llvm dev - Aug 2017 - [ScalarEvolution][SCEV] no-wrap flags dependent on order of getSCEV() calls

Hi all,

I'm looking into resolving a FIXME in the LoopDataPrefetch (and 
FalkorMarkStridedAccesses) pass by marking both of these passes as 
preserving the ScalarEvolution analysis.  Unfortunately, when this 
change is made, LSR will generate different code.  One of the root 
causes seems to be that SCEV will return different nsw/nuw flags for the 
same Value, depending on what order the SCEVs are computed, due to the 
fact that the SCEV object unique-ing doesn't take the nsw/nuw flags into 
account.  Since LoopDataPrefetch computes SCEVs in a different order 
than LSR, the nsw/nuw flags seen by LSR will differ based on whether the 
SCEVs are preserved from LoopDataPrefetch.

I believe this issue has been discussed before, but I just wanted to 
check if this is indeed the current expected behavior, and if anyone has 
any plans/ideas for addressing this issue.

For reference, below is a reduced loop where this problem occurs.  The 
SCEV for %i.07.i will have <nuw> or not depending on whether %idxprom.i 
was computed before it:

for.body.i:
   %i.07.i = phi i32 [ %inc.i, %for.body.i ], [ 0, %for.body.i.preheader ]
   %idxprom.i = zext i32 %i.07.i to i64
   %arrayidx.i = getelementptr inbounds i32, i32* %rot, i64 %idxprom.i
   %0 = load i32, i32* %arrayidx.i
   %inc.i = add i32 %i.07.i, 1
   %cmp.i = icmp ult i32 %inc.i, %size
   br i1 %cmp.i, label %for.body.i, label %do_bwe.exit


-- 
Geoff Berry
Employee of Qualcomm Datacenter Technologies, Inc.
  Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm 
Technologies, Inc.  Qualcomm Technologies, Inc. is a member of the Code 
Aurora Forum, a Linux Foundation Collaborative Project.

On 8/8/2017 10:22 AM, Geoff Berry via llvm-dev wrote:
> Hi all,
>
> I'm looking into resolving a FIXME in the LoopDataPrefetch (and 
> FalkorMarkStridedAccesses) pass by marking both of these passes as 
> preserving the ScalarEvolution analysis.  Unfortunately, when this 
> change is made, LSR will generate different code.  One of the root 
> causes seems to be that SCEV will return different nsw/nuw flags for 
> the same Value, depending on what order the SCEVs are computed, due to 
> the fact that the SCEV object unique-ing doesn't take the nsw/nuw 
> flags into account.  Since LoopDataPrefetch computes SCEVs in a 
> different order than LSR, the nsw/nuw flags seen by LSR will differ 
> based on whether the SCEVs are preserved from LoopDataPrefetch.
>
> I believe this issue has been discussed before, but I just wanted to 
> check if this is indeed the current expected behavior, and if anyone 
> has any plans/ideas for addressing this issue.
The general issue that SCEV nsw is weird is known... see, for example 
https://bugs.llvm.org/show_bug.cgi?id=23527.
> For reference, below is a reduced loop where this problem occurs.  The 
> SCEV for %i.07.i will have <nuw> or not depending on whether 
> %idxprom.i was computed before it:
%idxprom.i, the zext?  I'm not sure how you're getting that particular 
effect.  ScalarEvolution::getSCEV for a zext immediately calls getSCEV 
on its operand.

-Eli

-- 
Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux
Foundation Collaborative Project

On 8/8/2017 1:37 PM, Friedman, Eli wrote:
> On 8/8/2017 10:22 AM, Geoff Berry via llvm-dev wrote:
>> Hi all,
>>
>> I'm looking into resolving a FIXME in the LoopDataPrefetch (and
FalkorMarkStridedAccesses) pass by marking both of these passes as preserving
the ScalarEvolution analysis.  Unfortunately, when this change is made, LSR will
generate different code.  One of the root causes seems to be that SCEV will
return different nsw/nuw flags for the same Value, depending on what order the
SCEVs are computed, due to the fact that the SCEV object unique-ing doesn't
take the nsw/nuw flags into account.  Since LoopDataPrefetch computes SCEVs in a
different order than LSR, the nsw/nuw flags seen by LSR will differ based on
whether the SCEVs are preserved from LoopDataPrefetch.
>>
>> I believe this issue has been discussed before, but I just wanted to
check if this is indeed the current expected behavior, and if anyone has any
plans/ideas for addressing this issue.
> 
> The general issue that SCEV nsw is weird is known... see, for example
https://bugs.llvm.org/show_bug.cgi?id=23527.
> 
>> For reference, below is a reduced loop where this problem occurs.  The
SCEV for %i.07.i will have <nuw> or not depending on whether %idxprom.i
was computed before it:
> 
> %idxprom.i, the zext?  I'm not sure how you're getting that
particular effect.  ScalarEvolution::getSCEV for a zext immediately calls
getSCEV on its operand.
Here is an abridged record of the getSCEV results as seen by each pass
with/without preserving SCEVAnalysis.
In the first case, when the SCEV is invalidated, the SCEV for %i.07.i is
computed in LSR as {0,+,1}<%for.body.i>.  In the second case, the SCEV for
%i.07.i is computed in LSR the same way, but because the SCEV for %idxprom.i
from FalkorHWPFFix unique's to the same value but has the nuw flag set and
is still present in the foldingset, {0,+,1}<nuw><%for.body.i> is
returned for %i.07.i in LSR instead.  The SCEVs for other values differ too, but
I thought I'd start with this one.

********** FalkorHWPFFix **********
Created SCEV for %i.07.i = phi i32 [ %inc.i, %for.inc.i ], [ 0,
%for.body.i.preheader ]: {0,+,1}<%for.body.i>
Created SCEV for %idxprom.i = zext i32 %i.07.i to i64: (zext i32
{0,+,1}<%for.body.i> to i64)
Created SCEV for %arrayidx.i = getelementptr inbounds i32, i32* %rot, i64
%idxprom.i: ((4 * (zext i32 {0,+,1}<%for.body.i> to
i64))<nuw><nsw> + %rot)<nsw>
Created SCEV for %inc.i = add i32 %i.07.i, 1: {1,+,1}<%for.body.i>
Created SCEV for %idxprom.i = zext i32 %i.07.i to i64:
{0,+,1}<nuw><%for.body.i>
Created SCEV for %arrayidx.i = getelementptr inbounds i32, i32* %rot, i64
%idxprom.i: {%rot,+,4}<nw><%for.body.i>

********** SCEV invalidated **********

********** LSR **********
Created SCEV for %i.07.i = phi i32 [ %inc.i, %for.inc.i ], [ 0,
%for.body.i.preheader ]: {0,+,1}<%for.body.i>
Created SCEV for %idxprom.i = zext i32 %i.07.i to i64: (zext i32
{0,+,1}<%for.body.i> to i64)
Created SCEV for %arrayidx.i = getelementptr inbounds i32, i32* %rot, i64
%idxprom.i: ((4 * (zext i32 {0,+,1}<%for.body.i> to
i64))<nuw><nsw> + %rot)<nsw>
Created SCEV for %inc.i = add i32 %i.07.i, 1: {1,+,1}<%for.body.i>
Created SCEV for %cmp.i = icmp ult i32 %inc.i, %size: %cmp.i


vs.


********** FalkorHWPFFix **********
Created SCEV for %i.07.i = phi i32 [ %inc.i, %for.inc.i ], [ 0,
%for.body.i.preheader ]: {0,+,1}<%for.body.i>
Created SCEV for %idxprom.i = zext i32 %i.07.i to i64: (zext i32
{0,+,1}<%for.body.i> to i64)
Created SCEV for %arrayidx.i = getelementptr inbounds i32, i32* %rot, i64
%idxprom.i: ((4 * (zext i32 {0,+,1}<%for.body.i> to
i64))<nuw><nsw> + %rot)<nsw>
Created SCEV for %inc.i = add i32 %i.07.i, 1: {1,+,1}<%for.body.i>
Created SCEV for %idxprom.i = zext i32 %i.07.i to i64:
{0,+,1}<nuw><%for.body.i>
Created SCEV for %arrayidx.i = getelementptr inbounds i32, i32* %rot, i64
%idxprom.i: {%rot,+,4}<nw><%for.body.i>

********** SCEV preserved **********

********** LSR **********
Created SCEV for %i.07.i = phi i32 [ %inc.i, %for.inc.i ], [ 0,
%for.body.i.preheader ]: {0,+,1}<nuw><%for.body.i>
                                                                                
^^^^^
Created SCEV for %inc.i = add i32 %i.07.i, 1:
{1,+,1}<nuw><%for.body.i>
Existing SCEV for %idxprom.i = zext i32 %i.07.i to i64:
{0,+,1}<nuw><%for.body.i>
Existing SCEV for %arrayidx.i = getelementptr inbounds i32, i32* %rot, i64
%idxprom.i: {%rot,+,4}<nw><%for.body.i>
Created SCEV for %cmp.i = icmp ult i32 %inc.i, %size: %cmp.i
Created SCEV for %lsr.iv = phi i64 [ 0, %for.body.i.preheader ], [ %lsr.iv.next,
%for.inc.i ]: {0,+,1}<nuw><nsw><%for.body.i>



> 
> -Eli
> 
-- 
Geoff Berry
Employee of Qualcomm Datacenter Technologies, Inc.
  Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm
Technologies, Inc.  Qualcomm Technologies, Inc. is a member of the Code Aurora
Forum, a Linux Foundation Collaborative Project.