASTIC_SET(MPFR_SET, MPFR_VAR, VAL) \
__RAPTOR_CONCAT_MPFR(set_, MPFR_SET)(__raptor_stochastic_rnd.op_high_prec, \
VAL, __raptor_stochastic_rnd.high_prec_rnd); \
__raptor_stochastic_rnd.round(MPFR_VAR);

// Get GET_VAR of type GET_TY (that uses mpfr_get_##MPFR_GET) from MPFR_VAR
// with stochastic rounding
#define __RAPTOR_STOCHASTIC_GET(GET_TY, GET_VAR, MPFR_GET, MPFR_VAR) \
GET_TY GET_VAR = __RAPTOR_CONCAT_MPFR(get_, MPFR_GET)(MPFR_VAR, MPFR_RNDD);\
GET_TY tmp = __RAPTOR_CONCAT_MPFR(get_, MPFR_GET)(MPFR_VAR, MPFR_RNDU); \
double x = mpfr_get_d(MPFR_VAR, __raptor_stochastic_rnd.high_prec_rnd); \
double x_up = tmp; \
double x_down = GET_VAR; \
if (__raptor_stochastic_rnd.is_round_up(x, x_up, x_down)) { \
GET_VAR = tmp; \
}

// Call mpfr_##MPFR_FUNC_NAME and store result in MPFR_DST_VAR with stochastic
// rounding
#define __RAPTOR_STOCHASTIC_FUNC(MPFR_FUNC_NAME, MPFR_DST_VAR, ...) \
__RAPTOR_CONCAT_MPFR(, MPFR_FUNC_NAME)(__raptor_stochastic_rnd.op_high_prec\
,__VA_ARGS__, __raptor_stochastic_rnd.high_prec_rnd); \
__raptor_stochastic_rnd.round(MPFR_DST_VAR);
#else
#define __RATPTOR_USE_STOCHASTIC_ROUNDING false
// Do nothing, should not be used with if constexper(false)
#define __RAPTOR_STOCHASTIC_SET(MPFR_SET, MPFR_VAR, VAL)
// Just declare GET_VAR, should not be used with if constexper(false)
#define __RAPTOR_STOCHASTIC_GET(GET_TY, GET_VAR, MPFR_GET, MPFR_VAR) \
GET_TY GET_VAR;
// Do nothing, should not be used with if constexper(false)
#define __RAPTOR_STOCHASTIC_FUNC(MPFR_FUNC_NAME, MPFR_DST_VAR, ...)
#endif

__RAPTOR_MPFR_ATTRIBUTES
void __raptor_fprt_trunc_change(int64_t is_push, int64_t to_e, int64_t to_m,
int64_t mode, const char *loc, void *scratch) {
Expand Down Expand Up @@ -545,6 +628,11 @@ void raptor_fprt_op_clear();
ARG1 a, int64_t exponent, int64_t significand, int64_t mode, \
const char *loc, mpfr_t *scratch) { \
if (__raptor_fprt_is_op_mode(mode)) { \
if constexpr (__RATPTOR_USE_STOCHASTIC_ROUNDING) { \
__RAPTOR_STOCHASTIC_SET(MPFR_SET_ARG1, scratch[0], a); \
__RAPTOR_STOCHASTIC_GET(RET, c, si, scratch[0]); \
return c; \
} \
mpfr_set_##MPFR_SET_ARG1(scratch[0], a, ROUNDING_MODE); \
RET c = mpfr_get_si(scratch[0], ROUNDING_MODE); \
return c; \
Expand All @@ -562,6 +650,12 @@ void raptor_fprt_op_clear();
const char *loc, mpfr_t *scratch) { \
if (__raptor_fprt_is_op_mode(mode)) { \
__raptor_fprt_trunc_count(exponent, significand, mode, loc, scratch); \
if constexpr (__RATPTOR_USE_STOCHASTIC_ROUNDING) { \
__RAPTOR_STOCHASTIC_SET(MPFR_SET_ARG1, scratch[0], a); \
__RAPTOR_STOCHASTIC_FUNC(MPFR_FUNC_NAME, scratch[2], scratch[0]); \
__RAPTOR_STOCHASTIC_GET(RET, c, MPFR_GET, scratch[2]); \
return c; \
} \
mpfr_set_##MPFR_SET_ARG1(scratch[0], a, ROUNDING_MODE); \
mpfr_##MPFR_FUNC_NAME(scratch[2], scratch[0], ROUNDING_MODE); \
RET c = mpfr_get_##MPFR_GET(scratch[2], ROUNDING_MODE); \
Expand Down Expand Up @@ -592,6 +686,12 @@ void raptor_fprt_op_clear();
const char *loc, mpfr_t *scratch) { \
if (__raptor_fprt_is_op_mode(mode)) { \
__raptor_fprt_trunc_count(exponent, significand, mode, loc, scratch); \
if constexpr (__RATPTOR_USE_STOCHASTIC_ROUNDING) { \
__RAPTOR_STOCHASTIC_SET(MPFR_SET_ARG1, scratch[0], a); \
__RAPTOR_STOCHASTIC_FUNC(MPFR_FUNC_NAME, scratch[2], scratch[0], b); \
__RAPTOR_STOCHASTIC_GET(RET, c, MPFR_GET, scratch[2]); \
return c; \
} \
mpfr_set_##MPFR_SET_ARG1(scratch[0], a, ROUNDING_MODE); \
mpfr_##MPFR_FUNC_NAME(scratch[2], scratch[0], b, ROUNDING_MODE); \
RET c = mpfr_get_##MPFR_GET(scratch[2], ROUNDING_MODE); \
Expand Down Expand Up @@ -620,6 +720,14 @@ void raptor_fprt_op_clear();
const char *loc, mpfr_t *scratch) { \
if (__raptor_fprt_is_op_mode(mode)) { \
__raptor_fprt_trunc_count(exponent, significand, mode, loc, scratch); \
if constexpr (__RATPTOR_USE_STOCHASTIC_ROUNDING) { \
__RAPTOR_STOCHASTIC_SET(MPFR_SET_ARG1, scratch[0], a); \
__RAPTOR_STOCHASTIC_SET(MPFR_SET_ARG2, scratch[1], b); \
__RAPTOR_STOCHASTIC_FUNC(MPFR_FUNC_NAME, scratch[2], scratch[0], \
scratch[1]); \
__RAPTOR_STOCHASTIC_GET(RET, c, MPFR_GET, scratch[2]); \
return c; \
} \
mpfr_set_##MPFR_SET_ARG1(scratch[0], a, ROUNDING_MODE); \
mpfr_set_##MPFR_SET_ARG2(scratch[1], b, ROUNDING_MODE); \
mpfr_##MPFR_FUNC_NAME(scratch[2], scratch[0], scratch[1], \
Expand Down Expand Up @@ -653,6 +761,15 @@ void raptor_fprt_op_clear();
int64_t mode, const char *loc, mpfr_t *scratch) { \
if (__raptor_fprt_is_op_mode(mode)) { \
__raptor_fprt_trunc_count(exponent, significand, mode, loc, scratch); \
if constexpr (__RATPTOR_USE_STOCHASTIC_ROUNDING) { \
__RAPTOR_STOCHASTIC_SET(MPFR_TYPE, scratch[0], a); \
__RAPTOR_STOCHASTIC_SET(MPFR_TYPE, scratch[1], b); \
__RAPTOR_STOCHASTIC_SET(MPFR_TYPE, scratch[2], c); \
__RAPTOR_STOCHASTIC_FUNC(mul, scratch[0], scratch[0], scratch[1]); \
__RAPTOR_STOCHASTIC_FUNC(add, scratch[0], scratch[0], scratch[2]); \
__RAPTOR_STOCHASTIC_GET(TYPE, res, MPFR_TYPE, scratch[0]); \
return res; \
} \
mpfr_set_##MPFR_TYPE(scratch[0], a, ROUNDING_MODE); \
mpfr_set_##MPFR_TYPE(scratch[1], b, ROUNDING_MODE); \
mpfr_set_##MPFR_TYPE(scratch[2], c, ROUNDING_MODE); \
Expand Down Expand Up @@ -694,6 +811,12 @@ void raptor_fprt_op_clear();
const char *loc, mpfr_t *scratch) { \
if (__raptor_fprt_is_op_mode(mode)) { \
__raptor_fprt_trunc_count(exponent, significand, mode, loc, scratch); \
if constexpr (__RATPTOR_USE_STOCHASTIC_ROUNDING) { \
__RAPTOR_STOCHASTIC_SET(MPFR_GET, scratch[0], a); \
__RAPTOR_STOCHASTIC_SET(MPFR_GET, scratch[1], b); \
int ret = mpfr_cmp(scratch[0], scratch[1]); \
return ret CMP; \
} \
mpfr_set_##MPFR_GET(scratch[0], a, ROUNDING_MODE); \
mpfr_set_##MPFR_GET(scratch[1], b, ROUNDING_MODE); \
int ret = mpfr_cmp(scratch[0], scratch[1]); \
Expand Down
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