[Bug stdio/26649] New: printf should handle non-normal x86 long double numbers gracefully

["fweimer at redhat dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=26649

            Bug ID: 26649
           Summary: printf should handle non-normal x86 long double
                    numbers gracefully
           Product: glibc
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: stdio
          Assignee: unassigned at sourceware dot org
          Reporter: fweimer at redhat dot com
  Target Milestone: ---

The behavior of isnan/__builtin_isnan on bit patterns that do not correspond to
something that the CPU would produce from valid inputs is currently
under-defined in the toolchain. (The GCC built-in and glibc disagree.)

The isnan check in PRINTF_FP_FETCH in stdio-common/printf_fp.c assumes the GCC
behavior that returns true for non-normal numbers which are not specified as
NaN. (The glibc implementation returns false for such numbers.)

At present, passing non-normal numbers to __mppn_extract_long_double causes
this function to produce irregularly shaped multi-precision integers,
triggering undefined behavior in __printf_fp_l.

With GCC 10 and glibc 2.32, this behavior is not visible because
__builtin_isnan is used, which avoids calling __mppn_extract_long_double in
this case.

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