Start doing the significantly useful part of jump threading: handle cases

where a comparison has a phi input and that phi is a constant.  For example,
stuff like:

  Threading edge through bool from 'bb2149' to 'bb2231' with cost: 1, across block:
bb2237:		; preds = %bb2231, %bb2149
	%tmp2328.rle = phi i32 [ %tmp2232, %bb2231 ], [ %tmp2232439, %bb2149 ]		; <i32> [#uses=2]
	%done.0 = phi i32 [ %done.2, %bb2231 ], [ 0, %bb2149 ]		; <i32> [#uses=1]
	%tmp2239 = icmp eq i32 %done.0, 0		; <i1> [#uses=1]
	br i1 %tmp2239, label %bb2231, label %bb2327

or

bb38.i298:		; preds = %bb33.i295, %bb1693
	%tmp39.i296.rle = phi %struct.ibox* [ null, %bb1693 ], [ %tmp39.i296.rle1109, %bb33.i295 ]		; <%struct.ibox*> [#uses=2]
	%minspan.1.i291.reg2mem.1 = phi i32 [ 32000, %bb1693 ], [ %minspan.0.i288, %bb33.i295 ]		; <i32> [#uses=1]
	%tmp40.i297 = icmp eq %struct.ibox* %tmp39.i296.rle, null		; <i1> [#uses=1]
	br i1 %tmp40.i297, label %implfeeds.exit311, label %bb43.i301

This triggers thousands of times in spec.

llvm-svn: 50110
This commit is contained in:
Chris Lattner 2008-04-22 21:40:39 +00:00
parent c79c0591d6
commit 37e9c187b0
2 changed files with 113 additions and 7 deletions

View File

@ -61,6 +61,7 @@ namespace {
bool ProcessJumpOnPHI(PHINode *PN);
bool ProcessBranchOnLogical(Value *V, BasicBlock *BB, bool isAnd);
bool ProcessBranchOnCompare(CmpInst *Cmp, BasicBlock *BB);
};
char JumpThreading::ID = 0;
RegisterPass<JumpThreading> X("jump-threading", "Jump Threading");
@ -199,6 +200,14 @@ bool JumpThreading::ThreadBlock(BasicBlock *BB) {
return true;
}
// If we have "br (phi != 42)" and the phi node has any constant values as
// operands, we can thread through this block.
if (CmpInst *CondCmp = dyn_cast<CmpInst>(Condition))
if (isa<PHINode>(CondCmp->getOperand(0)) &&
isa<Constant>(CondCmp->getOperand(1)) &&
ProcessBranchOnCompare(CondCmp, BB))
return true;
return false;
}
@ -209,18 +218,14 @@ bool JumpThreading::ThreadBlock(BasicBlock *BB) {
bool JumpThreading::ProcessJumpOnPHI(PHINode *PN) {
// See if the phi node has any constant values. If so, we can determine where
// the corresponding predecessor will branch.
unsigned PredNo = ~0U;
ConstantInt *PredCst = 0;
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
if ((PredCst = dyn_cast<ConstantInt>(PN->getIncomingValue(i)))) {
PredNo = i;
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
if ((PredCst = dyn_cast<ConstantInt>(PN->getIncomingValue(i))))
break;
}
}
// If no incoming value has a constant, we don't know the destination of any
// predecessors.
if (PredNo == ~0U)
if (PredCst == 0)
return false;
// See if the cost of duplicating this block is low enough.
@ -325,6 +330,77 @@ bool JumpThreading::ProcessBranchOnLogical(Value *V, BasicBlock *BB,
return true;
}
/// ProcessBranchOnCompare - We found a branch on a comparison between a phi
/// node and a constant. If the PHI node contains any constants as inputs, we
/// can fold the compare for that edge and thread through it.
bool JumpThreading::ProcessBranchOnCompare(CmpInst *Cmp, BasicBlock *BB) {
PHINode *PN = cast<PHINode>(Cmp->getOperand(0));
Constant *RHS = cast<Constant>(Cmp->getOperand(1));
// If the phi isn't in the current block, an incoming edge to this block
// doesn't control the destination.
if (PN->getParent() != BB)
return false;
// We can do this simplification if any comparisons fold to true or false.
// See if any do.
Constant *PredCst = 0;
bool TrueDirection = false;
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
PredCst = dyn_cast<Constant>(PN->getIncomingValue(i));
if (PredCst == 0) continue;
Constant *Res;
if (ICmpInst *ICI = dyn_cast<ICmpInst>(Cmp))
Res = ConstantExpr::getICmp(ICI->getPredicate(), PredCst, RHS);
else
Res = ConstantExpr::getFCmp(cast<FCmpInst>(Cmp)->getPredicate(),
PredCst, RHS);
// If this folded to a constant expr, we can't do anything.
if (ConstantInt *ResC = dyn_cast<ConstantInt>(Res)) {
TrueDirection = ResC->getZExtValue();
break;
}
// If this folded to undef, just go the false way.
if (isa<UndefValue>(Res)) {
TrueDirection = false;
break;
}
// Otherwise, we can't fold this input.
PredCst = 0;
}
// If no match, bail out.
if (PredCst == 0)
return false;
// See if the cost of duplicating this block is low enough.
unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB);
if (JumpThreadCost > Threshold) {
DOUT << " Not threading BB '" << BB->getNameStart()
<< "' - Cost is too high: " << JumpThreadCost << "\n";
return false;
}
// If so, we can actually do this threading. Merge any common predecessors
// that will act the same.
BasicBlock *PredBB = FactorCommonPHIPreds(PN, PredCst);
// Next, get our successor.
BasicBlock *SuccBB = BB->getTerminator()->getSuccessor(!TrueDirection);
// And finally, do it!
DOUT << " Threading edge through bool from '" << PredBB->getNameStart()
<< "' to '" << SuccBB->getNameStart() << "' with cost: "
<< JumpThreadCost << ", across block:\n "
<< *BB << "\n";
ThreadEdge(BB, PredBB, SuccBB);
++NumThreads;
return true;
}
/// ThreadEdge - We have decided that it is safe and profitable to thread an
/// edge from PredBB to SuccBB across BB. Transform the IR to reflect this

View File

@ -0,0 +1,30 @@
; There should be no phi nodes left.
; RUN: llvm-as < %s | opt -jump-threading -simplifycfg -mem2reg | llvm-dis | not grep {phi i32}
declare i32 @f1()
declare i32 @f2()
declare void @f3()
define i32 @test(i1 %cond) {
br i1 %cond, label %T1, label %F1
T1:
%v1 = call i32 @f1()
br label %Merge
F1:
%v2 = call i32 @f2()
br label %Merge
Merge:
%B = phi i32 [%v1, %T1], [12, %F1]
%A = icmp ne i32 %B, 42
br i1 %A, label %T2, label %F2
T2:
call void @f3()
ret i32 1
F2:
ret i32 0
}