Changes in directory llvm/lib/Transforms/Scalar:
LoopUnswitch.cpp updated: 1.9 -> 1.10
---
Log message:
Implement unconditional unswitching of 'trivial' loops, those loops that contain
branches in their entry block that control whether or not the loop is a noop or
not.
---
Diffs of the changes: (+149 -18)
LoopUnswitch.cpp | 167 +++++++++++++++++++++++++++++++++++++++++++++++++------
1 files changed, 149 insertions(+), 18 deletions(-)
Index: llvm/lib/Transforms/Scalar/LoopUnswitch.cpp
diff -u llvm/lib/Transforms/Scalar/LoopUnswitch.cpp:1.9
llvm/lib/Transforms/Scalar/LoopUnswitch.cpp:1.10
--- llvm/lib/Transforms/Scalar/LoopUnswitch.cpp:1.9 Thu Feb 9 16:15:42 2006
+++ llvm/lib/Transforms/Scalar/LoopUnswitch.cpp Thu Feb 9 19:24:09 2006
@@ -65,9 +65,11 @@
}
private:
+ unsigned getLoopUnswitchCost(Loop *L, Value *LIC);
void VersionLoop(Value *LIC, Loop *L, Loop *&Out1, Loop *&Out2);
BasicBlock *SplitBlock(BasicBlock *BB, bool SplitAtTop);
void RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC, bool Val);
+ void UnswitchTrivialCondition(Loop *L, Value *Cond, ConstantBool
*LoopCond);
};
RegisterOpt<LoopUnswitch> X("loop-unswitch", "Unswitch loops");
}
@@ -88,13 +90,8 @@
}
-/// InsertPHINodesForUsesOutsideLoop - If this instruction is used outside of
-/// the specified loop, insert a PHI node in the appropriate exit block to
merge
-/// the values in the two different loop versions.
-///
-/// Most values are not used outside of the loop they are defined in, so be
-/// efficient for this case.
-///
+/// LoopValuesUsedOutsideLoop - Return true if there are any values defined in
+/// the loop that are used by instructions outside of it.
static bool LoopValuesUsedOutsideLoop(Loop *L) {
// We will be doing lots of "loop contains block" queries. Loop::contains is
// linear time, use a set to speed this up.
@@ -117,6 +114,89 @@
return false;
}
+/// IsTrivialUnswitchCondition - Check to see if this unswitch condition is
+/// trivial: that is, that the condition controls whether or not the loop does
+/// anything at all. If this is a trivial condition, unswitching produces no
+/// code duplications (equivalently, it produces a simpler loop and a new empty
+/// loop, which gets deleted).
+///
+/// If this is a trivial condition, return ConstantBool::True if the loop body
+/// runs when the condition is true, False if the loop body executes when the
+/// condition is false. Otherwise, return null to indicate a complex
condition.
+static ConstantBool *IsTrivialUnswitchCondition(Loop *L, Value *Cond) {
+ BasicBlock *Header = L->getHeader();
+ BranchInst *HeaderTerm = dyn_cast<BranchInst>(Header->getTerminator());
+ ConstantBool *RetVal = 0;
+
+ // If the header block doesn't end with a conditional branch on Cond, we
can't
+ // handle it.
+ if (!HeaderTerm || !HeaderTerm->isConditional() ||
+ HeaderTerm->getCondition() != Cond)
+ return 0;
+
+ // Check to see if the conditional branch goes to the latch block. If not,
+ // it's not trivial. This also determines the value of Cond that will
execute
+ // the loop.
+ BasicBlock *Latch = L->getLoopLatch();
+ if (HeaderTerm->getSuccessor(1) == Latch)
+ RetVal = ConstantBool::True;
+ else if (HeaderTerm->getSuccessor(0) == Latch)
+ RetVal = ConstantBool::False;
+ else
+ return 0; // Doesn't branch to latch block.
+
+ // The latch block must end with a conditional branch where one edge goes to
+ // the header (this much we know) and one edge goes OUT of the loop.
+ BranchInst *LatchBranch = dyn_cast<BranchInst>(Latch->getTerminator());
+ if (!LatchBranch || !LatchBranch->isConditional()) return 0;
+
+ if (LatchBranch->getSuccessor(0) == Header) {
+ if (L->contains(LatchBranch->getSuccessor(1))) return 0;
+ } else {
+ assert(LatchBranch->getSuccessor(1) == Header);
+ if (L->contains(LatchBranch->getSuccessor(0))) return 0;
+ }
+
+ // We already know that nothing uses any scalar values defined inside of this
+ // loop. As such, we just have to check to see if this loop will execute any
+ // side-effecting instructions (e.g. stores, calls, volatile loads) in the
+ // part of the loop that the code *would* execute.
+ for (BasicBlock::iterator I = Header->begin(), E = Header->end(); I != E;
++I)
+ if (I->mayWriteToMemory())
+ return 0;
+ for (BasicBlock::iterator I = Latch->begin(), E = Latch->end(); I != E; ++I)
+ if (I->mayWriteToMemory())
+ return 0;
+ return RetVal;
+}
+
+/// getLoopUnswitchCost - Return the cost (code size growth) that will happen
if
+/// we choose to unswitch the specified loop on the specified value.
+///
+unsigned LoopUnswitch::getLoopUnswitchCost(Loop *L, Value *LIC) {
+ // If the condition is trivial, always unswitch. There is no code growth for
+ // this case.
+ if (IsTrivialUnswitchCondition(L, LIC))
+ return 0;
+
+ unsigned Cost = 0;
+ // FIXME: this is brain dead. It should take into consideration code
+ // shrinkage.
+ for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
+ I != E; ++I) {
+ BasicBlock *BB = *I;
+ // Do not include empty blocks in the cost calculation. This happen due to
+ // loop canonicalization and will be removed.
+ if (BB->begin() == BasicBlock::iterator(BB->getTerminator()))
+ continue;
+
+ // Count basic blocks.
+ ++Cost;
+ }
+
+ return Cost;
+}
+
bool LoopUnswitch::visitLoop(Loop *L) {
bool Changed = false;
@@ -150,7 +230,7 @@
continue;
// Check to see if it would be profitable to unswitch this loop.
- if (L->getBlocks().size() > Threshold) {
+ if (getLoopUnswitchCost(L, BI->getCondition()) > Threshold) {
// FIXME: this should estimate growth by the amount of code shared by the
// resultant unswitched loops. This should have no code growth:
// for () { if (iv) {...} }
@@ -173,13 +253,22 @@
}
//std::cerr << "BEFORE:\n"; LI->dump();
- Loop *First = 0, *Second = 0;
- VersionLoop(BI->getCondition(), L, First, Second);
+ Loop *NewLoop1 = 0, *NewLoop2 = 0;
+
+ // If this is a trivial condition to unswitch (which results in no code
+ // duplication), do it now.
+ if (ConstantBool *V = IsTrivialUnswitchCondition(L, BI->getCondition())) {
+ UnswitchTrivialCondition(L, BI->getCondition(), V);
+ NewLoop1 = L;
+ } else {
+ VersionLoop(BI->getCondition(), L, NewLoop1, NewLoop2);
+ }
+
//std::cerr << "AFTER:\n"; LI->dump();
// Try to unswitch each of our new loops now!
- if (First) visitLoop(First);
- if (Second) visitLoop(Second);
+ if (NewLoop1) visitLoop(NewLoop1);
+ if (NewLoop2) visitLoop(NewLoop2);
return true;
}
@@ -193,6 +282,9 @@
///
/// This method updates the LoopInfo for this function to correctly reflect the
/// CFG changes made.
+///
+/// This routine returns the new basic block that was inserted, which is always
+/// the later part of the block.
BasicBlock *LoopUnswitch::SplitBlock(BasicBlock *BB, bool SplitAtTop) {
BasicBlock::iterator SplitPoint;
if (!SplitAtTop)
@@ -201,12 +293,12 @@
SplitPoint = BB->begin();
while (isa<PHINode>(SplitPoint)) ++SplitPoint;
}
-
+
BasicBlock *New = BB->splitBasicBlock(SplitPoint, BB->getName()+".tail");
// New now lives in whichever loop that BB used to.
if (Loop *L = LI->getLoopFor(BB))
L->addBasicBlockToLoop(New, *LI);
- return SplitAtTop ? BB : New;
+ return New;
}
@@ -247,6 +339,42 @@
return New;
}
+/// UnswitchTrivialCondition - Given a loop that has a trivial unswitchable
+/// condition in it (a cond branch from its header block to its latch block,
+/// where the path through the loop that doesn't execute its body has no
+/// side-effects), unswitch it. This doesn't involve any code duplication,
just
+/// moving the conditional branch outside of the loop and updating loop info.
+void LoopUnswitch::UnswitchTrivialCondition(Loop *L, Value *Cond,
+ ConstantBool *LoopCond) {
+ // First step, split the preahder, so that we know that there is a safe place
+ // to insert the conditional branch. We will change 'OrigPH' to have a
+ // conditional branch on Cond.
+ BasicBlock *OrigPH = L->getLoopPreheader();
+ BasicBlock *NewPH = SplitBlock(OrigPH, false);
+
+ // Now that we have a place to insert the conditional branch, create a place
+ // to branch to: this is the non-header successor of the latch block.
+ BranchInst *LatchBranch
=cast<BranchInst>(L->getLoopLatch()->getTerminator());
+ BasicBlock *ExitBlock =
+ LatchBranch->getSuccessor(LatchBranch->getSuccessor(0) == L->getHeader());
+ assert(!L->contains(ExitBlock) && "Exit block is in the loop?");
+
+ // Split this block now, so that the loop maintains its exit block.
+ BasicBlock *NewExit = SplitBlock(ExitBlock, true);
+
+ // Okay, now we have a position to branch from and a position to branch to,
+ // insert the new conditional branch.
+ bool EnterOnTrue = LoopCond->getValue();
+ new BranchInst(EnterOnTrue ? NewPH : NewExit, EnterOnTrue ? NewExit : NewPH,
+ Cond, OrigPH->getTerminator());
+ OrigPH->getTerminator()->eraseFromParent();
+
+ // Now that we know that the loop is never entered when this condition is a
+ // particular value, rewrite the loop with this info. We know that this will
+ // at least eliminate the old branch.
+ RewriteLoopBodyWithConditionConstant(L, Cond, EnterOnTrue);
+}
+
/// VersionLoop - We determined that the loop is profitable to unswitch and
/// contains a branch on a loop invariant condition. Split it into loop
@@ -254,7 +382,7 @@
/// created as Out1/Out2.
void LoopUnswitch::VersionLoop(Value *LIC, Loop *L, Loop *&Out1, Loop *&Out2) {
Function *F = L->getHeader()->getParent();
-
+
DEBUG(std::cerr << "loop-unswitch: Unswitching loop %"
<< L->getHeader()->getName() << " [" << L->getBlocks().size()
<< " blocks] in Function " << F->getName()
@@ -275,8 +403,10 @@
std::sort(ExitBlocks.begin(), ExitBlocks.end());
ExitBlocks.erase(std::unique(ExitBlocks.begin(), ExitBlocks.end()),
ExitBlocks.end());
- for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
- LoopBlocks.push_back(ExitBlocks[i] = SplitBlock(ExitBlocks[i], true));
+ for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
+ SplitBlock(ExitBlocks[i], true);
+ LoopBlocks.push_back(ExitBlocks[i]);
+ }
// Next step, clone all of the basic blocks that make up the loop (including
// the loop preheader and exit blocks), keeping track of the mapping between
@@ -337,6 +467,7 @@
// of properties correlated to it.
void LoopUnswitch::RewriteLoopBodyWithConditionConstant(Loop *L, Value *LIC,
bool Val) {
+ assert(!isa<Constant>(LIC) && "Why are we unswitching on a constant?");
// FIXME: Support correlated properties, like:
// for (...)
// if (li1 < li2)
@@ -347,7 +478,7 @@
std::vector<User*> Users(LIC->use_begin(), LIC->use_end());
for (unsigned i = 0, e = Users.size(); i != e; ++i)
- if (Instruction *U = dyn_cast<Instruction>(Users[i]))
+ if (Instruction *U = cast<Instruction>(Users[i]))
if (L->contains(U->getParent()))
U->replaceUsesOfWith(LIC, BoolVal);
}
_______________________________________________
llvm-commits mailing list
llvm-commits@cs.uiuc.edu
http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
