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[Jump Threading] Unfold a select insn that feeds a switch via a phi node 

Currently when a select has a constant value in one branch and the select feeds
a conditional branch (via a compare/ phi and compare) we unfold the select 
statement. This results in threading the conditional branch later on. Similar
opportunity exists when a select (with a constant in one branch) feeds a 
switch (via a phi node). The patch unfolds select under this condition. 
A testcase is provided.

llvm-svn: 350931
This commit is contained in:
Ehsan Amiri 2019-01-11 15:52:57 +00:00
parent 80378fd38b
commit f452f116d2
3 changed files with 152 additions and 28 deletions
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4
llvm/include/llvm/Transforms/Scalar/JumpThreading.h
View File

@ -139,7 +139,11 @@ public:
bool ProcessImpliedCondition(BasicBlock *BB);
bool SimplifyPartiallyRedundantLoad(LoadInst *LI);
void UnfoldSelectInstr(BasicBlock *Pred, BasicBlock *BB, SelectInst *SI,
PHINode *SIUse, unsigned Idx);
bool TryToUnfoldSelect(CmpInst *CondCmp, BasicBlock *BB);
bool TryToUnfoldSelect(SwitchInst *SI, BasicBlock *BB);
bool TryToUnfoldSelectInCurrBB(BasicBlock *BB);
bool ProcessGuards(BasicBlock *BB);

98
llvm/lib/Transforms/Scalar/JumpThreading.cpp
View File

@ -1171,6 +1171,9 @@ bool JumpThreadingPass::ProcessBlock(BasicBlock *BB) {
}
}
if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator()))
TryToUnfoldSelect(SI, BB);
// Check for some cases that are worth simplifying. Right now we want to look
// for loads that are used by a switch or by the condition for the branch. If
// we see one, check to see if it's partially redundant. If so, insert a PHI
@ -2388,6 +2391,72 @@ bool JumpThreadingPass::DuplicateCondBranchOnPHIIntoPred(
return true;
}
// Pred is a predecessor of BB with an unconditional branch to BB. SI is
// a Select instruction in Pred. BB has other predecessors and SI is used in
// a PHI node in BB. SI has no other use.
// A new basic block, NewBB, is created and SI is converted to compare and
// conditional branch. SI is erased from parent.
void JumpThreadingPass::UnfoldSelectInstr(BasicBlock *Pred, BasicBlock *BB,
SelectInst *SI, PHINode *SIUse,
unsigned Idx) {
// Expand the select.
//
// Pred --
// | v
// | NewBB
// | |
// |-----
// v
// BB
BranchInst *PredTerm = dyn_cast<BranchInst>(Pred->getTerminator());
BasicBlock *NewBB = BasicBlock::Create(BB->getContext(), "select.unfold",
BB->getParent(), BB);
// Move the unconditional branch to NewBB.
PredTerm->removeFromParent();
NewBB->getInstList().insert(NewBB->end(), PredTerm);
// Create a conditional branch and update PHI nodes.
BranchInst::Create(NewBB, BB, SI->getCondition(), Pred);
SIUse->setIncomingValue(Idx, SI->getFalseValue());
SIUse->addIncoming(SI->getTrueValue(), NewBB);
// The select is now dead.
SI->eraseFromParent();
DTU->applyUpdates({{DominatorTree::Insert, NewBB, BB},
{DominatorTree::Insert, Pred, NewBB}});
// Update any other PHI nodes in BB.
for (BasicBlock::iterator BI = BB->begin();
PHINode *Phi = dyn_cast<PHINode>(BI); ++BI)
if (Phi != SIUse)
Phi->addIncoming(Phi->getIncomingValueForBlock(Pred), NewBB);
}
bool JumpThreadingPass::TryToUnfoldSelect(SwitchInst *SI, BasicBlock *BB) {
PHINode *CondPHI = dyn_cast<PHINode>(SI->getCondition());
if (!CondPHI || CondPHI->getParent() != BB)
return false;
for (unsigned I = 0, E = CondPHI->getNumIncomingValues(); I != E; ++I) {
BasicBlock *Pred = CondPHI->getIncomingBlock(I);
SelectInst *PredSI = dyn_cast<SelectInst>(CondPHI->getIncomingValue(I));
// The second and third condition can be potentially relaxed. Currently
// the conditions help to simplify the code and allow us to reuse existing
// code, developed for TryToUnfoldSelect(CmpInst *, BasicBlock *)
if (!PredSI || PredSI->getParent() != Pred || !PredSI->hasOneUse())
continue;
BranchInst *PredTerm = dyn_cast<BranchInst>(Pred->getTerminator());
if (!PredTerm || !PredTerm->isUnconditional())
continue;
UnfoldSelectInstr(Pred, BB, PredSI, CondPHI, I);
return true;
}
return false;
}
/// TryToUnfoldSelect - Look for blocks of the form
/// bb1:
/// %a = select
@ -2438,34 +2507,7 @@ bool JumpThreadingPass::TryToUnfoldSelect(CmpInst *CondCmp, BasicBlock *BB) {
if ((LHSFolds != LazyValueInfo::Unknown ||
RHSFolds != LazyValueInfo::Unknown) &&
LHSFolds != RHSFolds) {
// Expand the select.
//
// Pred --
// | v
// | NewBB
// | |
// |-----
// v
// BB
BasicBlock *NewBB = BasicBlock::Create(BB->getContext(), "select.unfold",
BB->getParent(), BB);
// Move the unconditional branch to NewBB.
PredTerm->removeFromParent();
NewBB->getInstList().insert(NewBB->end(), PredTerm);
// Create a conditional branch and update PHI nodes.
BranchInst::Create(NewBB, BB, SI->getCondition(), Pred);
CondLHS->setIncomingValue(I, SI->getFalseValue());
CondLHS->addIncoming(SI->getTrueValue(), NewBB);
// The select is now dead.
SI->eraseFromParent();
DTU->applyUpdates({{DominatorTree::Insert, NewBB, BB},
{DominatorTree::Insert, Pred, NewBB}});
// Update any other PHI nodes in BB.
for (BasicBlock::iterator BI = BB->begin();
PHINode *Phi = dyn_cast<PHINode>(BI); ++BI)
if (Phi != CondLHS)
Phi->addIncoming(Phi->getIncomingValueForBlock(Pred), NewBB);
UnfoldSelectInstr(Pred, BB, SI, CondLHS, I);
return true;
}
}

78
llvm/test/Transforms/JumpThreading/select.ll
View File

@ -363,3 +363,81 @@ cond.false.15.i: ; preds = %cond.false.10.i
; CHECK: br i1 %cmp13.i, label %.exit, label %cond.false.15.i
; CHECK: br label %.exit
}
; When a select has a constant operand in one branch, and it feeds a phi node
; and the phi node feeds a switch we unfold the select
define void @test_func(i32* nocapture readonly %a, i32* nocapture readonly %b, i32* nocapture readonly %c, i32 %n) local_unnamed_addr #0 {
entry:
br label %for.cond
for.cond: ; preds = %sw.default, %entry
%i.0 = phi i32 [ 0, %entry ], [ %inc, %sw.default ]
%cmp = icmp slt i32 %i.0, %n
br i1 %cmp, label %for.body, label %for.cond.cleanup
for.cond.cleanup: ; preds = %for.cond
ret void
for.body: ; preds = %for.cond
%0 = zext i32 %i.0 to i64
%arrayidx = getelementptr inbounds i32, i32* %a, i64 %0
%1 = load i32, i32* %arrayidx, align 4
%cmp1 = icmp eq i32 %1, 4
br i1 %cmp1, label %land.lhs.true, label %if.end
land.lhs.true: ; preds = %for.body
%arrayidx3 = getelementptr inbounds i32, i32* %b, i64 %0
%2 = load i32, i32* %arrayidx3, align 4
%arrayidx5 = getelementptr inbounds i32, i32* %c, i64 %0
%3 = load i32, i32* %arrayidx5, align 4
%cmp6 = icmp eq i32 %2, %3
%spec.select = select i1 %cmp6, i32 2, i32 4
br label %if.end
if.end: ; preds = %land.lhs.true, %for.body
%local_var.0 = phi i32 [ %1, %for.body ], [ %spec.select, %land.lhs.true ]
switch i32 %local_var.0, label %sw.default [
i32 2, label %sw.bb
i32 4, label %sw.bb7
i32 5, label %sw.bb8
i32 7, label %sw.bb9
]
sw.bb: ; preds = %if.end
call void @foo()
br label %sw.bb7
sw.bb7: ; preds = %if.end, %sw.bb
call void @bar()
br label %sw.bb8
sw.bb8: ; preds = %if.end, %sw.bb7
call void @baz()
br label %sw.bb9
sw.bb9: ; preds = %if.end, %sw.bb8
call void @quux()
br label %sw.default
sw.default: ; preds = %if.end, %sw.bb9
call void @baz()
%inc = add nuw nsw i32 %i.0, 1
br label %for.cond
; CHECK-LABEL: @test_func(
; CHECK: [[REG:%[0-9]+]] = load
; CHECK-NOT: select
; CHECK: br i1
; CHECK-NOT: select
; CHECK: br i1 {{.*}}, label [[DEST1:%.*]], label [[DEST2:%.*]]
; The following line checks existence of a phi node, and makes sure
; it only has one incoming value. To do this, we check every '%'. Note
; that REG and REG2 each contain one '%;. There is another one in the
; beginning of the incoming block name. After that there should be no other '%'.
; CHECK: [[REG2:%.*]] = phi i32 {{[^%]*}}[[REG]]{{[^%]*%[^%]*}}
; CHECK: switch i32 [[REG2]]
; CHECK: i32 2, label [[DEST1]]
; CHECK: i32 4, label [[DEST2]]
}