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[Loop Unswitch] Patch to selective unswitch only the reachable branch instructions. 

Summary:
The iterative algorithm for Loop Unswitching may render some of the branches unreachable in the unswitched loops.
Given the exponential nature of the algorithm, this is quite an overhead.
This patch fixes this problem by selectively unswitching only those branches within a loop that are reachable from the loop header.

Reviewers: Michael Zolothukin, Anna Thomas, Weiming Zhao.
Subscribers: llvm-commits.

Differential Revision: http://reviews.llvm.org/D26299

llvm-svn: 287925
This commit is contained in:
Abhilash Bhandari 2016-11-25 14:07:44 +00:00
parent eddec09ee4
commit 54e5a1a4da
2 changed files with 98 additions and 1 deletions
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37
llvm/lib/Transforms/Scalar/LoopUnswitch.cpp
View File

@ -210,7 +210,7 @@ namespace {
bool runOnLoop(Loop *L, LPPassManager &LPM) override;
bool processCurrentLoop();
bool isUnreachableDueToPreviousUnswitching(BasicBlock *);
/// This transformation requires natural loop information & requires that
/// loop preheaders be inserted into the CFG.
///
@ -483,6 +483,35 @@ bool LoopUnswitch::runOnLoop(Loop *L, LPPassManager &LPM_Ref) {
return Changed;
}
// Return true if the BasicBlock BB is unreachable from the loop header.
// Return false, otherwise.
bool LoopUnswitch::isUnreachableDueToPreviousUnswitching(BasicBlock *BB) {
auto *Node = DT->getNode(BB)->getIDom();
BasicBlock *DomBB = Node->getBlock();
while (currentLoop->contains(DomBB)) {
BranchInst *BInst = dyn_cast<BranchInst>(DomBB->getTerminator());
Node = DT->getNode(DomBB)->getIDom();
DomBB = Node->getBlock();
if (!BInst || !BInst->isConditional())
continue;
Value *Cond = BInst->getCondition();
if (!isa<ConstantInt>(Cond))
continue;
BasicBlock *UnreachableSucc =
Cond == ConstantInt::getTrue(Cond->getContext())
? BInst->getSuccessor(1)
: BInst->getSuccessor(0);
if (DT->dominates(UnreachableSucc, BB))
return true;
}
return false;
}
/// Do actual work and unswitch loop if possible and profitable.
bool LoopUnswitch::processCurrentLoop() {
bool Changed = false;
@ -593,6 +622,12 @@ bool LoopUnswitch::processCurrentLoop() {
continue;
if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
// Some branches may be rendered unreachable because of previous
// unswitching.
// Unswitch only those branches that are reachable.
if (isUnreachableDueToPreviousUnswitching(*I))
continue;
// If this isn't branching on an invariant condition, we can't unswitch
// it.
if (BI->isConditional()) {

62
llvm/test/Transforms/LoopUnswitch/elseif-non-exponential-behavior.ll Normal file
View File

@ -0,0 +1,62 @@
; RUN: opt -loop-unswitch -S - < %s | FileCheck %s
;CHECK-LABEL: @b
;CHECK: [[Loop1:for\.end.*]]: ; preds = %for.cond.us
;CHECK-NEXT: %[[PhiVar1:pdt.*]] = phi i32 [ %pdt.0.us, %for.cond.us ]
;CHECK: [[Loop2:for\.end.*]]: ; preds = %for.cond.us1
;CHECK-NEXT: %[[PhiVar2:pdt.*]] = phi i32 [ %pdt.0.us2, %for.cond.us1 ]
;CHECK: [[Loop3:for\.end.*]]: ; preds = %for.cond
;CHECK-NEXT: %[[PhiVar3:pdt.*]] = phi i32 [ %pdt.0, %for.cond ]
;CHECK: [[Join1:for\.end.*]]: ; preds = %[[Loop2]], %[[Loop3]]
;CHECK-NEXT: %[[PhiRes1:pdt.*]] = phi i32 [ %[[PhiVar3]], %[[Loop3]] ], [ %[[PhiVar2]], %[[Loop2]] ]
;CHECK: for.end: ; preds = %[[Loop1]], %[[Join1]]
;CHECK-NEXT: %[[PhiRes2:pdt.*]] = phi i32 [ %[[PhiRes1]], %[[Join1]] ], [ %[[PhiVar1]], %[[Loop1]] ]
;CHECK-NEXT: ret i32 %[[PhiRes2]]
; Function Attrs: nounwind uwtable
define i32 @b(i32 %x, i32 %y) #0 {
entry:
br label %for.cond
for.cond: ; preds = %for.inc, %entry
%pdt.0 = phi i32 [ 1, %entry ], [ %pdt.2, %for.inc ]
%i.0 = phi i32 [ 0, %entry ], [ %inc, %for.inc ]
%cmp = icmp slt i32 %i.0, 100
br i1 %cmp, label %for.body, label %for.end
for.body: ; preds = %for.cond
%tobool = icmp ne i32 %x, 0
br i1 %tobool, label %if.then, label %if.else
if.then: ; preds = %for.body
%mul = mul nsw i32 %pdt.0, 2
br label %if.end6
if.else: ; preds = %for.body
%tobool1 = icmp ne i32 %y, 0
br i1 %tobool1, label %if.then2, label %if.else4
if.then2: ; preds = %if.else
%mul3 = mul nsw i32 %pdt.0, 3
br label %if.end
if.else4: ; preds = %if.else
%mul5 = mul nsw i32 %pdt.0, 4
br label %if.end
if.end: ; preds = %if.else4, %if.then2
%pdt.1 = phi i32 [ %mul3, %if.then2 ], [ %mul5, %if.else4 ]
br label %if.end6
if.end6: ; preds = %if.end, %if.then
%pdt.2 = phi i32 [ %mul, %if.then ], [ %pdt.1, %if.end ]
br label %for.inc
for.inc: ; preds = %if.end6
%inc = add nsw i32 %i.0, 1
br label %for.cond
for.end: ; preds = %for.cond
ret i32 %pdt.0
}