[llvm-dev] [SCEV][ScalarEvolution] SE limitation impacting LV
Caballero, Diego via llvm-dev
llvm-dev at lists.llvm.org
Wed Nov 22 15:43:39 PST 2017
Thanks for the feedback, Sanjoy.
> SCEV is fairly conservative around PHI nodes that aren't recurrences and aren't obviously equivalent to a min-max branch-phi idiom. Is that the limitation you're running into here?
Yes, that's exactly the problem. The problematic PHI nodes (%bc.resume.val and %bc.resume.val1) aren't either recurrences or related to min-max idioms. I have a tentative fix for the LV bug and I just wanted to know if this is something that should also be fixed at SCEV level to report a bug accordingly. Your previous comment sounds like "this is a well-known limitation in SCEV" so maybe there is nothing new to report.
Thanks,
Diego
-----Original Message-----
From: Sanjoy Das [mailto:sanjoy at playingwithpointers.com]
Sent: Wednesday, November 22, 2017 2:58 PM
To: Caballero, Diego <diego.caballero at intel.com>
Cc: llvm-dev at lists.llvm.org
Subject: Re: [llvm-dev] [SCEV][ScalarEvolution] SE limitation impacting LV
Hi,
SCEV is fairly conservative around PHI nodes that aren't recurrences and aren't obviously equivalent to a min-max branch-phi idiom. Is that the limitation you're running into here?
-- Sanjoy
On Tue, Nov 14, 2017 at 9:16 AM, Caballero, Diego via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> Hi!
>
>
>
> I would appreciate some feedback from someone with experience in SCEV/SE.
> D39346 tries to fix an issue in LV (PR34965) that exposes a limitation
> in SCEV/SE. The best solution to the LV issue might not be a fix at
> SCEV/SE level but we may want to report/address SCEV/SE limitation as well.
>
>
>
> For the snippet below, LV expects SE to return a SCEVAddRecExpr for %21.
> However, SE returns ((4 * (zext i32 (2 + %18) to i64))<nuw><nsw> +
> undef)<nsw>, probably because it’s not able to deduce that
> %bc.resume.val1 = %bc.resume.val + 1. You can find further information
> in D39346 and the full test in PR34965 (“IR_after_createVectorizedLoopSkeleton” attachment).
>
>
>
> I could open a new bug against SCEV/SE if a fix or extension is
> feasible at that level.
>
>
>
> Thanks,
>
> Diego
>
>
>
> ----
>
>
>
> .outer: ; preds = %2, %0
>
> …
>
> %.ph2 = phi i32 [ 62, %2 ], [ 110, %0 ]
>
> %5 = add i32 %.ph2, 1
>
> …
>
>
>
> vector.ph: ; preds = %.outer
>
> …
>
> %ind.end = add i32 %5, %n.vec
>
> %ind.end2 = add i32 %.ph2, %n.vec
>
> …
>
>
>
> middle.block: ; preds =
> %vector.body.latch
>
> …
>
>
>
> scalar.ph: ; preds = %middle.block,
> %.outer
>
> %bc.resume.val = phi i32 [ %ind.end, %middle.block ], [ %5, %.outer
> ]
>
> %bc.resume.val1 = phi i32 [ %ind.end2, %middle.block ], [ %.ph2,
> %.outer ]
>
> br label %16
>
>
>
> ; <label>:16: ; preds = %16, %scalar.ph
>
> %17 = phi i32 [ %bc.resume.val, %scalar.ph ], [ %23, %16 ]
>
> %18 = phi i32 [ %bc.resume.val1, %scalar.ph ], [ %17, %16 ]
>
> %19 = add i32 %18, 2
>
> %20 = zext i32 %19 to i64
>
> %21 = getelementptr inbounds i32, i32 addrspace(1)* undef, i64 %20
>
> %22 = ashr i32 undef, %4
>
> store i32 %22, i32 addrspace(1)* %21, align 4
>
> %23 = add i32 %17, 1
>
> %24 = icmp sgt i32 %23, 61
>
> br i1 %24, label %._crit_edge.loopexit.preheader, label %16
>
>
>
>
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