RE: Fix for "FAIL: tmpdir-gcc.dg-struct-layout-1/t028 c_compat_x_tst.o compile, (internal compiler error)"

["David Sherwood" <david dot sherwood at arm dot com>]

Hi Vladimir,

Sorry, I forgot to CC you on this as it's your code. It's my first attempt at
submitting patches to gcc so I'm still learning as I go!

Kind Regards,
David Sherwood.

-----Original Message-----
From: David Sherwood [mailto:david.sherwood@arm.com] 
Sent: 05 September 2014 15:52
To: 'gcc-patches@gcc.gnu.org'
Subject: Fix for "FAIL: tmpdir-gcc.dg-struct-layout-1/t028 c_compat_x_tst.o compile, (internal
compiler error)"

Hi,

I have a potential fix for a gcc testsuite failure for aarch64 in big endian, i.e.

FAIL: tmpdir-gcc.dg-struct-layout-1/t028 c_compat_x_tst.o compile, (internal compiler error)
FAIL: tmpdir-gcc.dg-struct-layout-1/t028 c_compat_y_tst.o compile, (internal compiler error)

It is caused by the inappropriate choice of hard registers for paradoxical sub registers in
big endian mode, for example if register 0 is chosen for a paradoxical TI subreg on big
endian then we may end up attempting to reference register -1. Similarly, on little endian
we could end up going beyond the upper bounds of the register file too.

My fix involves adding particular constraints in IRA on the choice of register once paradoxical 
sub registers are encountered. However, Richard Sandiford also proposed an alternative
solution that involves not constraining registers in IRA, but rather making use of cost analysis
instead and letting LRA do the work. Not sure what your preference is ....

Fix was tested on aarch64 on little and big endian with no regressions.

Regards,
David Sherwood.

2014-08-26  David Sherwood  <david.sherwood@arm.com>

        * ira-int.h (ira_allocno): Add "wmode" field.
        * ira-build.c (create_insn_allocnos): Add new "parent" function
        parameter.
        * ira-conflicts.c (ira_build_conflicts): Add conflicts for registers
        that cannot be accessed in wmode.