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Re: [PATCH][expr.c] PR 65358 Avoid clobbering partial argument during sibcall

On 2015-04-27 6:12 AM, Kyrill Tkachov wrote:


On 22/04/15 12:51, Kyrill Tkachov wrote:

On 21/04/15 18:33, Kyrill Tkachov wrote:

On 21/04/15 15:09, Jeff Law wrote:

On 04/21/2015 02:30 AM, Kyrill Tkachov wrote:
From reading config/stormy16/stormy-abi it seems to me that we don't pass arguments partially in stormy16, so this code would never be called 
there. That leaves pa as the potential problematic target.
I don't suppose there's an easy way to test on pa? My checkout of binutils 
doesn't seem to include a sim target for it.

No simulator, no machines in the testfarm, the box I had access to via
parisc-linux.org seems dead and my ancient PA overheats well before a
bootstrap could complete.  I often regret knowing about the backwards
way many things were done on the PA because it makes me think about
cases that only matter on dead architectures.

So what should be the action plan here? I can't add an assert on
positive result as a negative result is valid.

We want to catch the case where this would cause trouble on
pa, or change the patch until we're confident that it's fine
for pa.

That being said, reading the documentation of STACK_GROWS_UPWARD
and ARGS_GROW_DOWNWARD I'm having a hard time visualising a case
where this would cause trouble on pa.

Is the problem that in the function:

+/* Add SIZE to X and check whether it's greater than Y.
+ If it is, return the constant amount by which it's greater or smaller. + If the two are not statically comparable (for example, X and Y contain + different registers) return -1. This is used in expand_push_insn to + figure out if reading SIZE bytes from location X will end up reading from 
+   location Y.  */
+static int
+memory_load_overlap (rtx x, rtx y, HOST_WIDE_INT size)
+{
+  rtx tmp = plus_constant (Pmode, x, size);
+  rtx sub = simplify_gen_binary (MINUS, Pmode, tmp, y);
+
+  if (!CONST_INT_P (sub))
+    return -1;
+
+  return INTVAL (sub);
+}

for ARGS_GROW_DOWNWARD we would be reading 'backwards' from x,
so the function should something like the following?

static int
memory_load_overlap (rtx x, rtx y, HOST_WIDE_INT size)
{
#ifdef ARGS_GROW_DOWNWARD
     rtx tmp = plus_constant (Pmode, x, -size);
#else
     rtx tmp = plus_constant (Pmode, x, size);
#endif
     rtx sub = simplify_gen_binary (MINUS, Pmode, tmp, y);

     if (!CONST_INT_P (sub))
       return -1;

#ifdef ARGS_GROW_DOWNWARD
     return INTVAL (-sub);
#else
     return INTVAL (sub);
#endif
}

now, say for x == sp + 4,  y == sp + 8, size == 16:
This would be a problematic case for arm, so this code on arm
(where ARGS_GROW_DOWNWARD is *not* defined) would return
12, which is the number of bytes that overlap.

On a target where ARGS_GROW_DOWNWARD is defined this would return
-20, meaning that no overlap occurs (because we read in the descending
direction from x, IIUC).

Hi Jeff,

Here's an attempt to make this more concrete.
Only the memory_load_overlap function has changed.
This time I tried to take into account the case when
ARGS_GROW_DOWNWARD.

Take the case where x == sp, y == sp + 8, size == 16.
For arm, this would return 8 as that is the number of bytes
that overlap. On pa, since ARGS_GROW_DOWNWARD is defined it
would return -1 as we're reading down from x rather than up
towards y.

In the case when x == sp + 8, y == sp, size == 16
This would return -1 on arm since we're reading upwards from x
and thefore no overlap would happen.

On pa, this would return 8, which I think is the right thing.
But again, I don't have access to any pa means of testing.

What do you think of this approach?


Hi Dave,

Would it be possible for you to test this patch on a 64-bit hppa
or at least bootstrap it?
https://gcc.gnu.org/ml/gcc-patches/2015-04/msg01288.html
I started a build and test with your patch on hppa64-hp-hpux11.11 this morning. 



There is a concern that it may potentially affect the passing of
complex arguments partially on the stack and partially in regs
on pa because of the way the args and stack grow on that target.

Unfortunately I don't have access to any hardware or simulators.
It would help a lot with getting this patch in.

If you write to linux-parisc@vger.kernel.org, arrangements can be made for
an account on a Debian parisc linux machine for development testing.

Helge Deller has arranged for some new machines since we took over the
Debian buildd infrastructure  for parisc.  More info is here:
https://parisc.wiki.kernel.org/index.php/Main_Page



Thanks,
Kyrill




Thanks,
Kyrill

P.S. I've included the testcase from Honggyu in the patch.

2015-04-22  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>

      PR target/65358
      * expr.c (memory_load_overlap): New function.
      (emit_push_insn): When pushing partial args to the stack would
      clobber the register part load the overlapping part into a pseudo
      and put it into the hard reg after pushing.

2015-04-22  Honggyu Kim  <hong.gyu.kim@lge.com>

      PR target/65358
      * gcc.dg/pr65358.c: New test.



Thanks,
Kyrill


Jeff







Regards,
Dave

--
John David Anglin  dave.anglin@bell.net
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