[llvm-dev] visitShiftByConstant of DAGCombiner

[Jojo Ma via llvm-dev]

Thanks very much, Renato!

I looked into the regressions, there're many cases have similar context as
the case I mentioned but N just has only one use.
In this case, the canonicalisation won't make it more profitable, will only
prevent the possible folding or
make the sequence is not expected.
The regressions be eliminated, after I limit the expanding to not
"N->hasOneUse()".

And now only one regression left:

 FAIL: LLVM :: CodeGen/Thumb2/machine-licm.ll (18865 of 29222)
******************** TEST 'LLVM :: CodeGen/Thumb2/machine-licm.ll' FAILED
********************
Script:
--
/home/likewise-open/SPREADTRUM/
jojo.ma/jojoma/source/llvm/llvm-linaro/llvm/build/./bin/llc <
/home/likewise-open/SPREADTRUM/
jojo.ma/jojoma/source/llvm/llvm-linaro/llvm/llvm/test/CodeGen/Thumb2/machine-licm.ll
-mtriple=thumbv7-apple-darwin -mcpu=cortex-a8
-relocation-model=dynamic-no-pic -disable-fp-elim |
/home/likewise-open/SPREADTRUM/
jojo.ma/jojoma/source/llvm/llvm-linaro/llvm/build/./bin/FileCheck
/home/likewise-open/SPREADTRUM/
jojo.ma/jojoma/source/llvm/llvm-linaro/llvm/llvm/test/CodeGen/Thumb2/machine-licm.ll
/home/likewise-open/SPREADTRUM/
jojo.ma/jojoma/source/llvm/llvm-linaro/llvm/build/./bin/llc <
/home/likewise-open/SPREADTRUM/
jojo.ma/jojoma/source/llvm/llvm-linaro/llvm/llvm/test/CodeGen/Thumb2/machine-licm.ll
-mtriple=thumbv7-apple-darwin -mcpu=cortex-a8 -relocation-model=pic
-disable-fp-elim | /home/likewise-open/SPREADTRUM/
jojo.ma/jojoma/source/llvm/llvm-linaro/llvm/build/./bin/FileCheck
/home/likewise-open/SPREADTRUM/
jojo.ma/jojoma/source/llvm/llvm-linaro/llvm/llvm/test/CodeGen/Thumb2/machine-licm.ll
--check-prefix=PIC
--
Exit Code: 1

Command Output (stderr):
--
/home/likewise-open/SPREADTRUM/
jojo.ma/jojoma/source/llvm/llvm-linaro/llvm/llvm/test/CodeGen/Thumb2/machine-licm.ll:88:10:
error: expected string not found in input
; CHECK: movw {{(r[0-9])|(lr)}}, #32768
         ^
<stdin>:56:2: note: scanning from here
 movw r12, #32768
 ^

The outputs before and after the canonicalisation are:

   - before canonicalisation

 _t3:
@ BB#0:                                 @ %bb.nph
push {r7, lr}
mov r7, sp
movw lr, #32768
movs r2, #0
movw r9, #16386
movw r12, #65534
movt lr, #65535
LBB2_1:                                 @ %bb
                                        @ =>This Inner Loop Header: Depth=1
eor.w r3, r0, r1
adds r2, #1
ands r3, r3, #1
it ne
eorne.w r1, r1, r9
and.w r3, r1, r12
orr.w r1, lr, r3, lsr #1
it eq
lsreq r1, r3, #1
ubfx r0, r0, #1, #7
uxtb r3, r2
cmp r3, #8
bne LBB2_1
@ BB#2:                                 @ %bb8
uxth r0, r1
pop {r7, pc}


   - after canonicalisation

_t3:
@ BB#0:                                 @ %bb.nph
movw r12, #32768
movs r2, #0
movw r9, #16386
movt r12, #65535
LBB2_1:                                 @ %bb
                                        @ =>This Inner Loop Header: Depth=1
eor.w r3, r0, r1
adds r2, #1
ands r3, r3, #1
it ne
eorne.w r1, r1, r9
uxtb r3, r2
ubfx r1, r1, #1, #15
it ne
orrne.w r1, r1, r12
ubfx r0, r0, #1, #7
cmp r3, #8
bne LBB2_1
@ BB#2:                                 @ %bb8
uxth r0, r1
bx lr


The context of this is same with the case of CoreMa, so i can't rule it out
in my change.
I created a review on phabricator of my proposal:
https://reviews.llvm.org/D27916

Hope all the above could make the problem clear.
Welcome any review and advice.Thanks!

Best Regards,
Jojo

On 15 December 2016 at 20:10, Renato Golin <renato.golin at linaro.org> wrote:

> On 8 December 2016 at 02:34, Jojo Ma <jojo.ma at linaro.org> wrote:
> > It would be profitable as well if we could enable the canonicalisation on
> > it.
> > sequence before this canonicalisation (ARM):
> > test:
> > .fnstart
> > @ BB#0:                                 @ %entry
> > movw r1, #65534
> > and r1, r0, r1
> > ubfx r0, r0, #1, #15
> > add r0, r0, r1, lsr #1
> > bx lr
> >
> > sequence after this canonicalisation:
> > test:
> > .fnstart
> > @ BB#0:                                 @ %entry
> > ubfx r0, r0, #1, #15
> > add r0, r0, r0
> > bx lr
> >
> > But when I tried to expand the condition to this case, there are lots of
> > various regression fails.
>
> Hi Jojo,
>
> I think this canonicalisation is correct, at least in this case, so
> it's quite possible that the regressions are just bad tests, or you
> actually made the code better in other places that weren't expecting
> it.
>
> I'm adding some folks that have worked on the DAG combiner recently to
> have a look, but it would be good to know what regressions were there
> to see if they're really regressions (ie. worse code) or just
> different/better code.
>
> You may also have missed a check on your code (which would wrongly
> combine constants). Can you create a review on Phab of your proposed
> change?
>
> Also, if you could list the regressions and see if they have a common
> pattern (ie. "tests expecting an additional mask failed", or "the
> wrong mask was applied"), it would become clearer if they are real
> regressions or not.
>
> cheers,
> --renato
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20161219/3d896f51/attachment.html>