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py: Add MICROPY_USE_GCC_MUL_OVERFLOW_INTRINSIC.

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Most MCUs apart from Cortex-M0 with Thumb 1 have an instruction
for computing the "high part" of a multiplication (e.g., the upper
32 bits of a 32x32 multiply).

When they do, gcc uses this to implement a small and fast
overflow check using the __builtin_mul_overflow intrinsic, which
is preferable to the guard division method previously used in smallint.c.

However, in contrast to the previous mp_small_int_mul_overflow
routine, which checks that the result fits not only within mp_int_t
but is SMALL_INT_FITS(), __builtin_mul_overflow only checks for
overflow of the C type. As a result, a slight change in the code
flow is needed for MP_BINARY_OP_MULTIPLY.

Other sites using mp_small_int_mul_overflow already had the
result value flow through to a SMALL_INT_FITS check so they didn't
need any additional changes.

Do similarly for the _ll and _ull multiply overflows checks.

Signed-off-by: Jeff Epler <jepler@gmail.com>
This commit is contained in:
Jeff Epler
2025-07-23 16:14:22 -05:00
committed by Damien George
parent 3dd8073c29
commit a809132921
7 changed files with 105 additions and 86 deletions
Download Patch File Download Diff File Expand all files Collapse all files

49
py/misc.h
View File

@@ -35,7 +35,11 @@
#include <stdbool.h>
#include <stdint.h>
#include <stddef.h>
#if __cplusplus // Required on at least one compiler to get ULLONG_MAX
#include <climits>
#else
#include <limits.h>
#endif
typedef unsigned char byte;
typedef unsigned int uint;
@@ -454,7 +458,7 @@ static inline uint32_t mp_clz_mpi(mp_int_t x) {
#endif
}
// Overflow-checked operations for long long
// Overflow-checked operations
// Integer overflow builtins were added to GCC 5, but __has_builtin only in GCC 10
//
@@ -462,45 +466,28 @@ static inline uint32_t mp_clz_mpi(mp_int_t x) {
// functions below don't update the result if an overflow would occur (to avoid UB).
#define MP_GCC_HAS_BUILTIN_OVERFLOW (__GNUC__ >= 5)
#if __has_builtin(__builtin_umulll_overflow) || MP_GCC_HAS_BUILTIN_OVERFLOW
#if MICROPY_USE_GCC_MUL_OVERFLOW_INTRINSIC
#define mp_mul_ull_overflow __builtin_umulll_overflow
#define mp_mul_ll_overflow __builtin_smulll_overflow
static inline bool mp_mul_mp_int_t_overflow(mp_int_t x, mp_int_t y, mp_int_t *res) {
// __builtin_mul_overflow is a type-generic function, this inline ensures the argument
// types are checked to match mp_int_t.
return __builtin_mul_overflow(x, y, res);
}
#else
inline static bool mp_mul_ull_overflow(unsigned long long int x, unsigned long long int y, unsigned long long int *res) {
bool mp_mul_ll_overflow(long long int x, long long int y, long long int *res);
bool mp_mul_mp_int_t_overflow(mp_int_t x, mp_int_t y, mp_int_t *res);
static inline bool mp_mul_ull_overflow(unsigned long long int x, unsigned long long int y, unsigned long long int *res) {
if (y > 0 && x > (ULLONG_MAX / y)) {
return true; // overflow
}
*res = x * y;
return false;
}
#endif
#if __has_builtin(__builtin_smulll_overflow) || MP_GCC_HAS_BUILTIN_OVERFLOW
#define mp_mul_ll_overflow __builtin_smulll_overflow
#else
inline static bool mp_mul_ll_overflow(long long int x, long long int y, long long int *res) {
bool overflow;
// Check for multiply overflow; see CERT INT32-C
if (x > 0) { // x is positive
if (y > 0) { // x and y are positive
overflow = (x > (LLONG_MAX / y));
} else { // x positive, y nonpositive
overflow = (y < (LLONG_MIN / x));
} // x positive, y nonpositive
} else { // x is nonpositive
if (y > 0) { // x is nonpositive, y is positive
overflow = (x < (LLONG_MIN / y));
} else { // x and y are nonpositive
overflow = (x != 0 && y < (LLONG_MAX / x));
} // End if x and y are nonpositive
} // End if x is nonpositive
if (!overflow) {
*res = x * y;
}
return overflow;
}
#endif
#if __has_builtin(__builtin_saddll_overflow) || MP_GCC_HAS_BUILTIN_OVERFLOW

19
py/mpconfig.h
View File

@@ -2336,4 +2336,23 @@ typedef time_t mp_timestamp_t;
#define MP_WARN_CAT(x) (NULL)
#endif
// If true, use __builtin_mul_overflow (a gcc intrinsic supported by clang) for
// overflow checking when multiplying two small ints. Otherwise, use a portable
// algorithm.
//
// Most MCUs have a 32x32->64 bit multiply instruction, in which case the
// intrinsic is likely to be faster and generate smaller code. The main exception is
// cortex-m0 with __ARM_ARCH_ISA_THUMB == 1.
//
// The intrinsic is in GCC starting with version 5.
#ifndef MICROPY_USE_GCC_MUL_OVERFLOW_INTRINSIC
#if defined(__ARM_ARCH_ISA_THUMB) && (__GNUC__ >= 5)
#define MICROPY_USE_GCC_MUL_OVERFLOW_INTRINSIC (__ARM_ARCH_ISA_THUMB >= 2)
#elif (__GNUC__ >= 5)
#define MICROPY_USE_GCC_MUL_OVERFLOW_INTRINSIC (1)
#else
#define MICROPY_USE_GCC_MUL_OVERFLOW_INTRINSIC (0)
#endif
#endif
#endif // MICROPY_INCLUDED_PY_MPCONFIG_H

3
py/parsenum.c
View File

@@ -28,6 +28,7 @@
#include <stdlib.h>
#include "py/runtime.h"
#include "py/misc.h"
#include "py/parsenumbase.h"
#include "py/parsenum.h"
#include "py/smallint.h"
@@ -52,7 +53,7 @@ static MP_NORETURN void raise_exc(mp_obj_t exc, mp_lexer_t *lex) {
// to bigint parsing if supported)
typedef mp_int_t parsed_int_t;
#define PARSED_INT_MUL_OVERFLOW mp_small_int_mul_overflow
#define PARSED_INT_MUL_OVERFLOW mp_mul_mp_int_t_overflow
#define PARSED_INT_FITS MP_SMALL_INT_FITS
#else
// In the special case where bigint support is long long, we save code size by

27
py/runtime.c
View File

@@ -430,7 +430,7 @@ mp_obj_t MICROPY_WRAP_MP_BINARY_OP(mp_binary_op)(mp_binary_op_t op, mp_obj_t lhs
// Operations that can overflow:
// + result always fits in mp_int_t, then handled by SMALL_INT check
// - result always fits in mp_int_t, then handled by SMALL_INT check
// * checked explicitly
// * checked explicitly for fit in mp_int_t, then handled by SMALL_INT check
// / if lhs=MIN and rhs=-1; result always fits in mp_int_t, then handled by SMALL_INT check
// % if lhs=MIN and rhs=-1; result always fits in mp_int_t, then handled by SMALL_INT check
// << checked explicitly
@@ -489,31 +489,16 @@ mp_obj_t MICROPY_WRAP_MP_BINARY_OP(mp_binary_op)(mp_binary_op_t op, mp_obj_t lhs
break;
case MP_BINARY_OP_MULTIPLY:
case MP_BINARY_OP_INPLACE_MULTIPLY: {
// If long long type exists and is larger than mp_int_t, then
// we can use the following code to perform overflow-checked multiplication.
// Otherwise (eg in x64 case) we must use mp_small_int_mul_overflow.
#if 0
// compute result using long long precision
long long res = (long long)lhs_val * (long long)rhs_val;
if (res > MP_SMALL_INT_MAX || res < MP_SMALL_INT_MIN) {
// result overflowed SMALL_INT, so return higher precision integer
return mp_obj_new_int_from_ll(res);
} else {
// use standard precision
lhs_val = (mp_int_t)res;
}
#endif
mp_int_t int_res;
if (mp_small_int_mul_overflow(lhs_val, rhs_val, &int_res)) {
if (mp_mul_mp_int_t_overflow(lhs_val, rhs_val, &int_res)) {
// use higher precision
lhs = mp_obj_new_int_from_ll(lhs_val);
goto generic_binary_op;
} else {
// use standard precision
return MP_OBJ_NEW_SMALL_INT(int_res);
lhs_val = int_res;
}
break;
}
case MP_BINARY_OP_FLOOR_DIVIDE:
case MP_BINARY_OP_INPLACE_FLOOR_DIVIDE:
@@ -553,7 +538,7 @@ mp_obj_t MICROPY_WRAP_MP_BINARY_OP(mp_binary_op)(mp_binary_op_t op, mp_obj_t lhs
mp_int_t ans = 1;
while (rhs_val > 0) {
if (rhs_val & 1) {
if (mp_small_int_mul_overflow(ans, lhs_val, &ans)) {
if (mp_mul_mp_int_t_overflow(ans, lhs_val, &ans)) {
goto power_overflow;
}
}
@@ -562,7 +547,7 @@ mp_obj_t MICROPY_WRAP_MP_BINARY_OP(mp_binary_op)(mp_binary_op_t op, mp_obj_t lhs
}
rhs_val /= 2;
mp_int_t int_res;
if (mp_small_int_mul_overflow(lhs_val, lhs_val, &int_res)) {
if (mp_mul_mp_int_t_overflow(lhs_val, lhs_val, &int_res)) {
goto power_overflow;
}
lhs_val = int_res;

60
py/runtime_utils.c
View File

@@ -50,3 +50,63 @@ mp_obj_t mp_call_function_2_protected(mp_obj_t fun, mp_obj_t arg1, mp_obj_t arg2
return MP_OBJ_NULL;
}
}
#if !MICROPY_USE_GCC_MUL_OVERFLOW_INTRINSIC
bool mp_mul_ll_overflow(long long int x, long long int y, long long int *res) {
bool overflow;
// Check for multiply overflow; see CERT INT32-C
if (x > 0) { // x is positive
if (y > 0) { // x and y are positive
overflow = (x > (LLONG_MAX / y));
} else { // x positive, y nonpositive
overflow = (y < (LLONG_MIN / x));
} // x positive, y nonpositive
} else { // x is nonpositive
if (y > 0) { // x is nonpositive, y is positive
overflow = (x < (LLONG_MIN / y));
} else { // x and y are nonpositive
overflow = (x != 0 && y < (LLONG_MAX / x));
} // End if x and y are nonpositive
} // End if x is nonpositive
if (!overflow) {
*res = x * y;
}
return overflow;
}
#define MP_UINT_MAX (~(mp_uint_t)0)
#define MP_INT_MAX ((mp_int_t)(MP_UINT_MAX >> 1))
#define MP_INT_MIN (-MP_INT_MAX - 1)
bool mp_mul_mp_int_t_overflow(mp_int_t x, mp_int_t y, mp_int_t *res) {
// Check for multiply overflow; see CERT INT32-C
if (x > 0) { // x is positive
if (y > 0) { // x and y are positive
if (x > (MP_INT_MAX / y)) {
return true;
}
} else { // x positive, y nonpositive
if (y < (MP_INT_MIN / x)) {
return true;
}
} // x positive, y nonpositive
} else { // x is nonpositive
if (y > 0) { // x is nonpositive, y is positive
if (x < (MP_INT_MIN / y)) {
return true;
}
} else { // x and y are nonpositive
if (x != 0 && y < (MP_INT_MAX / x)) {
return true;
}
} // End if x and y are nonpositive
} // End if x is nonpositive
// Result doesn't overflow
*res = x * y;
return false;
}
#endif

29
py/smallint.c
View File

@@ -26,35 +26,6 @@
#include "py/smallint.h"
bool mp_small_int_mul_overflow(mp_int_t x, mp_int_t y, mp_int_t *res) {
// Check for multiply overflow; see CERT INT32-C
if (x > 0) { // x is positive
if (y > 0) { // x and y are positive
if (x > (MP_SMALL_INT_MAX / y)) {
return true;
}
} else { // x positive, y nonpositive
if (y < (MP_SMALL_INT_MIN / x)) {
return true;
}
} // x positive, y nonpositive
} else { // x is nonpositive
if (y > 0) { // x is nonpositive, y is positive
if (x < (MP_SMALL_INT_MIN / y)) {
return true;
}
} else { // x and y are nonpositive
if (x != 0 && y < (MP_SMALL_INT_MAX / x)) {
return true;
}
} // End if x and y are nonpositive
} // End if x is nonpositive
// Result doesn't overflow
*res = x * y;
return false;
}
mp_int_t mp_small_int_modulo(mp_int_t dividend, mp_int_t divisor) {
// Python specs require that mod has same sign as second operand
dividend %= divisor;

4
py/smallint.h
View File

@@ -68,10 +68,6 @@
// The number of bits in a MP_SMALL_INT including the sign bit.
#define MP_SMALL_INT_BITS (MP_IMAX_BITS(MP_SMALL_INT_MAX) + 1)
// Multiply two small ints.
// If returns false, the correct result is stored in 'res'
// If returns true, the multiplication would have overflowed. 'res' is unchanged.
bool mp_small_int_mul_overflow(mp_int_t x, mp_int_t y, mp_int_t *res);
mp_int_t mp_small_int_modulo(mp_int_t dividend, mp_int_t divisor);
mp_int_t mp_small_int_floor_divide(mp_int_t num, mp_int_t denom);