[llvm] r273763 - [LoopUnswitch] Avoid exponential behavior

Fri Jun 24 21:44:43 PDT 2016

Sanjoy Das via llvm-commits <llvm-commits at lists.llvm.org> writes:
> Author: sanjoy
> Date: Fri Jun 24 20:14:19 2016
> New Revision: 273763
>
> URL: http://llvm.org/viewvc/llvm-project?rev=273763&view=rev
> Log:
> [LoopUnswitch] Avoid exponential behavior
>
> Summary: (No semantic change intended).
>
> Reviewers: majnemer, bogner, mzolotukhin

I guess this is a phab thing? This line is very misleading and really
shouldn't be included in commit messages.

> Subscribers: mcrosier, llvm-commits, mzolotukhin

Similarly, this isn't very helpful.

I've filed llvm.org/PR28300 about the problem, but if there's an obvious
way to fix this on your end when committing I would appreciate it.

> Differential Revision: http://reviews.llvm.org/D21707
>
> Added:
>     llvm/trunk/test/Transforms/LoopUnswitch/exponential-behavior.ll
> Modified:
>     llvm/trunk/lib/Transforms/Scalar/LoopUnswitch.cpp
>
> Modified: llvm/trunk/lib/Transforms/Scalar/LoopUnswitch.cpp
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/LoopUnswitch.cpp?rev=273763&r1=273762&r2=273763&view=diff
> ==============================================================================
> --- llvm/trunk/lib/Transforms/Scalar/LoopUnswitch.cpp (original)
> +++ llvm/trunk/lib/Transforms/Scalar/LoopUnswitch.cpp Fri Jun 24 20:14:19 2016
> @@ -389,7 +389,11 @@ Pass *llvm::createLoopUnswitchPass(bool
>  
>  /// Cond is a condition that occurs in L. If it is invariant in the loop, or has
>  /// an invariant piece, return the invariant. Otherwise, return null.
> -static Value *FindLIVLoopCondition(Value *Cond, Loop *L, bool &Changed) {
> +static Value *FindLIVLoopCondition(Value *Cond, Loop *L, bool &Changed,
> +                                   DenseMap<Value *, Value *> &Cache) {
> +  auto CacheIt = Cache.find(Cond);
> +  if (CacheIt != Cache.end())
> +    return CacheIt->second;
>  
>    // We started analyze new instruction, increment scanned instructions counter.
>    ++TotalInsts;
> @@ -404,8 +408,10 @@ static Value *FindLIVLoopCondition(Value
>    // TODO: Handle: br (VARIANT|INVARIANT).
>  
>    // Hoist simple values out.
> -  if (L->makeLoopInvariant(Cond, Changed))
> +  if (L->makeLoopInvariant(Cond, Changed)) {
> +    Cache[Cond] = Cond;
>      return Cond;
> +  }
>  
>    if (BinaryOperator *BO = dyn_cast<BinaryOperator>(Cond))
>      if (BO->getOpcode() == Instruction::And ||
> @@ -413,15 +419,27 @@ static Value *FindLIVLoopCondition(Value
>        // If either the left or right side is invariant, we can unswitch on this,
>        // which will cause the branch to go away in one loop and the condition to
>        // simplify in the other one.
> -      if (Value *LHS = FindLIVLoopCondition(BO->getOperand(0), L, Changed))
> +      if (Value *LHS =
> +              FindLIVLoopCondition(BO->getOperand(0), L, Changed, Cache)) {
> +        Cache[Cond] = LHS;
>          return LHS;
> -      if (Value *RHS = FindLIVLoopCondition(BO->getOperand(1), L, Changed))
> +      }
> +      if (Value *RHS =
> +              FindLIVLoopCondition(BO->getOperand(1), L, Changed, Cache)) {
> +        Cache[Cond] = RHS;
>          return RHS;
> +      }
>      }
>  
> +  Cache[Cond] = nullptr;
>    return nullptr;
>  }
>  
> +static Value *FindLIVLoopCondition(Value *Cond, Loop *L, bool &Changed) {
> +  DenseMap<Value *, Value *> Cache;
> +  return FindLIVLoopCondition(Cond, L, Changed, Cache);
> +}
> +
>  bool LoopUnswitch::runOnLoop(Loop *L, LPPassManager &LPM_Ref) {
>    if (skipLoop(L))
>      return false;
>
> Added: llvm/trunk/test/Transforms/LoopUnswitch/exponential-behavior.ll
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/LoopUnswitch/exponential-behavior.ll?rev=273763&view=auto
> ==============================================================================
> --- llvm/trunk/test/Transforms/LoopUnswitch/exponential-behavior.ll (added)
> +++ llvm/trunk/test/Transforms/LoopUnswitch/exponential-behavior.ll Fri Jun 24 20:14:19 2016
> @@ -0,0 +1,51 @@
> +; RUN: opt -loop-unswitch -S < %s | FileCheck %s
> +
> +define void @f(i32 %n, i32* %ptr) {
> +; CHECK-LABEL: @f(
> +entry:
> +  br label %loop
> +
> +loop:
> +  %iv = phi i32 [ 0, %entry ], [ %iv.inc, %be ]
> +  %iv.inc = add i32 %iv, 1
> +  %unswitch_cond_root = icmp ne i32 %iv.inc, 42
> +  %us.0 = and i1 %unswitch_cond_root, %unswitch_cond_root
> +  %us.1 = and i1 %us.0, %us.0
> +  %us.2 = and i1 %us.1, %us.1
> +  %us.3 = and i1 %us.2, %us.2
> +  %us.4 = and i1 %us.3, %us.3
> +  %us.5 = and i1 %us.4, %us.4
> +  %us.6 = and i1 %us.5, %us.5
> +  %us.7 = and i1 %us.6, %us.6
> +  %us.8 = and i1 %us.7, %us.7
> +  %us.9 = and i1 %us.8, %us.8
> +  %us.10 = and i1 %us.9, %us.9
> +  %us.11 = and i1 %us.10, %us.10
> +  %us.12 = and i1 %us.11, %us.11
> +  %us.13 = and i1 %us.12, %us.12
> +  %us.14 = and i1 %us.13, %us.13
> +  %us.15 = and i1 %us.14, %us.14
> +  %us.16 = and i1 %us.15, %us.15
> +  %us.17 = and i1 %us.16, %us.16
> +  %us.18 = and i1 %us.17, %us.17
> +  %us.19 = and i1 %us.18, %us.18
> +  %us.20 = and i1 %us.19, %us.19
> +  %us.21 = and i1 %us.20, %us.20
> +  %us.22 = and i1 %us.21, %us.21
> +  %us.23 = and i1 %us.22, %us.22
> +  %us.24 = and i1 %us.23, %us.23
> +  %us.25 = and i1 %us.24, %us.24
> +  %us.26 = and i1 %us.25, %us.25
> +  %us.27 = and i1 %us.26, %us.26
> +  %us.28 = and i1 %us.27, %us.27
> +  %us.29 = and i1 %us.28, %us.28
> +  br i1 %us.29, label %leave, label %be
> +
> +be:
> +  store volatile i32 0, i32* %ptr
> +  %becond = icmp ult i32 %iv.inc, %n
> +  br i1 %becond, label %leave, label %loop
> +
> +leave:
> +  ret void
> +}
>
>
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