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Hal Finkel

2015-Jul-16 01:35 UTC

[LLVMdev] Improving loop vectorizer support for loops with a volatile iteration variable

----- Original Message -----
> From: "Chandler Carruth" <chandlerc at google.com>
> To: "Hyojin Sung" <hsung at us.ibm.com>, llvmdev at
cs.uiuc.edu
> Sent: Wednesday, July 15, 2015 7:34:54 PM
> Subject: Re: [LLVMdev] Improving loop vectorizer support for loops
> with a volatile iteration variable
> On Wed, Jul 15, 2015 at 12:55 PM Hyojin Sung < hsung at us.ibm.com >
> wrote:
> > Hi all,
> 
> > I would like to propose an improvement of the “almost dead” block
> > elimination in Transforms/Local.cpp so that it will preserve the
> > canonical loop form for loops with a volatile iteration variable.
> 
> > *** Problem statement
> 
> > Nested loops in LCALS Subset B (
> > https://codesign.llnl.gov/LCALS.php
> > ) are not vectorized with LLVM -O3 because the LLVM loop vectorizer
> > fails the test whether the loop latch and exiting block of a loop
> > is
> > the same. The loops are vectorizable, and get vectorized with LLVM
> > -O2
> 
> I would be interested to know why -O2 succeeds here.
> > and also with other commercial compilers (icc, xlc).
> 
> > *** Details
> 
> > These loops ended up with different loop latch and exiting block
> > after a series of optimizations including loop unswitching, jump
> > threading, simplify-the-CFG, and loop simplify. The fundamental
> > problem here is that the above optimizations cannot recognize a
> > loop
> > with a volatile iteration variable and do not preserve its
> > canonical
> > loop structure.
> 
> Ok, meta-level question first:
> Why do we care about performance of loops with a volatile iteration
> variable?I don't think we do, however, I think that misses the point. In this case,
the volatile iteration variable is just an easy way to expose this problem that
we have with nested loop canonicalization and the vectorizer. To be specific:

This we vectorizer just fine: 

void foo1(float * restrict x, float * restrict y, float * restrict z) { 
for (int i = 0; i < 1000; ++i) { 
for (int j = 0; j < 1600; ++j) { 
x[j] = y[j] + z[j]; 
} 
} 
} 

And, indeed, this we don't (the only change is adding volatile on i): 

void foo2(float * restrict x, float * restrict y, float * restrict z) { 
for (volatile int i = 0; i < 1000; ++i) { 
for (int j = 0; j < 1600; ++j) { 
x[j] = y[j] + z[j]; 
} 
} 
} 

However, this we don't either, and that's a big problem: 

int done(float *x, float *y, float *z); 
void foo3(float * restrict x, float * restrict y, float * restrict z) { 
while (!done(x, y, z)) { 
for (int j = 0; j < 1600; ++j) { 
x[j] = y[j] + z[j]; 
} 
} 
} 

And the underlying reason is the same. The IR at the point in time when the loop
vectorizer runs looks like this:

define void @foo3(float* noalias %x, float* noalias %y, float* noalias %z) #0 { 
entry: 
%call.14 = tail call i32 @done(float* %x, float* %y, float* %z) #1 
%lnot.15 = icmp eq i32 %call.14, 0 
br i1 %lnot.15, label %for.body.preheader, label %while.end 

for.body.preheader: ; preds = %entry 
br label %for.body 

while.cond.loopexit: ; preds = %for.body 
%call = tail call i32 @done(float* %x, float* %y, float* %z) #1 
%lnot = icmp eq i32 %call, 0 
br i1 %lnot, label %for.body.backedge, label %while.end.loopexit 

for.body: ; preds = %for.body.backedge, %for.body.preheader 
%indvars.iv = phi i64 [ 0, %for.body.preheader ], [ %indvars.iv.be,
%for.body.backedge ]
... 
%indvars.iv.next = add nuw nsw i64 %indvars.iv, 1 
%exitcond = icmp eq i64 %indvars.iv.next, 1600 
br i1 %exitcond, label %while.cond.loopexit, label %for.body.backedge 

for.body.backedge: ; preds = %for.body, %while.cond.loopexit 
%indvars.iv.be = phi i64 [ %indvars.iv.next, %for.body ], [ 0,
%while.cond.loopexit ]
br label %for.body 

while.end.loopexit: ; preds = %while.cond.loopexit 
br label %while.end 

while.end: ; preds = %while.end.loopexit, %entry 
ret void 
} 

and we can currently only vectorize loops where the loop latch is also the
loop's exiting block. In this case, as in the case with the volatile
variable, vectorization is blocked by this constraint (here the backedge is from
the terminator of %for.body.backedge, but the loop exiting block is %for.body).
The check in the vectorizer is explicit:

// We only handle bottom-tested loops, i.e. loop in which the condition is 
// checked at the end of each iteration. With that we can assume that all 
// instructions in the loop are executed the same number of times. 
if (TheLoop->getExitingBlock() != TheLoop->getLoopLatch()) { 
... 

-Hal 
> That seems both counter-intuitive and unlikely to be a useful goal.
> We simply don't optimize volatile operations well in *any* part of
> the optimizer, and I'm not sure why we need to start trying to fix
> that. This seems like an irreparably broken benchmark, but perhaps
> there is a motivation I don't yet see.
> Assuming that sufficient motivation arises to try to fix this, see my
> comments below:
> > (1) Loop unswitching generates several empty placeholder BBs only
> > with PHI nodes after separating out a shorter path with no inner
> > loop execution from a standard path.
> 
> > (2) Jump threading and simplify-the-CFG passes independently calls
> > TryToSimplifyUnconditionalBranchFromEmptyBlock() in
> > Transforms/Utils/Local.cpp to get rid of almost empty BBs.
> 
> > (3) TryToSimplifyUnconditionalBranchFromEmtpyBlock() eliminates the
> > placeholder BBs after loop unswitching and merges them into
> > subsequent blocks including the header of the inner loop. Before
> > eliminating the blocks, the function checks if the block is a loop
> > header by looking at its PHI nodes so that it can be saved, but the
> > test fails with the loops with a volatile iteration variable.
> 
> Why does this fail for a volatile iteration variable but not for a
> non-volatile one? I think understanding that will be key to
> understanding how it should be fixed.
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
-- 

Hal Finkel 
Assistant Computational Scientist 
Leadership Computing Facility 
Argonne National Laboratory 
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Chandler Carruth

2015-Jul-16 06:06 UTC

head link

[LLVMdev] Improving loop vectorizer support for loops with a volatile iteration variable

On Wed, Jul 15, 2015 at 6:36 PM Hal Finkel <hfinkel at anl.gov> wrote:
>
> ------------------------------
>
> *From: *"Chandler Carruth" <chandlerc at google.com>
> *To: *"Hyojin Sung" <hsung at us.ibm.com>, llvmdev at
cs.uiuc.edu
> *Sent: *Wednesday, July 15, 2015 7:34:54 PM
> *Subject: *Re: [LLVMdev] Improving loop vectorizer support for loops with
> a volatile iteration variable
>
>
>
> On Wed, Jul 15, 2015 at 12:55 PM Hyojin Sung <hsung at us.ibm.com>
wrote:
>
>> Hi all,
>>
>> I would like to propose an improvement of the “almost dead” block
>> elimination in Transforms/Local.cpp so that it will preserve the
canonical
>> loop form for loops with a volatile iteration variable.
>>
>> *** Problem statement
>> Nested loops in LCALS Subset B (*https://codesign.llnl.gov/LCALS.php*
>>
<https://urldefense.proofpoint.com/v2/url?u=https-3A__codesign.llnl.gov_LCALS.php&d=AwMGaQ&c=8hUWFZcy2Z-Za5rBPlktOQ&r=Mfk2qtn1LTDThVkh6-oGglNfMADXfJdty4_bhmuhMHA&m=aWKfvN4c8lvUSvVn8J0Z2ajTctlBJf0198Au28epBr0&s=4d9dt5ODcDWHHatSrwu5ZYT9ebgVzNEtpOlIR87izCM&e=>)
>> are not vectorized with LLVM -O3 because the LLVM loop vectorizer fails
the
>> test whether the loop latch and exiting block of a loop is the same.
The
>> loops are vectorizable, and get vectorized with LLVM -O2
>>
> I would be interested to know why -O2 succeeds here.
>
>
>> and also with other commercial compilers (icc, xlc).
>>
>> *** Details
>> These loops ended up with different loop latch and exiting block after
a
>> series of optimizations including loop unswitching, jump threading,
>> simplify-the-CFG, and loop simplify. The fundamental problem here is
that
>> the above optimizations cannot recognize a loop with a volatile
iteration
>> variable and do not preserve its canonical loop structure.
>>
> Ok, meta-level question first:
>
> Why do we care about performance of loops with a volatile iteration
> variable?
>
> I don't think we do, however, I think that misses the point. In this
case,
> the volatile iteration variable is just an easy way to expose this problem
> that we have with nested loop canonicalization and the vectorizer. To be
> specific:
>
> This we vectorizer just fine:
>
> void foo1(float * restrict x, float * restrict y, float * restrict z) {
>   for (int i = 0; i < 1000; ++i) {
>     for (int j = 0; j < 1600; ++j) {
>       x[j] = y[j] + z[j];
>     }
>   }
> }
>
> And, indeed, this we don't (the only change is adding volatile on i):
>
> void foo2(float * restrict x, float * restrict y, float * restrict z) {
>   for (volatile int i = 0; i < 1000; ++i) {
>     for (int j = 0; j < 1600; ++j) {
>       x[j] = y[j] + z[j];
>     }
>   }
> }
>
> However, this we don't either, and that's a big problem:
>
> int done(float *x, float *y, float *z);
> void foo3(float * restrict x, float * restrict y, float * restrict z) {
>   while (!done(x, y, z)) {
>     for (int j = 0; j < 1600; ++j) {
>       x[j] = y[j] + z[j];
>     }
>   }
> }
>
> And the underlying reason is the same. The IR at the point in time when
> the loop vectorizer runs looks like this:
>
> define void @foo3(float* noalias %x, float* noalias %y, float* noalias %z)
> #0 {
> entry:
>   %call.14 = tail call i32 @done(float* %x, float* %y, float* %z) #1
>   %lnot.15 = icmp eq i32 %call.14, 0
>   br i1 %lnot.15, label %for.body.preheader, label %while.end
>
> for.body.preheader:                               ; preds = %entry
>   br label %for.body
>
> while.cond.loopexit:                              ; preds = %for.body
>   %call = tail call i32 @done(float* %x, float* %y, float* %z) #1
>   %lnot = icmp eq i32 %call, 0
>   br i1 %lnot, label %for.body.backedge, label %while.end.loopexit
>
> for.body:                                         ; preds >
%for.body.backedge, %for.body.preheader
>   %indvars.iv = phi i64 [ 0, %for.body.preheader ], [ %indvars.iv.be,
> %for.body.backedge ]
>   ...
>   %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
>   %exitcond = icmp eq i64 %indvars.iv.next, 1600
>   br i1 %exitcond, label %while.cond.loopexit, label %for.body.backedge
>
> for.body.backedge:                                ; preds = %for.body,
> %while.cond.loopexit
>   %indvars.iv.be = phi i64 [ %indvars.iv.next, %for.body ], [ 0,
> %while.cond.loopexit ]
>   br label %for.body
>
> while.end.loopexit:                               ; preds >
%while.cond.loopexit
>   br label %while.end
>
> while.end:                                        ; preds >
%while.end.loopexit, %entry
>   ret void
> }
>
> and we can currently only vectorize loops where the loop latch is also the
> loop's exiting block. In this case, as in the case with the volatile
> variable, vectorization is blocked by this constraint (here the backedge is
> from the terminator of %for.body.backedge, but the loop exiting block is
> %for.body). The check in the vectorizer is explicit:
>
>   // We only handle bottom-tested loops, i.e. loop in which the condition
> is
>   // checked at the end of each iteration. With that we can assume that all
>   // instructions in the loop are executed the same number of times.
>   if (TheLoop->getExitingBlock() != TheLoop->getLoopLatch()) {
>     ...
>
Thanks for the detailed explanation. This makes much more sense why we need
to handle it. I think its much better to look at nested loops of this form
than anything to do with volatile -- the latter is too prone to other
random optimizations turning off.

Regarding this problem, it would be interesting to know based on this
explanation what the desired fix would be. I see at least these options:

1) Canonicalize loops harder to make them look the way the vectorizer
wants. If this can be done without causing significant problems, it seems
likely the best approach.

2) Teach the vectorizer to vectorize without this constraint by instead
establishing the actual invariant it cares about.

Maybe there is another strategy?



>  -Hal
>
> That seems both counter-intuitive and unlikely to be a useful goal. We
> simply don't optimize volatile operations well in *any* part of the
> optimizer, and I'm not sure why we need to start trying to fix that.
This
> seems like an irreparably broken benchmark, but perhaps there is a
> motivation I don't yet see.
>
>
> Assuming that sufficient motivation arises to try to fix this, see my
> comments below:
>
>
>>
>>
>> (1) Loop unswitching generates several empty placeholder BBs only with
>> PHI nodes after separating out a shorter path with no inner loop
execution
>> from a standard path.
>>
>> (2) Jump threading and simplify-the-CFG passes independently calls
>> TryToSimplifyUnconditionalBranchFromEmptyBlock() in
>> Transforms/Utils/Local.cpp to get rid of almost empty BBs.
>>
>> (3) TryToSimplifyUnconditionalBranchFromEmtpyBlock() eliminates the
>> placeholder BBs after loop unswitching and merges them into subsequent
>> blocks including the header of the inner loop. Before eliminating the
>> blocks, the function checks if the block is a loop header by looking at
its
>> PHI nodes so that it can be saved, but the test fails with the loops
with a
>> volatile iteration variable.
>>
> Why does this fail for a volatile iteration variable but not for a
> non-volatile one? I think understanding that will be key to understanding
> how it should be fixed.
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
>
>
>
> --
> Hal Finkel
> Assistant Computational Scientist
> Leadership Computing Facility
> Argonne National Laboratory
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Hal Finkel

2015-Jul-16 06:46 UTC

head link

[LLVMdev] Improving loop vectorizer support for loops with a volatile iteration variable

----- Original Message -----
> From: "Chandler Carruth" <chandlerc at google.com>
> To: "Hal Finkel" <hfinkel at anl.gov>
> Cc: "Hyojin Sung" <hsung at us.ibm.com>, llvmdev at
cs.uiuc.edu
> Sent: Thursday, July 16, 2015 1:06:03 AM
> Subject: Re: [LLVMdev] Improving loop vectorizer support for loops
> with a volatile iteration variable
> On Wed, Jul 15, 2015 at 6:36 PM Hal Finkel < hfinkel at anl.gov >
wrote:
> > > From: "Chandler Carruth" < chandlerc at google.com
>
> > 
> 
> > > To: "Hyojin Sung" < hsung at us.ibm.com >,
llvmdev at cs.uiuc.edu
> > 
> 
> > > Sent: Wednesday, July 15, 2015 7:34:54 PM
> > 
> 
> > > Subject: Re: [LLVMdev] Improving loop vectorizer support for
> > > loops
> > > with a volatile iteration variable
> > 
> 
> > > On Wed, Jul 15, 2015 at 12:55 PM Hyojin Sung < hsung at
us.ibm.com >
> > > wrote:
> > 
> 
> > > > Hi all,
> > > 
> > 
> 
> > > > I would like to propose an improvement of the “almost dead”
> > > > block
> > > > elimination in Transforms/Local.cpp so that it will preserve
> > > > the
> > > > canonical loop form for loops with a volatile iteration
> > > > variable.
> > > 
> > 
> 
> > > > *** Problem statement
> > > 
> > 
> 
> > > > Nested loops in LCALS Subset B (
> > > > https://codesign.llnl.gov/LCALS.php
> > > > ) are not vectorized with LLVM -O3 because the LLVM loop
> > > > vectorizer
> > > > fails the test whether the loop latch and exiting block of a
> > > > loop
> > > > is
> > > > the same. The loops are vectorizable, and get vectorized
with
> > > > LLVM
> > > > -O2
> > > 
> > 
> 
> > > I would be interested to know why -O2 succeeds here.
> > 
> 
> > > > and also with other commercial compilers (icc, xlc).
> > > 
> > 
> 
> > > > *** Details
> > > 
> > 
> 
> > > > These loops ended up with different loop latch and exiting
> > > > block
> > > > after a series of optimizations including loop unswitching,
> > > > jump
> > > > threading, simplify-the-CFG, and loop simplify. The
fundamental
> > > > problem here is that the above optimizations cannot
recognize a
> > > > loop
> > > > with a volatile iteration variable and do not preserve its
> > > > canonical
> > > > loop structure.
> > > 
> > 
> 
> > > Ok, meta-level question first:
> > 
> 
> > > Why do we care about performance of loops with a volatile
> > > iteration
> > > variable?
> > 
> 
> > I don't think we do, however, I think that misses the point. In
> > this
> > case, the volatile iteration variable is just an easy way to expose
> > this problem that we have with nested loop canonicalization and the
> > vectorizer. To be specific:
> 
> > This we vectorizer just fine:
> 
> > void foo1(float * restrict x, float * restrict y, float * restrict
> > z)
> > {
> 
> > for (int i = 0; i < 1000; ++i) {
> 
> > for (int j = 0; j < 1600; ++j) {
> 
> > x[j] = y[j] + z[j];
> 
> > }
> 
> > }
> 
> > }
> 
> > And, indeed, this we don't (the only change is adding volatile on
> > i):
> 
> > void foo2(float * restrict x, float * restrict y, float * restrict
> > z)
> > {
> 
> > for (volatile int i = 0; i < 1000; ++i) {
> 
> > for (int j = 0; j < 1600; ++j) {
> 
> > x[j] = y[j] + z[j];
> 
> > }
> 
> > }
> 
> > }
> 
> > However, this we don't either, and that's a big problem:
> 
> > int done(float *x, float *y, float *z);
> 
> > void foo3(float * restrict x, float * restrict y, float * restrict
> > z)
> > {
> 
> > while (!done(x, y, z)) {
> 
> > for (int j = 0; j < 1600; ++j) {
> 
> > x[j] = y[j] + z[j];
> 
> > }
> 
> > }
> 
> > }
> 
> > And the underlying reason is the same. The IR at the point in time
> > when the loop vectorizer runs looks like this:
> 
> > define void @foo3(float* noalias %x, float* noalias %y, float*
> > noalias %z) #0 {
> 
> > entry:
> 
> > %call.14 = tail call i32 @done(float* %x, float* %y, float* %z) #1
> 
> > %lnot.15 = icmp eq i32 %call.14, 0
> 
> > br i1 %lnot.15, label %for.body.preheader, label %while.end
> 
> > for.body.preheader: ; preds = %entry
> 
> > br label %for.body
> 
> > while.cond.loopexit: ; preds = %for.body
> 
> > %call = tail call i32 @done(float* %x, float* %y, float* %z) #1
> 
> > %lnot = icmp eq i32 %call, 0
> 
> > br i1 %lnot, label %for.body.backedge, label %while.end.loopexit
> 
> > for.body: ; preds = %for.body.backedge, %for.body.preheader
> 
> > %indvars.iv = phi i64 [ 0, %for.body.preheader ], [ % indvars.iv.be
> > ,
> > %for.body.backedge ]
> 
> > ...
> 
> > %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
> 
> > %exitcond = icmp eq i64 %indvars.iv.next, 1600
> 
> > br i1 %exitcond, label %while.cond.loopexit, label
> > %for.body.backedge
> 
> > for.body.backedge: ; preds = %for.body, %while.cond.loopexit
> 
> > % indvars.iv.be = phi i64 [ %indvars.iv.next, %for.body ], [ 0,
> > %while.cond.loopexit ]
> 
> > br label %for.body
> 
> > while.end.loopexit: ; preds = %while.cond.loopexit
> 
> > br label %while.end
> 
> > while.end: ; preds = %while.end.loopexit, %entry
> 
> > ret void
> 
> > }
> 
> > and we can currently only vectorize loops where the loop latch is
> > also the loop's exiting block. In this case, as in the case with
> > the
> > volatile variable, vectorization is blocked by this constraint
> > (here
> > the backedge is from the terminator of %for.body.backedge, but the
> > loop exiting block is %for.body). The check in the vectorizer is
> > explicit:
> 
> > // We only handle bottom-tested loops, i.e. loop in which the
> > condition is
> 
> > // checked at the end of each iteration. With that we can assume
> > that
> > all
> 
> > // instructions in the loop are executed the same number of times.
> 
> > if (TheLoop->getExitingBlock() != TheLoop->getLoopLatch()) {
> 
> > ...
> 
> Thanks for the detailed explanation. This makes much more sense why
> we need to handle it. I think its much better to look at nested
> loops of this form than anything to do with volatile -- the latter
> is too prone to other random optimizations turning off.
> Regarding this problem, it would be interesting to know based on this
> explanation what the desired fix would be. I see at least these
> options:
> 1) Canonicalize loops harder to make them look the way the vectorizer
> wants. If this can be done without causing significant problems, it
> seems likely the best approach.I agree. In this case, we could certainly fold the trivial %for.body.backedge
block into %for.body, meaning transforming this:

for.body.backedge: ; preds = %while.cond.loopexit, %for.body 
%indvars.iv.be = phi i64 [ %indvars.iv.next, %for.body ], [ 0,
%while.cond.loopexit ]
br label %for.body 

for.body: ; preds = %for.body.backedge, %for.body.preheader 
%indvars.iv = phi i64 [ 0, %for.body.preheader ], [ %indvars.iv.be,
%for.body.backedge ]
... 
%indvars.iv.next = add nuw nsw i64 %indvars.iv, 1 
%exitcond = icmp eq i64 %indvars.iv.next, 1600 
br i1 %exitcond, label %while.cond.loopexit, label %for.body.backedge 

into this: 

for.body: ; preds = %for.body.backedge, %for.body.preheader 
%indvars.iv.be = phi i64 [ %indvars.iv.next, %for.body ], [ 0,
%while.cond.loopexit ]
%indvars.iv = phi i64 [ 0, %for.body.preheader ], [ %indvars.iv.be, %for.body ] 
... 
%indvars.iv.next = add nuw nsw i64 %indvars.iv, 1 
%exitcond = icmp eq i64 %indvars.iv.next, 1600 
br i1 %exitcond, label %while.cond.loopexit, label %for.body 

and this seems pretty trivial when %for.body.backedge is completely empty (as in
this case), but if it had non-PHI instructions in it on which the existing PHIs
in %for.body depended, then maybe this becomes less trivial?
> 2) Teach the vectorizer to vectorize without this constraint by
> instead establishing the actual invariant it cares about.It really cares that there's no code that comes in between the latch and the
exit, because such code is not really part of the loop (it only runs once), or
code in between the exit and the latch (because such code runs in one fewer
iterations than the code before the exit). At least nothing with side effects I
presume.

-Hal 
> Maybe there is another strategy?
> > -Hal
> 
> > > That seems both counter-intuitive and unlikely to be a useful
> > > goal.
> > > We simply don't optimize volatile operations well in *any*
part
> > > of
> > > the optimizer, and I'm not sure why we need to start trying
to
> > > fix
> > > that. This seems like an irreparably broken benchmark, but
> > > perhaps
> > > there is a motivation I don't yet see.
> > 
> 
> > > Assuming that sufficient motivation arises to try to fix this,
> > > see
> > > my
> > > comments below:
> > 
> 
> > > > (1) Loop unswitching generates several empty placeholder BBs
> > > > only
> > > > with PHI nodes after separating out a shorter path with no
> > > > inner
> > > > loop execution from a standard path.
> > > 
> > 
> 
> > > > (2) Jump threading and simplify-the-CFG passes independently
> > > > calls
> > > > TryToSimplifyUnconditionalBranchFromEmptyBlock() in
> > > > Transforms/Utils/Local.cpp to get rid of almost empty BBs.
> > > 
> > 
> 
> > > > (3) TryToSimplifyUnconditionalBranchFromEmtpyBlock()
eliminates
> > > > the
> > > > placeholder BBs after loop unswitching and merges them into
> > > > subsequent blocks including the header of the inner loop.
> > > > Before
> > > > eliminating the blocks, the function checks if the block is
a
> > > > loop
> > > > header by looking at its PHI nodes so that it can be saved,
but
> > > > the
> > > > test fails with the loops with a volatile iteration
variable.
> > > 
> > 
> 
> > > Why does this fail for a volatile iteration variable but not for
> > > a
> > > non-volatile one? I think understanding that will be key to
> > > understanding how it should be fixed.
> > 
> 
> > > _______________________________________________
> > 
> 
> > > LLVM Developers mailing list
> > 
> 
> > > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu
> > 
> 
> > > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> > 
> 
> > --
> 
> > Hal Finkel
> 
> > Assistant Computational Scientist
> 
> > Leadership Computing Facility
> 
> > Argonne National Laboratory
> 
-- 

Hal Finkel 
Assistant Computational Scientist 
Leadership Computing Facility 
Argonne National Laboratory 
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llvm dev - Jul 2015 - [LLVMdev] Improving loop vectorizer support for loops with a volatile iteration variable

[LLVMdev] Improving loop vectorizer support for loops with a volatile iteration variable

[LLVMdev] Improving loop vectorizer support for loops with a volatile iteration variable

[LLVMdev] Improving loop vectorizer support for loops with a volatile iteration variable