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Implement SimplifyCFG/switch_switch_fold.ll

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This case occurs many times in various benchmarks, especially when combined
with the previous patch.  This allows it to get stuff like:
  if (X == 4 || X == 3)
    if (X == 5 || X == 8)

and

switch (X) {
case 4: case 5: case 6:
  if (X == 4 || X == 5)


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@11797 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner 2004-02-24 07:23:58 +00:00
parent 2189a76024
commit d52c261cf8
1 changed files with 150 additions and 2 deletions
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152
lib/Transforms/Utils/SimplifyCFG.cpp
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@ -18,6 +18,7 @@
#include "llvm/Support/CFG.h"
#include <algorithm>
#include <functional>
#include <set>
using namespace llvm;
// PropagatePredecessorsForPHIs - This gets "Succ" ready to have the
@ -286,6 +287,47 @@ static void ErasePossiblyDeadInstructionTree(Instruction *I) {
}
}
/// SafeToMergeTerminators - Return true if it is safe to merge these two
/// terminator instructions together.
///
static bool SafeToMergeTerminators(TerminatorInst *SI1, TerminatorInst *SI2) {
if (SI1 == SI2) return false; // Can't merge with self!
// It is not safe to merge these two switch instructions if they have a common
// successor, and if that successor has a PHI node, and if that PHI node has
// conflicting incoming values from the two switch blocks.
BasicBlock *SI1BB = SI1->getParent();
BasicBlock *SI2BB = SI2->getParent();
std::set<BasicBlock*> SI1Succs(succ_begin(SI1BB), succ_end(SI1BB));
for (succ_iterator I = succ_begin(SI2BB), E = succ_end(SI2BB); I != E; ++I)
if (SI1Succs.count(*I))
for (BasicBlock::iterator BBI = (*I)->begin();
PHINode *PN = dyn_cast<PHINode>(BBI); ++BBI)
if (PN->getIncomingValueForBlock(SI1BB) !=
PN->getIncomingValueForBlock(SI2BB))
return false;
return true;
}
/// AddPredecessorToBlock - Update PHI nodes in Succ to indicate that there will
/// now be entries in it from the 'NewPred' block. The values that will be
/// flowing into the PHI nodes will be the same as those coming in from
/// ExistPred, and existing predecessor of Succ.
static void AddPredecessorToBlock(BasicBlock *Succ, BasicBlock *NewPred,
BasicBlock *ExistPred) {
assert(std::find(succ_begin(ExistPred), succ_end(ExistPred), Succ) !=
succ_end(ExistPred) && "ExistPred is not a predecessor of Succ!");
if (!isa<PHINode>(Succ->begin())) return; // Quick exit if nothing to do
for (BasicBlock::iterator I = Succ->begin();
PHINode *PN = dyn_cast<PHINode>(I); ++I) {
Value *V = PN->getIncomingValueForBlock(ExistPred);
PN->addIncoming(V, NewPred);
}
}
// SimplifyCFG - This function is used to do simplification of a CFG. For
// example, it adjusts branches to branches to eliminate the extra hop, it
// eliminates unreachable basic blocks, and does other "peephole" optimization
@ -302,7 +344,8 @@ bool llvm::SimplifyCFG(BasicBlock *BB) {
assert(&BB->getParent()->front() != BB && "Can't Simplify entry block!");
// Remove basic blocks that have no predecessors... which are unreachable.
if (pred_begin(BB) == pred_end(BB)) {
if (pred_begin(BB) == pred_end(BB) ||
*pred_begin(BB) == BB && ++pred_begin(BB) == pred_end(BB)) {
//cerr << "Removing BB: \n" << BB;
// Loop through all of our successors and make sure they know that one
@ -439,7 +482,6 @@ bool llvm::SimplifyCFG(BasicBlock *BB) {
// We know there are no successors, so just nuke the block.
M->getBasicBlockList().erase(BB);
return true;
}
}
@ -470,6 +512,112 @@ bool llvm::SimplifyCFG(BasicBlock *BB) {
Preds.pop_back();
}
} else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->begin())) {
// If the only instruction in this block is a switch instruction, see if we
// can fold the switch instruction into a switch in a predecessor block.
std::vector<BasicBlock*> Preds(pred_begin(BB), pred_end(BB));
while (!Preds.empty()) {
BasicBlock *Pred = Preds.back();
Preds.pop_back();
// If the two blocks are switching on the same value, we can merge this
// switch into the predecessor's switch.
if (SwitchInst *PSI = dyn_cast<SwitchInst>(Pred->getTerminator()))
if (PSI->getCondition() == SI->getCondition() &&
SafeToMergeTerminators(SI, PSI)) {
// Figure out which 'cases' to copy from SI to PSI.
std::vector<std::pair<Constant*, BasicBlock*> > Cases;
BasicBlock *NewDefault = 0;
if (PSI->getDefaultDest() == BB) {
// If this is the default destination from PSI, only the edges in SI
// that don't occur in PSI, or that branch to BB will be activated.
std::set<Constant*> PSIHandled;
for (unsigned i = 1, e = PSI->getNumSuccessors(); i != e; ++i)
if (PSI->getSuccessor(i) != BB)
PSIHandled.insert(PSI->getCaseValue(i));
else {
// This entry will be replaced.
PSI->removeCase(i);
--i; --e;
}
NewDefault = SI->getDefaultDest();
for (unsigned i = 1, e = SI->getNumSuccessors(); i != e; ++i) {
Constant *C = SI->getCaseValue(i);
if (!PSIHandled.count(C))
Cases.push_back(std::make_pair(C, SI->getSuccessor(i)));
}
} else {
// If this is not the default destination from PSI, only the edges
// in SI that occur in PSI with a destination of BB will be
// activated.
std::set<Constant*> PSIHandled;
for (unsigned i = 1, e = PSI->getNumSuccessors(); i != e; ++i)
if (PSI->getSuccessor(i) == BB) {
// We know that BB doesn't have any PHI nodes in it, so just
// drop the edges.
PSIHandled.insert(PSI->getCaseValue(i));
PSI->removeCase(i);
--i; --e;
}
// Okay, now we know which constants were sent to BB from the
// predecessor. Figure out where they will all go now.
for (unsigned i = 1, e = SI->getNumSuccessors(); i != e; ++i) {
Constant *C = SI->getCaseValue(i);
if (PSIHandled.count(C)) {
// If this is one we are capable of getting...
Cases.push_back(std::make_pair(C, SI->getSuccessor(i)));
PSIHandled.erase(C); // This constant is taken care of
}
}
// If there are any constants vectored to BB that SI doesn't handle,
// they must go to the default destination of SI.
for (std::set<Constant*>::iterator I = PSIHandled.begin(),
E = PSIHandled.end(); I != E; ++I)
Cases.push_back(std::make_pair(*I, SI->getDefaultDest()));
}
// Okay, at this point, we know which cases need to be added to the
// PSI switch and which destinations they go to. If PSI needs its
// default destination changed, NewDefault is set. Start changing
// stuff now.
if (NewDefault) {
AddPredecessorToBlock(NewDefault, Pred, BB);
PSI->setSuccessor(0, NewDefault);
}
// Okay, add all of the cases now.
for (unsigned i = 0, e = Cases.size(); i != e; ++i) {
AddPredecessorToBlock(Cases[i].second, Pred, BB);
PSI->addCase(Cases[i].first, Cases[i].second);
}
// Okay, last check. If BB is still a successor of PSI, then we must
// have an infinite loop case. If so, add an infinitely looping block
// to handle the case to preserve the behavior of the code.
BasicBlock *InfLoopBlock = 0;
for (unsigned i = 0, e = PSI->getNumSuccessors(); i != e; ++i)
if (PSI->getSuccessor(i) == BB) {
if (InfLoopBlock == 0) {
// Insert it at the end of the loop, because it's either code,
// or it won't matter if it's hot. :)
InfLoopBlock = new BasicBlock("infloop", BB->getParent());
new BranchInst(InfLoopBlock, InfLoopBlock);
}
PSI->setSuccessor(i, InfLoopBlock);
}
Changed = true;
}
}
// If we removed all predecessors of this block, recursively call
// SimplifyCFG to remove it.
if (pred_begin(BB) == pred_end(BB))
return SimplifyCFG(BB);
}
// Merge basic blocks into their predecessor if there is only one distinct