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#include "slang-ir-simplify-cfg.h"
#include "slang-ir-insts.h"
#include "slang-ir.h"
#include "slang-ir-dominators.h"
#include "slang-ir-restructure.h"
namespace Slang
{
struct CFGSimplificationContext
{
RefPtr<RegionTree> regionTree;
RefPtr<IRDominatorTree> domTree;
};
static BreakableRegion* findBreakableRegion(Region* region)
{
for (;;)
{
if (auto b = as<BreakableRegion>(region))
return b;
region = region->getParent();
if (!region)
return nullptr;
}
}
// Test if a loop is trivial: a trivial loop runs for a single iteration without any back edges, and
// there is only one break out of the loop at the very end. The function generates `regionTree` if
// it is needed and hasn't been generated yet.
static bool isTrivialSingleIterationLoop(
IRGlobalValueWithCode* func,
IRLoop* loop,
CFGSimplificationContext& inoutContext)
{
auto targetBlock = loop->getTargetBlock();
if (targetBlock->getPredecessors().getCount() != 1) return false;
if (*targetBlock->getPredecessors().begin() != loop->getParent()) return false;
int useCount = 0;
for (auto use = loop->getBreakBlock()->firstUse; use; use = use->nextUse)
{
if (use->getUser() == loop)
continue;
useCount++;
if (useCount > 1)
return false;
}
// The loop has passed simple test.
//
// We need to verify this is a trivial loop by checking if there is any multi-level breaks
// that skips out of this loop.
if (!inoutContext.domTree)
inoutContext.domTree = computeDominatorTree(func);
if (!inoutContext.regionTree)
inoutContext.regionTree = generateRegionTreeForFunc(func, nullptr);
SimpleRegion* targetBlockRegion = nullptr;
if (!inoutContext.regionTree->mapBlockToRegion.TryGetValue(targetBlock, targetBlockRegion))
return false;
BreakableRegion* loopBreakableRegion = findBreakableRegion(targetBlockRegion);
LoopRegion* loopRegion = as<LoopRegion>(loopBreakableRegion);
if (!loopRegion)
return false;
for (auto block : func->getBlocks())
{
if (!inoutContext.domTree->dominates(loop->getTargetBlock(), block))
continue;
if (inoutContext.domTree->dominates(loop->getBreakBlock(), block))
continue;
SimpleRegion* region = nullptr;
if (!inoutContext.regionTree->mapBlockToRegion.TryGetValue(block, region))
return false;
for (auto branchTarget : block->getSuccessors())
{
SimpleRegion* targetRegion = nullptr;
if (!inoutContext.regionTree->mapBlockToRegion.TryGetValue(branchTarget, targetRegion))
return false;
// If multi-level break out that skips over this loop exists, then this is not a trivial loop.
if (targetRegion->isDescendentOf(loopRegion))
continue;
if (targetBlock != loop->getBreakBlock())
return false;
if (findBreakableRegion(region) != loopRegion)
{
// If the break is initiated from a nested region, this is not trivial.
return false;
}
}
}
return true;
}
static bool processFunc(IRGlobalValueWithCode* func)
{
auto firstBlock = func->getFirstBlock();
if (!firstBlock)
return false;
// Lazily generated region tree.
CFGSimplificationContext simplificationContext;
SharedIRBuilder sharedBuilder(func->getModule());
IRBuilder builder(&sharedBuilder);
bool changed = false;
List<IRBlock*> workList;
HashSet<IRBlock*> processedBlock;
workList.add(func->getFirstBlock());
while (workList.getCount())
{
auto block = workList.getFirst();
workList.fastRemoveAt(0);
while (block)
{
if (auto loop = as<IRLoop>(block->getTerminator()))
{
// If continue block is unreachable, remove it.
auto continueBlock = loop->getContinueBlock();
if (continueBlock && !continueBlock->hasMoreThanOneUse())
{
loop->continueBlock.set(loop->getTargetBlock());
continueBlock->removeAndDeallocate();
}
// If there isn't any actual back jumps into loop target and there is a trivial
// break at the end of the loop, we can remove the header and turn it into
// a normal branch.
auto targetBlock = loop->getTargetBlock();
if (isTrivialSingleIterationLoop(func, loop, simplificationContext))
{
builder.setInsertBefore(loop);
List<IRInst*> args;
for (UInt i = 0; i < loop->getArgCount(); i++)
{
args.add(loop->getArg(i));
}
builder.emitBranch(targetBlock, args.getCount(), args.getBuffer());
loop->removeAndDeallocate();
}
}
// If `block` does not end with an unconditional branch, bail.
if (block->getTerminator()->getOp() != kIROp_unconditionalBranch)
break;
auto branch = as<IRUnconditionalBranch>(block->getTerminator());
auto successor = branch->getTargetBlock();
// Only perform the merge if `block` is the only predecessor of `successor`.
// We also need to make sure not to merge a block that serves as the
// merge point in CFG. Such blocks will have more than one use.
if (successor->hasMoreThanOneUse())
break;
if (block->hasMoreThanOneUse())
break;
changed = true;
Index paramIndex = 0;
auto inst = successor->getFirstDecorationOrChild();
while (inst)
{
auto next = inst->getNextInst();
if (inst->getOp() == kIROp_Param)
{
inst->replaceUsesWith(branch->getArg(paramIndex));
paramIndex++;
}
else
{
inst->removeFromParent();
inst->insertAtEnd(block);
}
inst = next;
}
branch->removeAndDeallocate();
assert(!successor->hasUses());
successor->removeAndDeallocate();
}
for (auto successor : block->getSuccessors())
{
if (processedBlock.Add(successor))
{
workList.add(successor);
}
}
}
return changed;
}
bool simplifyCFG(IRModule* module)
{
bool changed = false;
for (auto inst : module->getGlobalInsts())
{
if (auto genericInst = as<IRGeneric>(inst))
{
inst = findGenericReturnVal(genericInst);
}
if (auto func = as<IRFunc>(inst))
{
changed |= processFunc(func);
}
}
return changed;
}
bool simplifyCFG(IRGlobalValueWithCode* func)
{
return processFunc(func);
}
} // namespace Slang
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