Re: [x86] Fix gcc.c-torture/compile/20080806-1.c failures

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Mon, Nov 5, 2012 at 2:21 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
> On Mon, Nov 5, 2012 at 1:01 AM, Uros Bizjak <ubizjak@gmail.com> wrote:
>> On Mon, Nov 5, 2012 at 9:30 AM, Richard Sandiford
>> <rdsandiford@googlemail.com> wrote:
>>> This is a case where we had:
>>>
>>>   (set (reg:HI foo) (plus:HI (reg:HI sp) (const_int X)))
>>>   (clobber (reg:CC FLAGS_REG))
>>>
>>> and the splitters decided to convert it to an LEA:
>>>
>>>   (set (reg:SI foo) (plus:SI (subreg:SI (reg:HI sp) 0) (const_int X)))
>>>
>>> But this fails to match, because ix86_address_subreg_operand
>>> doesn't allow subregs of the stack pointer.
>>>
>>> This shows up an inconsistency in the way the generic code handles
>>> subregs of the stack pointer.  Normally we refuse to fold them, even
>>> after reload, but the final_p case of alter_subreg folds them anyway.
>>> That's how we ended up with the rather odd 16-bit sp.
>>>
>>> However, even if the special alter_subreg case was removed
>>> (so that we continued to use stack_pointer_rtx itself), we'd have:
>>>
>>>   (set (reg:HI foo) (plus:HI (subreg:HI (reg:DI sp) 0) (const_int X)))
>>>   (clobber (reg:CC FLAGS_REG))
>>>
>>> which would get converted to:
>>>
>>>   (set (reg:SI foo) (plus:SI (subreg:SI (reg:DI sp) 0) (const_int X)))
>>>
>>> and we'd ICE in the same way.
>>>
>>> The reason x86 rejects subregs of the stack pointer is this same refusal
>>> to fold.  ix86_print_operand_address tries to simplify a SUBREG to a REG
>>> and simplify_subreg wouldn't do anything for sp.
>>>
>>> simplify_subreg isn't a lot of help at the output stage though.
>>> If the insn stream contains a subreg that could be simplified but
>>> hasn't been, then IMO that's a bug.  The cases we have to handle here
>>> are those that can't be simplified (unless we decide at some point that
>>> all registers must be simplifiable after reload, in which case we shouldn't
>>> need to handle SUBREGs at all).
>>>
>>> As things stand, I think we should be using true_regnum in this case instead.
>>>
>>> Tested on x86_64-linux-gnu.  OK to install?
>>
>> Let's ask H.J. to test this change on x32.
>>
>>> gcc/
>>>         PR target/55204
>>>         * config/i386/i386.c (ix86_address_subreg_operand): Remove stack
>>>         pointer check.
>>>         (print_reg): Use true_regnum rather than REGNO.
>>>         (ix86_print_operand_address): Remove SUBREG handling.
>>
>> The patch is OK for mainline and 4.7, if it passes H.J.'s tests with
>> -maddress-mode={short,long} on x32.
>>
>
> I am on it.  It will take a while.
>

I tested it with -maddress-mode={short,long} on x32.  There are
no regressions in GCC nor glibc testsuite.

Thanks.

-- 
H.J.