On 24/10/16 17:15, Andrew Pinski wrote:
On Mon, Oct 24, 2016 at 7:27 AM, Kyrill Tkachov
<kyrylo.tkac...@foss.arm.com> wrote:
Hi all,
When storing a 64-bit immediate that has equal bottom and top halves we
currently
synthesize the repeating 32-bit pattern twice and perform a single X-store.
With this patch we synthesize the 32-bit pattern once into a W register and
store
that twice using an STP. This reduces codesize bloat from synthesising the
same
constant multiple times at the expense of converting a store to a
store-pair.
It will only trigger if we can save two or more instructions, so it will
only transform:
mov x1, 49370
movk x1, 0xc0da, lsl 32
str x1, [x0]
into:
mov w1, 49370
stp w1, w1, [x0]
when optimising for -Os, whereas it will always transform a 4-insn synthesis
sequence into a two-insn sequence + STP (see comments in the patch).
This patch triggers already but will trigger more with the store merging
pass
that I'm working on since that will generate more of these repeating 64-bit
constants.
This helps improve codegen on 456.hmmer where store merging can sometimes
create very
complex repeating constants and target-specific expand needs to break them
down.
Doing STP might be worse on ThunderX 1 than the mov/movk. Or this
might cause an ICE with -mcpu=thunderx; I can't remember if the check
for slow unaligned store pair word is with the pattern or not.
I can't get it to ICE with -mcpu=thunderx.
The restriction is just on the STP forming code in the sched-fusion hooks AFAIK.
Basically the rule is
1) if 4 byte aligned, then it is better to do two str.
2) If 8 byte aligned, then doing stp is good
3) Otherwise it is better to do two str.
Ok, then I'll make the function just emit two stores and depend on the
sched-fusion
machinery to fuse them into an STP when appropriate since that has the logic
that
takes thunderx into account.
Thanks for the info.
Kyrill
Thanks,
Andrew
Bootstrapped and tested on aarch64-none-linux-gnu.
Ok for trunk?
Thanks,
Kyrill
2016-10-24 Kyrylo Tkachov <kyrylo.tkac...@arm.com>
* config/aarch64/aarch64.md (mov<mode>): Call
aarch64_split_dimode_const_store on DImode constant stores.
* config/aarch64/aarch64-protos.h (aarch64_split_dimode_const_store):
New prototype.
* config/aarch64/aarch64.c (aarch64_split_dimode_const_store): New
function.
2016-10-24 Kyrylo Tkachov <kyrylo.tkac...@arm.com>
* gcc.target/aarch64/store_repeating_constant_1.c: New test.
* gcc.target/aarch64/store_repeating_constant_2.c: Likewise.
