https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59666
--- Comment #4 from joseph at codesourcery dot com <joseph at codesourcery dot com> --- On Tue, 1 Mar 2016, vincent-gcc at vinc17 dot net wrote: > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=59666 > > --- Comment #2 from Vincent Lefèvre <vincent-gcc at vinc17 dot net> --- > (In reply to Joseph S. Myers from comment #0) > > I think the right approach for a fix will probably involve setting > > round-to-nearest temporarily within the functions, then adjusting > > overflowing and underflowing results based on the original rounding mode. > > This would not be sufficient. Even without overflow/underflow, the rounding > direction needs to be honored for the basic arithmetic operations. For a non-IEEE type where the accuracy is specified as within 3ulp (depending on the operation in question), there is no definition of what honouring the rounding mode means. Such types fall within 5.2.4.2.2#6, "The accuracy of the floating-point operations (+, -, *, /) ... is implementation-defined ... The implementation may state that the accuracy is unknown.". Note the proposed TC for DR#441: "The value of FLT_ROUNDS applies to all IEC 60559 types supported by the implementation, but need not apply to non-IEC 60559 types.". Temporary setting of round-to-nearest and adjusting results suffices for uses in glibc; I have a libgcc patch to that effect (not suitable for inclusion because it doesn't condition things on a non-default configure option), together with an in-progress set of glibc patches to fix most of the glibc bugs shown up by test-ldouble failures. I think roughly four such glibc fixes plus libm-test-ulps regeneration should allow test-ldouble to pass with patched libgcc (with unpatched libgcc needing also some extension of the existing set of XFAILs for this bug, before a default glibc build can pass test-ldouble on powerpc by default). (Passing with patched libgcc without XFAILs enables the use of random test generation to find bugs in glibc libm. Passing with unpatched libgcc with XFAILs enabled the use of this test to detect regressions.)
