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2020-06-30Fix strtod multiple-precision division bug (bug 26137).Joseph Myers
Bug 26137 reports spurious "inexact" exceptions from strtod, on 32-bit systems only, for a decimal argument that is exactly 1 + 2^-32. In fact the same issue also appears for 1 + 2^-64 and 1 + 2^-96 as arguments to strtof128 on 32-bit systems, and 1 + 2^-64 as an argument to strtof128 on 64-bit systems. In FE_DOWNWARD or FE_TOWARDZERO mode, the return value is also incorrect. The problem is in the multiple-precision division logic used in the case of dividing by a denominator that occupies at least three GMP limbs. There was a comment "The division does not work if the upper limb of the two-limb mumerator is greater than the denominator.", but in fact there were problems for the case of equality (that is, where the high limbs are equal, offset by some multiple of the GMP limb size) as well. In such cases, the code used "quot = ~(mp_limb_t) 0;" (with subsequent correction if that is an overestimate), because udiv_qrnnd does not support the case of equality, but it's possible for the shifted numerator to be greater than or equal to the denominator, in which case that is an underestimate. To avoid that, this patch changes the ">" condition to ">=", meaning the first division is done with a zero high word. The tests added are all 1 + 2^-n for n from 1 to 113 except for those that were already present in tst-strtod-round-data. Tested for x86_64 and x86.
2018-06-13Fix strtod overflow detection (bug 23279).Joseph Myers
As shown by bug 23279, strtod's round_and_return has an off-by-one error in its overflow detection, only counting an exponent greater than MAX_EXP as overflowing when an exponent of MAX_EXP also means overflow (recall the ISO C definition of DBL_MAX_EXP etc. is based on a floating-point model where 2^exp is multiplied by a value in the interval [0.5, 1), so 2^MAX_EXP is not representable). For decimal arguments to strtod, a separate overflow check in the main implementation covers the case where the integer part of the argument (truncated to the nearest integer towards zero) has more than MAX_EXP bits, meaning that this issue in round_and_return only affects cases (arguments with absolute value strictly between the maximum representable value and 2^MAX_EXP) where overflow depends on the rounding mode; in such cases, the returned value would still have been correct on overflow but without the overflow exception being raised or errno being set to ERANGE. For hex float arguments, however, other cases can arise, as shown in bug 23279, where a value with exponent already set to MAX_EXP is passed into round_and_return and a result can wrongly end up being NaN, or infinity instead of the largest finite value. This patch fixes the off-by-one error, adds testing of overflow exceptions to the tst-strtod-round framework, and adds tests of these issues. Tested for x86_64. Also ran the tst-strtod-round tests for powerpc to make sure the new tests didn't introduce any new failures for IBM long double. [BZ #23279] * stdlib/strtod_l.c (round_and_return): Handle an exponent of MAX_EXP as overflowing. * stdlib/gen-tst-strtod-round.c (string_to_fp): Clear MPFR overflow flag. (round_str): Output also whether result overflows in each rounding mode. * stdlib/tst-strtod-round-data: Add more tests. * stdlib/tst-strtod-round-data.h: Regenerated. * stdlib/tst-strtod-round-skeleton.c (_XNTRY): Update comment. (TEST): Handle extra arguments for overflow flags. (struct test_overflow): New type. [!FE_OVERFLOW] (FE_OVERFLOW): Define to 0. (GEN_ONE_TEST): Clear all exceptions. Test overflow flag. (test_in_one_mode): Take argument with overflow information. (do_test): Update calls to test_in_one_mode.
2015-04-10Fix strtof decimal rounding close to half least subnormal (bug 18247).Joseph Myers
Bug 18247 is an off-by-one error in strtof's determination of a decimal exponent such that any value with that decimal exponent is at most half the least subnormal and so the appropriate underflowing value for the rounding mode can be determined with no multiple-precision computations. (Whether the value is in fact safe despite the off-by-one depends on the floating-point format in question. It's wrong for float and for m68k ldbl-96 but not for other supported formats.) This patch corrects the computation of the exponent in question to be safe in general, adding a comment explaining the new computation. Tested for x86_64. [BZ #18247] * stdlib/strtod_l.c (____STRTOF_INTERNAL): Decrease minimum decimal exponent by 1. * stdlib/tst-strtod-round-data: Add more tests. * stdlib/tst-strtod-round.c (tests): Regenerated.
2013-11-13Fix strtod rounding of half the least subnormal (bug 16151).Joseph Myers
2012-08-27Fix sign of zero on strtod underflow (bug 14519).Joseph Myers
2012-08-27Fix strtod rounding (bug 3479).Joseph Myers