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

[Hal Finkel]

----- 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.
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-- 

Hal Finkel 
Assistant Computational Scientist 
Leadership Computing Facility 
Argonne National Laboratory 
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