[Bug ports/15792] New: ARM dynamic linker should save/restore coprocessor registers

["jsm28 at gcc dot gnu.org" <sourceware-bugzilla at sourceware dot org>]

http://sourceware.org/bugzilla/show_bug.cgi?id=15792

            Bug ID: 15792
           Summary: ARM dynamic linker should save/restore coprocessor
                    registers
           Product: glibc
           Version: 2.17
            Status: NEW
          Severity: normal
          Priority: P2
         Component: ports
          Assignee: unassigned at sourceware dot org
          Reporter: jsm28 at gcc dot gnu.org
                CC: carlos at redhat dot com, roland at gnu dot org
              Host: arm*-*-*

The ARM dynamic linker should save and restore the call-clobbered NEON
registers, d0-d7 and d16-d31, if they exist, in _dl_runtime_resolve and
_dl_runtime_profile and the TLS descriptors code.  This requires runtime checks
for whether the registers exist using HWCAPs (unless compiler predefines allow
such checks to be elided).  (Likewise, for iWMMXt call-clobbered registers.)

For d0-d7 this is needed when built with the VFP ABI because they are used for
argument passing in that case.  For all the call-clobbered registers, in all
cases, this is needed because of restricted clobber sets for the TLS descriptor
functions and for functions such as __aeabi_memcpy that may be called via the
dynamic linker.

IFUNCs are the main practical way of triggering this issue, at least for the
case of d0-d7.  It might be worth explicitly disabling VFP and iWMMXt for the
dynamic linker if possible to avoid the cost of checking for and saving the
other registers on every program execution to address what's a fairly
theoretical issue - if the dynamic linker (including memcpy etc. called
therein) never uses VFP or iWMMXt registers, then the only case needing extra
register saving is the case of building for the VFP ABI, which needs to save
d0-d7 only because of the IFUNC issue, with no dynamic checks needed.

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