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

[Kyrill Tkachov <kyrylo.tkachov@arm.com>]

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).


Thanks,
Kyrill

>
>
> Jeff
>