[llvm-dev] Scalar Evolution Expressions Involving Sibling Loops
Bardia Mahjour via llvm-dev
llvm-dev at lists.llvm.org
Thu Apr 16 13:50:38 PDT 2020
Hi Jimmy,
It's good to know that the problem is not specific to the case I ran into.
May be you can provide your example as well, since Philip seems to be
interested in some specific examples. If the assertion in getAddrExpr is
deemed necessary, then I think a condition check would be the next best
solution as it helps client code guard against such cases and make
alternative arrangements to avoid an assertion or miscompile.
Bardia Mahjour
Compiler Optimizations
IBM Toronto Software Lab
From: Jimmy Zhongduo Lin <jimmy.zhongduo.lin at huawei.com>
To: Bardia Mahjour <bmahjour at ca.ibm.com>, Philip Reames
<listmail at philipreames.com>, "llvm-dev at lists.llvm.org"
<llvm-dev at lists.llvm.org>
Date: 2020/04/16 04:34 PM
Subject: [EXTERNAL] RE: [llvm-dev] Scalar Evolution Expressions
Involving Sibling Loops
Hi Bardia,
I am encountering a similar problem and totally agree that getAddExpr
shouldn’t generate any assertion error or at least provide condition check.
Even if this is something to avoid, would it be better to return nullptr
instead of assertion error?
Thanks,
Jimmy
From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of Bardia
Mahjour via llvm-dev
Sent: Monday, March 30, 2020 4:59 PM
To: Philip Reames <listmail at philipreames.com>
Cc: LLVM Development List <llvm-dev at lists.llvm.org>
Subject: Re: [llvm-dev] Scalar Evolution Expressions Involving Sibling
Loops
> I'm not following your example. If you have two sibling loops with the
same parent, one will frequently, but not always dominate the other. Can
you give a specific example of when forming a recurrence between two
siblings (without one dominating the other), is useful?
The situation can happen with guarded loops or with a user guard like
below:
if (c) {
for (i = 0; i < n; i++)
...
}
for (j = 0; j < n; j++)
...
The specific example that we ran into is described in
https://reviews.llvm.org/D75628. Basically we have two triangular loops
that are siblings and we'd like to run Banerjee MIV tests on the memory
accesses in those loops. The loop looks like:
void foo(int *restrict A, int n1, int n2, int n3) {
for (int i1 = 0; i1 < n1; i1++) {
for (int i2 = 2; i2 < n2; i2++) {
for (int i3 = i2 + 1; i3 < n3; i3++) {
A[i2 + i3*n2] = 11;
}
}
for (int i4 = 2; i4 < n3; i4++) {
for (int i5 = 1; i5 < i4 - 1; i5++) {
A[i5] = 22;
}
}
}
}
To check the bounds of the dependence function we need to create a symbolic
expression that involves AddRecs for i2 and i4.
Bardia Mahjour
Inactive hide details for Philip Reames ---2020/03/30 02:50:45 PM---On
3/30/20 11:27 AM, Bardia Mahjour via llvm-dev wrote: >Philip Reames
---2020/03/30 02:50:45 PM---On 3/30/20 11:27 AM, Bardia Mahjour via
llvm-dev wrote: >
From: Philip Reames <listmail at philipreames.com>
To: Bardia Mahjour <bmahjour at ca.ibm.com>, LLVM Development List <
llvm-dev at lists.llvm.org>
Date: 2020/03/30 02:50 PM
Subject: [EXTERNAL] Re: [llvm-dev] Scalar Evolution Expressions Involving
Sibling Loops
On 3/30/20 11:27 AM, Bardia Mahjour via llvm-dev wrote:
Forwarding to the dev list, in case others ran into similar
issues and/or have input on this topic.
Bardia Mahjour
----- Forwarded by Bardia Mahjour/Toronto/IBM on 2020/03/30
02:25 PM -----
From: Bardia Mahjour/Toronto/IBM
To: listmail at philipreames.com
Cc: "Michael Kruse" <llvm at meinersbur.de>
Date: 2020/03/26 11:47 AM
Subject: Scalar Evolution Expressions Involving Sibling Loops
Hi Philip,
I hope you are doing well.
We've recently run into an issue with SCEV in the context of
dependence analysis, and would like your opinion on it.
Background discussion can be found here
https://reviews.llvm.org/D75628#inline-689527.
Basically in this case, the dependence equation requires us to
symbolically create an expression involving two or more
recurrences that recur with non-dominating loops (sibling
loops).
I'm not following your example. If you have two sibling loops with the
same parent, one will frequently, but not always dominate the other. Can
you give a specific example of when forming a recurrence between two
siblings (without one dominating the other), is useful?
Currently creating such a SCEV expression trips asserts in `
CompareSCEVComplexity()` and `isKnownViaInduction()` saying
that a given SCEV expression cannot be composed of recurrences
that have no dominance relationship between them.
Michael tried explaining to me why there is this restriction
about dominance, and I'm beginning to understand why such
restriction may be necessary when evaluating or expanding SCEV
expressions in outer scopes (eg. `getSCEVAtScope(nullptr)`) but
I still don't understand why this restriction is imposed at
construction. Shouldn't this restriction be asserted on when
calling getSCEVAtScope instead of when creating AddRecs, given
that simplification steps may remove identical terms involving
non-dominating loops?
Well, SCEV construction is generally done to parallel IR. SSA requires
dominance, so having the SCEV operands require dominance would seem like a
reasonable thing. If you want to change this, you'll need to motivate the
change. (i.e. see above question)
We would appreciate any other insight you might have about this
issue.
Regards,
Bardia Mahjour
Compiler Optimizations
IBM Toronto Software Lab
bmahjour at ca.ibm.com
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