On Sat, Jan 4, 2014 at 8:16 AM, Joseph S. Myers <jos...@codesourcery.com> wrote: > This patch fixes various cases of spurious overflow exceptions in the > IBM long double support code. The generic issue is that an initial > approximation is computed by using the relevant arithmetic operation > on the high parts of the operands - but this may overflow double in > some cases where the final result is large but still a long way (up to > around 2^53 ulp) from overflowing long double. For division overflow > could occur not just from the initial a / c division but also from the > subsequent multiplication of the result by c (in some cases where a is > DBL_MAX, say), when the final result of the division need not be large > at all. > > __gcc_qadd already tried to handle such overflow cases, but detected > them by examining the result of the addition of high parts - which > leaves a spurious overflow exception raised even if it returns the > correct non-overflowing value. This patch instead checks the operands > and does appropriate scaling, in all of __gcc_qadd, __gcc_qmul and > __gcc_qdiv, to avoid spurious overflow exceptions arising as well as > avoiding the bad results arising from such overflows. > > Tested with no regressions with cross to powerpc-linux-gnu (and also > ran the glibc libm tests, which provide a rather more thorough test of > floating-point arithmetic than the GCC testsuite; this patch fixes, at > least, bad results from cbrtl (LDBL_MAX) that arose from the second > division issue mentioned, as well as the specific cases shown in the > tests added to the GCC testsuite). OK to commit?
Before we go farther down this path, IBM needs to internally decide on the end goal and the amount of language / library conformance that makes sense for the IBM long double format. Thanks, David
