On 06/09/2021 12:13, Richard Biener wrote:
On Mon, Sep 6, 2021 at 1:08 PM Richard Earnshaw
<richard.earns...@foss.arm.com> wrote:
On 06/09/2021 11:58, Richard Biener via Gcc-patches wrote:
On Mon, Sep 6, 2021 at 12:40 PM Richard Earnshaw <rearn...@arm.com> wrote:
GCC was recently changed to prevent simplify_subreg from simplifying
a subreg of a mem when the mode of the new mem would have stricter alignment
constraints than the inner mem already has when the target requires
STRICT_ALIGNMENT.
However, such targets may have specialist patterns that can handle
unaligned accesses and this restriction turns out to be unduly restrictive.
So limit this restriction to only apply when the inner mem is naturally
aligned to the inner mode.
Hmm, I think this can end up either generating wrong code or
recog fails. The specific combination of alignment and mode of 'op'
has been validated to be supported, replacing the mode with sth
else would need re-validation of the combination. I'm not sure
we can for example just query movmisalign support here and
hope for LRA to reload the mem with that.
So - where do you run into this? Is it possible to catch the
situation on a higher level where more context as in the whole insn
is visible?
I ran into it with patch 2 of this series when calling gen_highpart on a
misaligned mem. IIRC gen_highpart would end up returning (subreg:SI
(mem:DI (addr [A8])) 4), while gen_lowpart would simplify the operation
to (mem:SI (addr [A8])) as expected.
(subreg:SI (mem:DI (addr [A8])) 4) is really problematic, because it's
not a memory_operand (from the manual: it will get reloaded into a
register later on). But that's no good here, I don't want this
reloading into a wide register later, I need it to be narrowed to the
component part now.
So maybe calling gen_highpart is not what you want then?
adjust_address is IIRC what one uses to offset a MEM and change
its mode.
It was based on looking at the patch for PR 100106
(r12-163-c33db31d9ad9). That patch added the MEM_ALIGN constraint when
previously there was none here and my call would have been simplified.
Are you saying that GCC was always wrong in this respect? All I've done
was to tightened the check that Bernd added.
R.
Richard.
R.
Thanks,
Richard.
gcc/ChangeLog:
PR target/102125
* simplify-rtx.c (simplify_context::simplify_subreg): Allow
simplifying (subreg (mem())) when the inner mem is already
misaligned for its type.
---
gcc/simplify-rtx.c | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)
