Candidate fix for ia64 compat/struct-return-19 failure

[Mark Mitchell]

Zack Weinberg wrote:

>The compat/struct-return-19 testcase has been failing on ia64-hpux
>(and I imagine all other ia64 targets as well) for some time.  I have
>tracked the problem to an improperly handled case in several parts of
>the code generation for function calls.
>
>On ia64, this structure 
>
>struct Ssds { struct Sd { double d; } sd; };
>
>is passed in registers and returned in registers.  Floating-point
>registers.  (It's a 'homogenous floating-point aggregate'.)  GCC
>emits code to load it into the appropriate floating point register for
>both call and return, but it uses the wrong move instruction: the one
>you're supposed to use to load an integer into a floating-point
>register.  (ldf8 instead of ldfd; setf.sig instead of setf.d; etc.)
>This garbles the value.
>
>The basic issue is that calls.c, expr.c, function.c, and ia64.c all
>assume in various places that a BLKmode quantity is to be loaded into
>a register, any register, using word_mode (DImode, in this case)
>moves.  This is wrong when loading a DFmode value into one of the
>ia64's floating point registers; it's gotta be loaded as DFmode or it
>will be mangled.
>
>The simplest fix for this problem that I can find is to give the
>structure type DFmode instead of BLKmode.  This is the default
>behavior, but it's being overridden in ia64/hpux.h by
>MEMBER_TYPE_FORCES_BLK, which evaluates true for any nested structure
>no matter what its contents.  As far as I can tell, the point of that
>definition is to get a structure containing a short integer to be
>passed in the high half of the register as required for big-endian
>IA64.  (Y'all may puke now.)  Giving BLKmode to the above structure is
>unintentional.  This can easily be corrected by having
>MEMBER_TYPE_FORCES_BLK look only at its mode argument, and that is
>what the patch does.
>
>There is no PR for this failure but I would like to suggest it for 3.4
>anyway since it fixes a calling-convention bug.  (There are no ABI
>consequences - it's passing the wrong value in the right place.)  The
>patch by definition cannot affect any target other than ia64-hpux.
>
>Bootstrapped on ia64-hp-hpux11.23 (3.4 branch, C/C++ only) with no
>regressions.  OK mainline?  3.4 branch?
>
Yup, this is OK for mainline and 3.4.0.

Thanks for detailed explanation.

-- 
Mark Mitchell
CodeSourcery, LLC
(916) 791-8304
mark@codesourcery.com