diff options
Diffstat (limited to 'src/mpfr-impl.h')
| -rw-r--r-- | src/mpfr-impl.h | 41 |
1 files changed, 25 insertions, 16 deletions
diff --git a/src/mpfr-impl.h b/src/mpfr-impl.h index cb7c752a5..64a43e516 100644 --- a/src/mpfr-impl.h +++ b/src/mpfr-impl.h @@ -2522,16 +2522,21 @@ __MPFR_DECLSPEC extern int __gmpfr_cov_sum_tmd[MPFR_RND_MAX][2][2][3][2][2]; extern "C" { #endif -/* An UBF is like a MPFR number, but with an additional mpz_t member, - which is assumed to be present (with a value in it) when the usual - exponent field has the value MPFR_EXP_UBF. The goal of this compatible - representation is to easily be able to support UBF in "normal" code - using the public API. This is some form of "subtyping". +/* An UBF is like a MPFR number, but with an additional member _mpfr_zexp + of type mpz_t, which is assumed to be present (with a value in it) + when the usual exponent field has the value MPFR_EXP_UBF. The goal of + this compatible representation is to easily be able to support UBF in + "normal" code using the public API. This is some form of "subtyping". Unfortunately this breaks aliasing rules, and C does not provide any way to avoid that (except with additional syntax ugliness and API breakage): https://news.ycombinator.com/item?id=11753236 + A workaround should be to use mpfr_ptr to access the usual mpfr_t members + and mpfr_ubf_ptr to access the additional member _mpfr_zexp. And never + use __mpfr_ubf_struct as a declared type; otherwise this would force + __mpfr_ubf_struct to be the effective type of the whole object. + The alignment requirement for __mpfr_ubf_struct (UBF) needs to be at least as strong as the one for __mpfr_struct (MPFR number); this is not required by the C standard, but this should always be the case in practice, since @@ -2545,13 +2550,9 @@ extern "C" { with variadic functions, as the compiler will not be able to check in general. See fmma.c as an example of usage. - In general, the type used for values that may be UBF must be either - mpfr_ubf_t or mpfr_ubf_ptr. The type mpfr_ptr or mpfr_srcptr may be - used for UBF only in the case where the pointer has been converted - from mpfr_ubf_ptr, in order to ensure valid alignment. For instance, - in mpfr_fmma_aux, one uses mpfr_ubf_t to generate the exact products - as UBF; then the corresponding pointers are converted to mpfr_srcptr - for mpfr_add (even though they point to UBF). + When generating a pointer of a value that may be a UBF, make sure + that the equivalent of mpfr_ubf_ptr is used, but be careful with + the aliasing rules. Functions that can accept either MPFR arguments (mpfr_ptr type) or UBF arguments (mpfr_ubf_ptr type) must use a pointer type that can @@ -2572,9 +2573,16 @@ typedef struct { mpz_t _mpfr_zexp; } __mpfr_ubf_struct; -typedef __mpfr_ubf_struct mpfr_ubf_t[1]; typedef __mpfr_ubf_struct *mpfr_ubf_ptr; +/* The following is a trick to allocate a UBF as an automatic variable + with the required alignment but without forcing the effective type + to __mpfr_ubf_struct, which would break the aliasing rules. */ +typedef union { + __mpfr_ubf_struct u[1]; + __mpfr_struct m[1]; +} mpfr_ubf_t; + __MPFR_DECLSPEC void mpfr_ubf_mul_exact (mpfr_ubf_ptr, mpfr_srcptr, mpfr_srcptr); __MPFR_DECLSPEC int mpfr_ubf_exp_less_p (mpfr_srcptr, mpfr_srcptr); @@ -2589,11 +2597,12 @@ __MPFR_DECLSPEC mpfr_exp_t mpfr_ubf_diff_exp (mpfr_srcptr, mpfr_srcptr); For practical reasons, the type of the argument x can be either mpfr_ubf_ptr or mpfr_ptr, since the latter is used in functions that accept both MPFR numbers and UBF's; this is checked by the - code "(x)->_mpfr_exp". + code "(x)->_mpfr_exp" (the "sizeof" prevents an access, which + could be undefined behavior, depending on the type of x). This macro can be used when building a UBF. So we do not check that the _mpfr_exp field has the value MPFR_EXP_UBF. */ #define MPFR_ZEXP(x) \ - ((void) (x)->_mpfr_exp, \ + ((void) sizeof ((x)->_mpfr_exp), \ ((mpfr_ubf_ptr) (x))->_mpfr_zexp) /* If x is a UBF, clear its mpz_t exponent. */ @@ -2604,6 +2613,6 @@ __MPFR_DECLSPEC mpfr_exp_t mpfr_ubf_diff_exp (mpfr_srcptr, mpfr_srcptr); the interval [MPFR_EXP_MIN,MPFR_EXP_MAX]). */ #define MPFR_UBF_GET_EXP(x) \ (MPFR_IS_UBF (x) ? mpfr_ubf_zexp2exp (MPFR_ZEXP (x)) : \ - MPFR_GET_EXP ((mpfr_ptr) (x))) + MPFR_GET_EXP (x)) #endif /* __MPFR_IMPL_H__ */ |