llvm dev - Mar 2019 - EuroLLVM Numerics info

Hi,

I found issues with INDVARS <-> LSR passes interactions after
https://reviews.llvm.org/rL346397.

There were two main changes in this commit:
1. Previously we were propagating SCEV add expressions even if DefInst of exit
value has hard uses inside the loop. After rL346397 we forbid propagation of
SCEV add expressions if DefInst of exit value has hard use inside the loop.
2. Previously when checking for hard uses, we were only looking for immediate
uses of DefInst. After rL346397 we are going down through the Def-Use chain.

As I understand the motivation for this was that if Def instruction of the exit
value will stay in the loop due to hard use (i.e. it can't be easily
eliminated), there is no benefit in rewriting exit value.

But I think it is profitable in some cases, because it helps reducing the loop
strength. And in that sense we regressed here, because before rL346397 we would
rewrite the exit value of SCEV add expression even if there is hard use inside
the loop.

Here is the reproducer:

target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"

@c = external dso_local local_unnamed_addr global i32, align 4
@a = external dso_local local_unnamed_addr global i32*, align 8
@b = external dso_local local_unnamed_addr global i32, align 4
@d = external dso_local local_unnamed_addr global i32, align 4

define i32 @foo(){
entry:
  %0 = load i32*, i32** @a, align 8
  %.pre = load i32, i32* @c, align 4
  %.pre2 = load i32, i32* @d, align 4
  br label %do.body

do.body:                                          ; preds = %do.body, %entry
  %1 = phi i32 [ %dec, %do.body ], [ %.pre2, %entry ]
  %2 = phi i32 [ %inc, %do.body ], [ %.pre, %entry ]
  %inc = add nsw i32 %2, 1
  store i32 %inc, i32* @c, align 4
  %3 = load i32, i32* %0, align 4
  store i32 %3, i32* @b, align 4
  %dec = add nsw i32 %1, -1
  store i32 %dec, i32* @d, align 4
  %tobool = icmp eq i32 %dec, 0
  br i1 %tobool, label %do.end, label %do.body

do.end:                                           ; preds = %do.body
  %.lcssa = phi i32 [ %2, %do.body ]
  %inc1 = add nsw i32 %.lcssa, 2
  store i32 %inc1, i32* @c, align 4
  ret i32 undef
}

# Run this before and after rL346397:
$ opt a.ll -indvars -loop-reduce -S -o b.ll

After rL346397 we have additional ADD instruction inside the loop which is a
performance regression.

I think after rL346397 LSR is not able to rewrite all the uses of %2 because phi
node (%.lcssa) was not rewritten by indvars pass.

Simple fix would be just returning back original check for SCEVType in
lib/Transforms/Scalar/IndVarSimplify.cpp:
-        if (!isa<SCEVConstant>(ExitValue) &&
hasHardUserWithinLoop(L, Inst))
+        if ((ExitValue->getSCEVType() >= scMulExpr) &&
hasHardUserWithinLoop(L, Inst))

Note, there is another bug opened for the mentioned patch:
https://bugs.llvm.org/show_bug.cgi?id=39673, but it only fixes the problem when
the SCEV expression type is constant.

I'm not opening a new bug yet, but rather want to hear your comments.

Best regards,
Denis Bakhvalov.

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In general, exit value rewriting isn't really handled in a rigorous way; we
have hasHardUserWithinLoop like you noted, and there have been some proposed
changes to isHighCostExpansion recently.  But we don't try to weigh the
overall cost of various expansions versus the original code anywhere; maybe
something to look at improving in the future.

It's probably not worth trying to save one add instruction if that blocks
other loop optimizations.  If the expansion is more expensive, it gets more
complicated; we probably don't want to be generating a bunch of code outside
a loop if we're not sure it actually saves code inside the loop. Not sure at
first glance what cases your patch would trigger on.

-Eli

From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of
Bakhvalov, Denis via llvm-dev
Sent: Monday, March 25, 2019 11:58 AM
To: max.kazantsev at azul.com; llvm-dev <llvm-dev at lists.llvm.org>
Subject: [EXT] [llvm-dev] [IndVars] Rewriting exit value of SCEV add expressions

Hi,

I found issues with INDVARS <-> LSR passes interactions after
https://reviews.llvm.org/rL346397.

There were two main changes in this commit:
1. Previously we were propagating SCEV add expressions even if DefInst of exit
value has hard uses inside the loop. After rL346397 we forbid propagation of
SCEV add expressions if DefInst of exit value has hard use inside the loop.
2. Previously when checking for hard uses, we were only looking for immediate
uses of DefInst. After rL346397 we are going down through the Def-Use chain.

As I understand the motivation for this was that if Def instruction of the exit
value will stay in the loop due to hard use (i.e. it can't be easily
eliminated), there is no benefit in rewriting exit value.

But I think it is profitable in some cases, because it helps reducing the loop
strength. And in that sense we regressed here, because before rL346397 we would
rewrite the exit value of SCEV add expression even if there is hard use inside
the loop.

Here is the reproducer:

target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"

@c = external dso_local local_unnamed_addr global i32, align 4
@a = external dso_local local_unnamed_addr global i32*, align 8
@b = external dso_local local_unnamed_addr global i32, align 4
@d = external dso_local local_unnamed_addr global i32, align 4

define i32 @foo(){
entry:
  %0 = load i32*, i32** @a, align 8
  %.pre = load i32, i32* @c, align 4
  %.pre2 = load i32, i32* @d, align 4
  br label %do.body

do.body:                                          ; preds = %do.body, %entry
  %1 = phi i32 [ %dec, %do.body ], [ %.pre2, %entry ]
  %2 = phi i32 [ %inc, %do.body ], [ %.pre, %entry ]
  %inc = add nsw i32 %2, 1
  store i32 %inc, i32* @c, align 4
  %3 = load i32, i32* %0, align 4
  store i32 %3, i32* @b, align 4
  %dec = add nsw i32 %1, -1
  store i32 %dec, i32* @d, align 4
  %tobool = icmp eq i32 %dec, 0
  br i1 %tobool, label %do.end, label %do.body

do.end:                                           ; preds = %do.body
  %.lcssa = phi i32 [ %2, %do.body ]
  %inc1 = add nsw i32 %.lcssa, 2
  store i32 %inc1, i32* @c, align 4
  ret i32 undef
}

# Run this before and after rL346397:
$ opt a.ll -indvars -loop-reduce -S -o b.ll

After rL346397 we have additional ADD instruction inside the loop which is a
performance regression.

I think after rL346397 LSR is not able to rewrite all the uses of %2 because phi
node (%.lcssa) was not rewritten by indvars pass.

Simple fix would be just returning back original check for SCEVType in
lib/Transforms/Scalar/IndVarSimplify.cpp:
-        if (!isa<SCEVConstant>(ExitValue) &&
hasHardUserWithinLoop(L, Inst))
+        if ((ExitValue->getSCEVType() >= scMulExpr) &&
hasHardUserWithinLoop(L, Inst))

Note, there is another bug opened for the mentioned patch:
https://bugs.llvm.org/show_bug.cgi?id=39673, but it only fixes the problem when
the SCEV expression type is constant.

I'm not opening a new bug yet, but rather want to hear your comments.

Best regards,
Denis Bakhvalov.

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All:  There will be a BoF talk at the EuroLLVM conference regarding Numerics
(FMF and module flags which control fp behavior and optimization).

Even if you are not going to be in attendance, please reply to this thread as we
are collecting open issues and ideas for future direction in all layers of LLVM
for which optimizations are controlled by numerics flags.
Please read over the numerics blog if you like for reference material:

http://blog.llvm.org/2019/03/llvm-numerics-blog.html

Regards,
Michael