[s390] increase stack alignment

[s390] increase stack alignment

[DJ Delorie]

The CDSG opcode requires 16 byte alignment, not just 8.  Ok to apply?

2008-10-03  DJ Delorie  <dj@redhat.com>

	* config/s390/s390.h (STACK_BOUNDARY): Increase stack alignment to
	128 bits.

Index: config/s390/s390.h
===================================================================
--- config/s390/s390.h	(revision 140862)
+++ config/s390/s390.h	(working copy)
@@ -220,13 +220,13 @@ if (INTEGRAL_MODE_P (MODE) &&	        	 
 	  }
 
 /* Allocation boundary (in *bits*) for storing arguments in argument list.  */
 #define PARM_BOUNDARY (TARGET_64BIT ? 64 : 32)
 
 /* Boundary (in *bits*) on which stack pointer should be aligned.  */
-#define STACK_BOUNDARY 64
+#define STACK_BOUNDARY 128
 
 /* Allocation boundary (in *bits*) for the code of a function.  */
 #define FUNCTION_BOUNDARY 32
 
 /* There is no point aligning anything to a rounder boundary than this.  */
 #define BIGGEST_ALIGNMENT 64

Re: [s390] increase stack alignment

[Andreas Krebbel]

On Fri, Oct 03, 2008 at 09:08:59PM -0400, DJ Delorie wrote:
> 
> The CDSG opcode requires 16 byte alignment, not just 8.  Ok to apply?

You are right regarding the cdsg instruction.  But I don't think this
is a correct fix. I think this would be an ABI change.  Linking code
with the old stack alignment together with new code would not sustain
the 16 byte alignment anymore. Also the packed stack layout would need
adjustment to apply the proper alignment when saving an odd number of
GPRS.

I also consider an atomic operation on an auto variable to be a
relatively rare case.  For other variables the required alignment can
easily be enforced with the respective attribute.  So could you please
elaborate why you think this is necessary.

Bye,

-Andreas-

Re: [s390] increase stack alignment

[Andrew Pinski]

On Wed, Oct 8, 2008 at 1:55 PM, DJ Delorie <dj@redhat.com> wrote:
>
>> So could you please elaborate why you think this is necessary.
>
> The CDSG opcode needs 16 byte alignment when accessing stack
> variables, currently it doesn't get it, I'm not really picky about how
> it gets fixed ;-)
>
> In the test case I have (from a customer), the cdsg opcode is in an
> inline asm, so gcc can't know that that stack is "special".  However,
> the variables on stack are tagged for 32-byte alignment, and only get
> 8-byte alignment.  Perhaps something in gcc could note when stack
> variables need greater alignment than the stack normally provides, and
> adjust the stack at runtime accordingly?

See http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16660.  I had posted a
patch which realigned the stack variables if needed but it was not
commented on except by HJL.  Intel implemented something different for
x86 where they realign the whole stack.

Thanks,
Andrew Pinski

Re: [s390] increase stack alignment

[DJ Delorie]

> So could you please elaborate why you think this is necessary.

The CDSG opcode needs 16 byte alignment when accessing stack
variables, currently it doesn't get it, I'm not really picky about how
it gets fixed ;-)

In the test case I have (from a customer), the cdsg opcode is in an
inline asm, so gcc can't know that that stack is "special".  However,
the variables on stack are tagged for 32-byte alignment, and only get
8-byte alignment.  Perhaps something in gcc could note when stack
variables need greater alignment than the stack normally provides, and
adjust the stack at runtime accordingly?

Other suggestions are also welcome :-)

Re: [s390] increase stack alignment

[H.J. Lu]

On Wed, Oct 8, 2008 at 1:55 PM, DJ Delorie <dj@redhat.com> wrote:
>
>> So could you please elaborate why you think this is necessary.
>
> The CDSG opcode needs 16 byte alignment when accessing stack
> variables, currently it doesn't get it, I'm not really picky about how
> it gets fixed ;-)
>
> In the test case I have (from a customer), the cdsg opcode is in an
> inline asm, so gcc can't know that that stack is "special".  However,
> the variables on stack are tagged for 32-byte alignment, and only get
> 8-byte alignment.  Perhaps something in gcc could note when stack
> variables need greater alignment than the stack normally provides, and
> adjust the stack at runtime accordingly?
>

The middle end does collect all alignment requirements on stack. But
the backend has to use this info to align the stack properly. Right now
only x86 backend can align the stack dynamically.


-- 
H.J.

Re: [s390] increase stack alignment

[H.J. Lu]

On Wed, Oct 8, 2008 at 1:59 PM, Andrew Pinski <pinskia@gmail.com> wrote:
> On Wed, Oct 8, 2008 at 1:55 PM, DJ Delorie <dj@redhat.com> wrote:
>>
>>> So could you please elaborate why you think this is necessary.
>>
>> The CDSG opcode needs 16 byte alignment when accessing stack
>> variables, currently it doesn't get it, I'm not really picky about how
>> it gets fixed ;-)
>>
>> In the test case I have (from a customer), the cdsg opcode is in an
>> inline asm, so gcc can't know that that stack is "special".  However,
>> the variables on stack are tagged for 32-byte alignment, and only get
>> 8-byte alignment.  Perhaps something in gcc could note when stack
>> variables need greater alignment than the stack normally provides, and
>> adjust the stack at runtime accordingly?
>
> See http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16660.  I had posted a
> patch which realigned the stack variables if needed but it was not
> commented on except by HJL.  Intel implemented something different for
> x86 where they realign the whole stack.

Andrew, maybe you can update your patch with SUPPORTS_STACK_ALIGNMENT.
If SUPPORTS_STACK_ALIGNMENT is true, stack variables can be
aligned properly.

-- 
H.J.