Hi Ramana,
On 01/06/16 10:07, Ramana Radhakrishnan wrote:
On 01/06/16 10:02, Ramana Radhakrishnan wrote:
On 09/03/16 12:56, Kyrill Tkachov wrote:
Hi all,
I notice that the output code for our store exclusive patterns accesses
unallocated memory.
It wants to output an strexd instruction with a pair of consecutive registers
corresponding
to a DImode value. For that it creates the SImode top half of the DImode
register and puts it
into operands[3]. But the pattern only defines entries only up to operands[2],
with no match_dup 3
or like that, so operands[3] should technically be out of bounds.
We already have a mechanism for printing the top half of a DImode register,
that's the 'H' output modifier.
So this patch changes those patterns to use that, eliminating the out of bounds
access and making
the code a bit simpler as well.
Bootstrapped and tested on arm-none-linux-gnueabihf.
Ok for trunk?
Thanks,
Kyrill
2016-03-09 Kyrylo Tkachov <kyrylo.tkac...@arm.com>
* config/arm/sync.md (arm_store_exclusive<mode>):
Use 'H' output modifier on operands[2] rather than creating a new
entry in out-of-bounds memory of the operands array.
(arm_store_release_exclusivedi): Likewise.
Ok,
Ah hang on - is this safe for big-endian - remind me again why we shouldn't use
'Q' and 'R' here ?
Thanks for looking at this.
The existing code does an STRD of REG and REG+1 without any concern for
endianness.
This patch simplifies that logic (since %H is just REG+1), so there's no change
in behaviour.
The question is whether the current behaviour is wrong for big-endian.
I've looked at sections A8.8.214 and A3.3 in the Architecture Reference Manual
(I'm using Rev C.c)
and my understanding is that doubleword quantities should not have their words
swapped when storing
and that STREXD (and STRD) stores REG at [address] and REG+1 at [address+4]
regardless of endianness.
So I think it's correct here to always store REG followed by REG+1.
I note that this is also what we do when storing a normal DImode value with
STRD.
In output_move_double we use an STRD of the lower numbered register followed by
the next one without
any checks on endianness.
Looking at the uses of Q and R and their comment in arm_print_operand they are
only used when performing
arithmetic operations on the DImode values where choosing the most significant
or the least significant
word matters.
Hope this makes sense,
Kyrill
Ramana
Thanks,
ramana
