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expt: avoid undefined integer overflow in fixnum implementation

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Also, extend fixnum fast path to work more often in 64-bit mode.

More review would be appreciated to help ensure that the revised
implementation avoids undefined behavior in C.
This commit is contained in:
Matthew Flatt 2014-01-26 20:13:24 -07:00
parent 58b898bc13
commit e96d592735
1 changed files with 19 additions and 12 deletions
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31
racket/src/racket/src/number.c
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@ -50,9 +50,11 @@
#ifdef SIXTY_FOUR_BIT_INTEGERS #ifdef SIXTY_FOUR_BIT_INTEGERS
# define MAX_SHIFT_TRY 61 # define MAX_SHIFT_TRY 61
# define MAX_SHIFT_EVER 64 # define MAX_SHIFT_EVER 64
# define MAX_FIXNUM_SQRT 3037000498
#else #else
# define MAX_SHIFT_TRY 29 # define MAX_SHIFT_TRY 29
# define MAX_SHIFT_EVER 32 # define MAX_SHIFT_EVER 32
# define MAX_FIXNUM_SQRT 46339
#endif #endif
/* locals */ /* locals */
@ -3217,31 +3219,36 @@ static Scheme_Object *fixnum_expt(intptr_t x, intptr_t y)
if ((x == 2) && (y <= MAX_SHIFT_TRY)) if ((x == 2) && (y <= MAX_SHIFT_TRY))
return scheme_make_integer((intptr_t)1 << y); return scheme_make_integer((intptr_t)1 << y);
else else {
{
intptr_t result = 1; intptr_t result = 1;
int odd_result = (x < 0) && (y & 0x1); int neg_result = (x < 0) && (y & 0x1);
if (x < 0) if (x < 0)
x = -x; x = -x;
while (y > 0)
{ while (y > 0) {
/* x^y*result is invariant and result <= x */ /* x^y*result is invariant and result <= x */
if (x > 46339 && y > 1) /* x * x won't fit in 31 bits */ if (x > MAX_FIXNUM_SQRT && y > 1) /* x * x won't fit in fixnum */
return scheme_generic_integer_power(scheme_make_integer_value(orig_x), scheme_make_integer_value(orig_y)); return scheme_generic_integer_power(scheme_make_integer_value(orig_x),
scheme_make_integer_value(orig_y));
if (y & 0x1) /* if (odd?) */ if (y & 0x1) /* if (odd?) */
{ {
intptr_t next_result = x * result; uintptr_t next_result = (uintptr_t)x * (uintptr_t)result;
if (y == 1 && x > 46339 && !(next_result / x == result)) if ((y == 1)
return scheme_generic_integer_power(scheme_make_integer_value(orig_x), scheme_make_integer_value(orig_y)); && (x > MAX_FIXNUM_SQRT)
&& (((intptr_t)next_result < 0)
|| !(next_result / (uintptr_t)x == (uintptr_t)result)))
return scheme_generic_integer_power(scheme_make_integer_value(orig_x),
scheme_make_integer_value(orig_y));
else else
result = next_result; result = (intptr_t)next_result;
} }
y = y >> 1; y = y >> 1;
x = x * x; x = x * x;
} }
return scheme_make_integer_value(odd_result ? -result : result);
return scheme_make_integer_value(neg_result ? -result : result);
} }
} }