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JumpThreading: Turn a select instruction into branching if it allows to thread one half of the select.

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This is a common pattern coming out of simplifycfg generating gross code.

a:                                       ; preds = %entry
  %sel = select i1 %cmp1, double %add, double 0.000000e+00
  br label %b

b:
  %cond5 = phi double [ %sel, %a ], [ %sub, %entry ]
  %cmp6 = fcmp oeq double %cond5, 0.000000e+00
  br i1 %cmp6, label %if.then, label %if.end

becomes

a:
  br i1 %cmp1, label %b, label %if.then

b:
  %cond5 = phi double [ %sub, %entry ], [ %add, %a ]
  %cmp6 = fcmp oeq double %cond5, 0.000000e+00
  br i1 %cmp6, label %if.then, label %if.end

Skipping block b completely if possible.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187880 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Benjamin Kramer 2013-08-07 10:29:38 +00:00
parent e4a4aecffb
commit c11b107f21
2 changed files with 146 additions and 0 deletions
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83
lib/Transforms/Scalar/JumpThreading.cpp
View File

@ -130,6 +130,7 @@ namespace {
bool ProcessBranchOnXOR(BinaryOperator *BO);
bool SimplifyPartiallyRedundantLoad(LoadInst *LI);
bool TryToUnfoldSelect(CmpInst *CondCmp, BasicBlock *BB);
};
}
@ -776,7 +777,11 @@ bool JumpThreading::ProcessBlock(BasicBlock *BB) {
return true;
}
}
}
if (CondBr && CondConst && TryToUnfoldSelect(CondCmp, BB))
return true;
}
// Check for some cases that are worth simplifying. Right now we want to look
@ -1615,3 +1620,81 @@ bool JumpThreading::DuplicateCondBranchOnPHIIntoPred(BasicBlock *BB,
++NumDupes;
return true;
}
/// TryToUnfoldSelect - Look for blocks of the form
/// bb1:
/// %a = select
/// br bb
///
/// bb2:
/// %p = phi [%a, %bb] ...
/// %c = icmp %p
/// br i1 %c
///
/// And expand the select into a branch structure if one of its arms allows %c
/// to be folded. This later enables threading from bb1 over bb2.
bool JumpThreading::TryToUnfoldSelect(CmpInst *CondCmp, BasicBlock *BB) {
BranchInst *CondBr = dyn_cast<BranchInst>(BB->getTerminator());
PHINode *CondLHS = dyn_cast<PHINode>(CondCmp->getOperand(0));
Constant *CondRHS = cast<Constant>(CondCmp->getOperand(1));
if (!CondBr || !CondBr->isConditional() || !CondLHS ||
CondLHS->getParent() != BB)
return false;
for (unsigned I = 0, E = CondLHS->getNumIncomingValues(); I != E; ++I) {
BasicBlock *Pred = CondLHS->getIncomingBlock(I);
SelectInst *SI = dyn_cast<SelectInst>(CondLHS->getIncomingValue(I));
// Look if one of the incoming values is a select in the corresponding
// predecessor.
if (!SI || SI->getParent() != Pred || !SI->hasOneUse())
continue;
BranchInst *PredTerm = dyn_cast<BranchInst>(Pred->getTerminator());
if (!PredTerm || !PredTerm->isUnconditional())
continue;
// Now check if one of the select values would allow us to constant fold the
// terminator in BB. We don't do the transform if both sides fold, those
// cases will be threaded in any case.
LazyValueInfo::Tristate LHSFolds =
LVI->getPredicateOnEdge(CondCmp->getPredicate(), SI->getOperand(1),
CondRHS, Pred, BB);
LazyValueInfo::Tristate RHSFolds =
LVI->getPredicateOnEdge(CondCmp->getPredicate(), SI->getOperand(2),
CondRHS, Pred, 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();
// 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);
return true;
}
}
return false;
}

63
test/Transforms/JumpThreading/select.ll
View File

@ -157,3 +157,66 @@ L3:
L4:
ret void
}
define void @unfold1(double %x, double %y) nounwind {
entry:
%sub = fsub double %x, %y
%cmp = fcmp ogt double %sub, 1.000000e+01
br i1 %cmp, label %cond.end4, label %cond.false
cond.false: ; preds = %entry
%add = fadd double %x, %y
%cmp1 = fcmp ogt double %add, 1.000000e+01
%add. = select i1 %cmp1, double %add, double 0.000000e+00
br label %cond.end4
cond.end4: ; preds = %entry, %cond.false
%cond5 = phi double [ %add., %cond.false ], [ %sub, %entry ]
%cmp6 = fcmp oeq double %cond5, 0.000000e+00
br i1 %cmp6, label %if.then, label %if.end
if.then: ; preds = %cond.end4
call void @foo()
br label %if.end
if.end: ; preds = %if.then, %cond.end4
ret void
; CHECK-LABEL: @unfold1
; CHECK: br i1 %cmp, label %cond.end4, label %cond.false
; CHECK: br i1 %cmp1, label %cond.end4, label %if.then
; CHECK: br i1 %cmp6, label %if.then, label %if.end
; CHECK: br label %if.end
}
define void @unfold2(i32 %x, i32 %y) nounwind {
entry:
%sub = sub nsw i32 %x, %y
%cmp = icmp sgt i32 %sub, 10
br i1 %cmp, label %cond.end4, label %cond.false
cond.false: ; preds = %entry
%add = add nsw i32 %x, %y
%cmp1 = icmp sgt i32 %add, 10
%add. = select i1 %cmp1, i32 0, i32 %add
br label %cond.end4
cond.end4: ; preds = %entry, %cond.false
%cond5 = phi i32 [ %add., %cond.false ], [ %sub, %entry ]
%cmp6 = icmp eq i32 %cond5, 0
br i1 %cmp6, label %if.then, label %if.end
if.then: ; preds = %cond.end4
call void @foo()
br label %if.end
if.end: ; preds = %if.then, %cond.end4
ret void
; CHECK-LABEL: @unfold2
; CHECK: br i1 %cmp, label %if.end, label %cond.false
; CHECK: br i1 %cmp1, label %if.then, label %cond.end4
; CHECK: br i1 %cmp6, label %if.then, label %if.end
; CHECK: br label %if.end
}