private bool AssertDecodeBranchArguments(
IEnumerable <LNode> nodes,
Dictionary <Symbol, ValueTag> valueTags,
out IReadOnlyList <BranchArgument> args)
{
var results = new List <BranchArgument>();
foreach (var argNode in nodes)
{
if (!FeedbackHelpers.AssertIsId(argNode, Log))
{
args = null;
return(false);
}
var name = argNode.Name;
if (name == CodeSymbols.Result)
{
results.Add(BranchArgument.TryResult);
}
else if (name == EncoderState.tryFlowExceptionSymbol)
{
results.Add(BranchArgument.TryException);
}
else
{
results.Add(BranchArgument.FromValue(GetValueTag(argNode.Name, valueTags)));
}
}
args = results;
return(true);
}
public override void ReplaceInstruction(FlowGraph graph, IReadOnlyList <ValueTag> arguments)
{
if (!IsValid)
{
throw new InvalidOperationException("Cannot replace an invalid instruction reference.");
}
var tryFlow = (TryFlow)Flow;
// Include `graph` in the defining graph. Wrap exception-throwing instructions
// in 'try' flow.
var entryTag = Block.Graph.Include(
graph,
(retFlow, enclosingBlock) =>
{
// Rewrite 'return' flow by replacing it with a branch to the success
// block.
var resultTag = enclosingBlock.AppendInstruction(retFlow.ReturnValue);
return(new JumpFlow(
tryFlow.SuccessBranch.MapArguments(
arg => arg.IsTryResult ? BranchArgument.FromValue(resultTag) : arg)));
},
tryFlow.ExceptionBranch);
// Jump to `graph`'s entry point.
Block.Flow = new JumpFlow(entryTag, arguments);
// Invalidate the instruction reference.
Flow = null;
}
/// <summary>
/// Replaces a block's flow with an unconditional jump to an exception
/// branch. Replaces 'try' flow exception arguments in that exception
/// branch with a captured exception value.
/// </summary>
/// <param name="block">The block to rewrite.</param>
/// <param name="exceptionBranch">The exception branch to jump to unconditionally.</param>
/// <param name="capturedException">
/// The captured exception to pass instead of a 'try' flow exception argument.
/// </param>
private static void JumpToExceptionBranch(
BasicBlockBuilder block,
Branch exceptionBranch,
ValueTag capturedException)
{
var branch = exceptionBranch.WithArguments(
exceptionBranch.Arguments
.Select(arg => arg.IsTryException ? BranchArgument.FromValue(capturedException) : arg)
.ToArray());
block.Flow = new JumpFlow(branch);
}
private BlockFlow AsThreadableFlow(Branch branch, FlowGraphBuilder graph, HashSet <BasicBlockTag> processedBlocks)
{
ThreadJumps(graph.GetBasicBlock(branch.Target), processedBlocks);
// Block arguments are a bit of an obstacle for jump threading.
//
// * We can easily thread jumps to blocks that have block parameters
// if those parameters are never used outside of the block: in that case,
// we just substitute arguments for parameters.
//
// * Jumps to blocks that have block parameters that are used outside
// of the block that defines them are trickier to handle. We'll just bail
// when we encounter them, because these don't occur when the control-flow
// graph is in register forwarding form.
var target = graph.GetBasicBlock(branch.Target);
var uses = graph.GetAnalysisResult <ValueUses>();
// Only jump and switch flow are threadable. We don't jump-thread through
// instructions.
// TODO: consider copying instructions to enable more aggressive jump threading.
if ((!(target.Flow is JumpFlow) && !(target.Flow is SwitchFlow)) ||
target.InstructionTags.Count > 0 ||
target.ParameterTags.Any(
tag => uses.GetInstructionUses(tag).Count > 0 ||
uses.GetFlowUses(tag).Any(block => block != target.Tag)))
{
return(null);
}
var valueSubstitutionMap = new Dictionary <ValueTag, ValueTag>();
var argSubstitutionMap = new Dictionary <BranchArgument, BranchArgument>();
foreach (var kvPair in branch.ZipArgumentsWithParameters(graph))
{
if (kvPair.Value.IsValue)
{
valueSubstitutionMap[kvPair.Key] = kvPair.Value.ValueOrNull;
}
argSubstitutionMap[BranchArgument.FromValue(kvPair.Key)] = kvPair.Value;
}
return(target.Flow
.MapArguments(argSubstitutionMap)
.MapValues(valueSubstitutionMap));
}
/// <inheritdoc/>
public override FlowGraph Apply(FlowGraph graph)
{
var builder = graph.ToBuilder();
// Analyze all local value definitions and find imported values.
var definitions = new Dictionary <BasicBlockTag, Dictionary <ValueTag, ValueTag> >();
var extraArgs = new Dictionary <BasicBlockTag, List <ValueTag> >();
var imports = new Dictionary <BasicBlockTag, HashSet <ValueTag> >();
foreach (var block in builder.BasicBlocks)
{
var localDefs = new Dictionary <ValueTag, ValueTag>();
var localImports = new HashSet <ValueTag>();
foreach (var parameter in block.ParameterTags)
{
localDefs[parameter] = parameter;
}
foreach (var instruction in block.NamedInstructions)
{
localDefs[instruction] = instruction;
localImports.UnionWith(instruction.Arguments);
}
foreach (var instruction in block.Flow.Instructions)
{
localImports.UnionWith(instruction.Arguments);
}
foreach (var branch in block.Flow.Branches)
{
foreach (var arg in branch.Arguments)
{
if (arg.IsValue)
{
localImports.Add(arg.ValueOrNull);
}
}
}
definitions[block] = localDefs;
imports[block] = localImports;
extraArgs[block] = new List <ValueTag>();
}
var predecessors = graph.GetAnalysisResult <BasicBlockPredecessors>();
// Now import definitions until we reach a fixpoint.
var worklist = new HashSet <BasicBlockTag>(builder.BasicBlockTags);
while (worklist.Count > 0)
{
var block = builder.GetBasicBlock(worklist.First());
worklist.Remove(block);
var blockDefs = definitions[block];
var blockArgs = extraArgs[block];
var blockImports = imports[block];
blockImports.ExceptWith(blockDefs.Keys);
imports[block] = new HashSet <ValueTag>();
foreach (var value in blockImports)
{
var rffName = value.Name + ".rff." + block.Tag.Name;
NamedInstruction valueInstruction;
if (graph.TryGetInstruction(value, out valueInstruction) &&
valueInstruction.Prototype is ConstantPrototype)
{
// Create duplicate definitions of constants instead of
// importing them using phis.
blockDefs[value] = block.InsertInstruction(0, valueInstruction.Instruction, rffName);
continue;
}
// Import a definition by introducing a new parameter and recursively
// importing it the value in predecessor blocks.
blockDefs[value] = block.AppendParameter(
builder.GetValueType(value),
rffName);
blockArgs.Add(value);
foreach (var pred in predecessors.GetPredecessorsOf(block))
{
if (!definitions[pred].ContainsKey(value))
{
imports[pred].Add(value);
worklist.Add(pred);
}
}
}
}
// We have introduced basic block parameters and know which values are
// used by blocks. We finish by replacing value uses and appending
// branch arguments.
foreach (var block in builder.BasicBlocks)
{
var blockDefs = definitions[block];
foreach (var insn in block.NamedInstructions)
{
insn.Instruction = insn.Instruction.MapArguments(blockDefs);
}
block.Flow = block.Flow
.MapValues(blockDefs)
.MapBranches(branch =>
{
var args = new List <BranchArgument>(branch.Arguments);
foreach (var arg in extraArgs[branch.Target])
{
args.Add(BranchArgument.FromValue(blockDefs[arg]));
}
return(branch.WithArguments(args));
});
}
return(builder.ToImmutable());
}
private void ThreadJumps(BasicBlockBuilder block, HashSet <BasicBlockTag> processedBlocks)
{
if (!processedBlocks.Add(block))
{
return;
}
var flow = block.Flow;
if (flow is JumpFlow)
{
// Jump flow is usually fairly straightforward to thread.
var jumpFlow = (JumpFlow)flow;
var threadFlow = AsThreadableFlow(jumpFlow.Branch, block.Graph, processedBlocks);
if (threadFlow != null && jumpFlow.Branch.Target != block.Tag)
{
block.Flow = threadFlow;
if (block.Flow is SwitchFlow)
{
// If the jump flow gets replaced by switch flow, then we want to
// jump-thread the block again.
processedBlocks.Remove(block);
ThreadJumps(block, processedBlocks);
return;
}
}
// Now would also be a good time to try and fuse this block with
// its successor, if this jump is the only branch to the successor.
var predecessors = block.Graph.GetAnalysisResult <BasicBlockPredecessors>();
BasicBlockTag tail;
if (BlockFusion.TryGetFusibleTail(block, block.Graph.ToImmutable(), predecessors, out tail))
{
// Fuse the blocks.
FuseBlocks(block, tail);
// Fusing these blocks may have exposed additional information
// that will enable us to thread more jumps.
processedBlocks.Remove(block);
ThreadJumps(block, processedBlocks);
}
}
else if (flow is TryFlow)
{
// We might be able to turn try flow into a jump, which we can thread
// recursively.
var tryFlow = (TryFlow)flow;
// Start off by threading the try flow's branches.
var successBranch = tryFlow.SuccessBranch;
var failureBranch = tryFlow.ExceptionBranch;
var successFlow = AsThreadableFlow(tryFlow.SuccessBranch, block.Graph, processedBlocks);
var failureFlow = AsThreadableFlow(tryFlow.ExceptionBranch, block.Graph, processedBlocks);
if (successFlow is JumpFlow)
{
successBranch = ((JumpFlow)successFlow).Branch;
}
if (failureFlow is JumpFlow)
{
failureBranch = ((JumpFlow)failureFlow).Branch;
}
var exceptionSpecs = block.Graph.GetAnalysisResult <InstructionExceptionSpecs>();
if (!exceptionSpecs.GetExceptionSpecification(tryFlow.Instruction).CanThrowSomething ||
IsRethrowBranch(failureBranch, block.Graph, processedBlocks))
{
// If the "risky" instruction absolutely cannot throw, then we can
// just rewrite the try as a jump.
// Similarly, if the exception branch simply re-throws the exception,
// then we are also free to make the same transformation, but for
// different reasons.
var value = block.AppendInstruction(tryFlow.Instruction);
var branch = successBranch.WithArguments(
successBranch.Arguments
.Select(arg => arg.IsTryResult ? BranchArgument.FromValue(value) : arg)
.ToArray());
block.Flow = new JumpFlow(branch);
// Jump-thread this block again.
processedBlocks.Remove(block);
ThreadJumps(block, processedBlocks);
}
else if (IsRethrowIntrinsic(tryFlow.Instruction))
{
// If the "risky" instruction is really just a 'rethrow' intrinsic,
// then we can branch directly to the exception branch.
var capturedException = tryFlow.Instruction.Arguments[0];
JumpToExceptionBranch(block, failureBranch, capturedException);
// Jump-thread this block again.
processedBlocks.Remove(block);
ThreadJumps(block, processedBlocks);
}
else if (IsThrowIntrinsic(tryFlow.Instruction))
{
// If the "risky" instruction is really just a 'throw' intrinsic,
// then we can insert an instruction to capture the exception and
// jump directly to the exception block.
var exception = tryFlow.Instruction.Arguments[0];
var capturedExceptionParam = failureBranch
.ZipArgumentsWithParameters(block.Graph)
.FirstOrDefault(pair => pair.Value.IsTryException)
.Key;
if (capturedExceptionParam == null)
{
// If the exception branch does not pass an '#exception' parameter,
// then we can replace the try by a simple jump.
block.Flow = new JumpFlow(failureBranch);
}
else
{
// Otherwise, we actually need to capture the exception.
var capturedException = block.AppendInstruction(
Instruction.CreateCaptureIntrinsic(
block.Graph.GetValueType(capturedExceptionParam),
block.Graph.GetValueType(exception),
exception));
JumpToExceptionBranch(block, failureBranch, capturedException);
}
// Jump-thread this block again.
processedBlocks.Remove(block);
ThreadJumps(block, processedBlocks);
}
else
{
// If we can't replace the 'try' flow with something better, then
// the least we can do is try and thread the 'try' flow's branches.
block.Flow = new TryFlow(tryFlow.Instruction, successBranch, failureBranch);
}
}
else if (flow is SwitchFlow && IncludeSwitches)
{
// If we're allowed to, then we might be able to thread jumps in switch flow
// branches.
bool changed = false;
var switchFlow = (SwitchFlow)flow;
// Rewrite switch cases.
var cases = new List <SwitchCase>();
foreach (var switchCase in switchFlow.Cases)
{
var caseFlow = AsThreadableFlow(switchCase.Branch, block.Graph, processedBlocks);
if (caseFlow != null &&
switchCase.Branch.Target != block.Tag)
{
if (caseFlow is JumpFlow)
{
cases.Add(
new SwitchCase(
switchCase.Values,
((JumpFlow)caseFlow).Branch));
changed = true;
continue;
}
else if (caseFlow is SwitchFlow)
{
var threadedSwitch = (SwitchFlow)caseFlow;
if (threadedSwitch.SwitchValue == switchFlow.SwitchValue)
{
var valuesToBranches = threadedSwitch.ValueToBranchMap;
foreach (var value in switchCase.Values)
{
Branch branchForValue;
if (!valuesToBranches.TryGetValue(value, out branchForValue))
{
branchForValue = threadedSwitch.DefaultBranch;
}
cases.Add(
new SwitchCase(
ImmutableHashSet.Create(value),
branchForValue));
}
changed = true;
continue;
}
}
}
cases.Add(switchCase);
}
// Rewrite default branch if possible.
var defaultBranch = switchFlow.DefaultBranch;
if (defaultBranch.Target != block.Tag)
{
var defaultFlow = AsThreadableFlow(defaultBranch, block.Graph, processedBlocks);
if (defaultFlow is JumpFlow)
{
defaultBranch = ((JumpFlow)defaultFlow).Branch;
changed = true;
}
else if (defaultFlow is SwitchFlow)
{
var threadedSwitch = (SwitchFlow)defaultFlow;
if (threadedSwitch.SwitchValue == switchFlow.SwitchValue)
{
var valueSet = ImmutableHashSet.CreateRange(switchFlow.ValueToBranchMap.Keys);
foreach (var switchCase in threadedSwitch.Cases)
{
cases.Add(
new SwitchCase(
switchCase.Values.Except(valueSet),
switchCase.Branch));
}
defaultBranch = threadedSwitch.DefaultBranch;
changed = true;
}
}
}
if (changed)
{
block.Flow = new SwitchFlow(
switchFlow.SwitchValue,
cases,
defaultBranch);
}
// Also simplify the block's switch flow. If the switch
// flow decays to jump flow, then we want to try threading
// this block again.
if (SwitchSimplification.TrySimplifySwitchFlow(block) &&
block.Flow is JumpFlow)
{
processedBlocks.Remove(block);
ThreadJumps(block, processedBlocks);
}
}
}
