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[LoopUnswitch] Avoid exponential behavior

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Summary: (No semantic change intended).

Reviewers: majnemer, bogner, mzolotukhin

Subscribers: mcrosier, llvm-commits, mzolotukhin

Differential Revision: http://reviews.llvm.org/D21707

llvm-svn: 273763
This commit is contained in:
Sanjoy Das 2016-06-25 01:14:19 +00:00
parent cc99e4e75c
commit 8881a593c6
2 changed files with 73 additions and 4 deletions
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26
lib/Transforms/Scalar/LoopUnswitch.cpp
View File

@ -389,7 +389,11 @@ Pass *llvm::createLoopUnswitchPass(bool Os) {
/// 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 *Cond, Loop *L, bool &Changed) {
// 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 *Cond, Loop *L, bool &Changed) {
// 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;

51
test/Transforms/LoopUnswitch/exponential-behavior.ll Normal file
View File

@ -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
}