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JIT: elide redundant wasm Virtual IP updates #131688

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JIT: elide redundant wasm Virtual IP updates #131688

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132 changes: 120 additions & 12 deletions src/coreclr/jit/fgwasm.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -3413,9 +3413,10 @@ PhaseStatus Compiler::fgWasmVirtualIP()
// In the main method we update the VirtualIP local
// (which will end up at $fp[4]).
//
auto updateVirtualIPOnFrame = [&](FuncInfoDsc* func, BasicBlock* block, GenTree* beforeNode = nullptr) {
auto updateVirtualIPOnFrame = [&](FuncInfoDsc* func, BasicBlock* block, unsigned vipValue,
GenTree* beforeNode = nullptr) {
const unsigned virtualIPlclNum = getVirtualIPLclNum();
GenTree* const virtualIPValue = gtNewIconNode(virtualIP);
GenTree* const virtualIPValue = gtNewIconNode(vipValue);
GenTree* setVirtualIP = nullptr;

if (func->IsFunclet())
Expand Down Expand Up @@ -3452,9 +3453,33 @@ PhaseStatus Compiler::fgWasmVirtualIP()
updatesAdded++;
};

// Elide redundant Virtual IP stores: a store is redundant when the required value is
// already on the frame on every path into the block.
//
struct PerBlockData
{
enum : unsigned
{
VIP_UNSET = UINT_MAX, // no value yet (meet identity)
VIP_MANY = UINT_MAX - 1, // conflicting or unknown value
};

unsigned requiredVip = 0; // virtual IP this block requires on the frame
unsigned funcIndex = 0; // owning func index
unsigned availableIn = VIP_UNSET; // virtual IP available on entry
unsigned availableOut = VIP_UNSET; // virtual IP available on exit
bool isRequired = false; // block needs a correct value on the frame
bool isFuncEntry = false; // first block of its func
};

const unsigned numBlocks = fgBBNumMax + 1;
PerBlockData* const blockData = new (this, CMK_WasmEH) PerBlockData[numBlocks];
unsigned vipFuncIdx = 0;

for (FuncInfoDsc* const func : Funcs())
{
func->startVirtualIP = virtualIP;
bool vipFirstInFunc = true;

if (func->IsMethod())
{
Expand Down Expand Up @@ -3514,18 +3539,16 @@ PhaseStatus Compiler::fgWasmVirtualIP()
clauses[clauseIndex].clause.ClassToken = virtualIP;
}

// For now, just refresh the stack Virtual IP at the start of each non-empty
// block (later we can refine this to something like: blocks that have calls
// or will inspire calls during codegen).
//
// Also refresh BBJ_CALLFINALLY blocks: the implicit call_indirect to the
// finally funclet is emitted at codegen time and the runtime EH walker
// would otherwise see the stale try-region virtualIP and re-dispatch the
// same finally during unwind.
// Record the required Virtual IP and store-site/entry flags for each block.
//
if (!block->isEmpty() || block->KindIs(BBJ_CALLFINALLY))
PerBlockData& data = blockData[block->bbNum];
data.requiredVip = virtualIP;
data.funcIndex = vipFuncIdx;
data.isRequired = !block->isEmpty() || block->KindIs(BBJ_CALLFINALLY);
if (vipFirstInFunc)
{
updateVirtualIPOnFrame(func, block);
data.isFuncEntry = true;
vipFirstInFunc = false;
}

// If this is the end of the region, update its extent.
Expand Down Expand Up @@ -3578,6 +3601,91 @@ PhaseStatus Compiler::fgWasmVirtualIP()
}

func->endVirtualIP = virtualIP;
vipFuncIdx++;
}

// Solve the available-Virtual-IP dataflow: a block's incoming value is the meet of its
// in-func predecessors' outgoing values (cross-func edges and function entries are unknown).
//
class VirtualIPMerge
{
PerBlockData* const m_data;
unsigned m_merge;

void MeetInto(unsigned value)
{
if (value == PerBlockData::VIP_UNSET)
{
return;
}
if (m_merge == PerBlockData::VIP_UNSET)
{
m_merge = value;
}
else if ((m_merge == PerBlockData::VIP_MANY) || (value == PerBlockData::VIP_MANY) || (m_merge != value))
{
m_merge = PerBlockData::VIP_MANY;
}
}

public:
VirtualIPMerge(PerBlockData* data)
: m_data(data)
, m_merge(PerBlockData::VIP_UNSET)
{
}

void StartMerge(BasicBlock* block)
{
m_merge = m_data[block->bbNum].isFuncEntry ? PerBlockData::VIP_MANY : PerBlockData::VIP_UNSET;
}

void Merge(BasicBlock* block, BasicBlock* pred, unsigned dupCount)
{
const bool crossFunc = m_data[pred->bbNum].funcIndex != m_data[block->bbNum].funcIndex;
MeetInto(crossFunc ? PerBlockData::VIP_MANY : m_data[pred->bbNum].availableOut);
}

void MergeHandler(BasicBlock* block, BasicBlock* tryBeg, BasicBlock* tryLast)
{
// Handler funclets have a fresh frame; nothing is available on entry.
//
MeetInto(PerBlockData::VIP_MANY);
}

bool EndMerge(BasicBlock* block)
{
PerBlockData& data = m_data[block->bbNum];
data.availableIn = m_merge;
const unsigned out = data.isRequired ? data.requiredVip : m_merge;
if (out != data.availableOut)
{
data.availableOut = out;
return true;
}
return false;
}
};

VirtualIPMerge vipMerge(blockData);
DataFlow vipFlow(this);
vipFlow.ForwardAnalysis(vipMerge);

// ForwardAnalysis builds the generic DFS tree; drop it so later wasm phases recompute theirs.
fgInvalidateDfsTree();

// Emit a store only where a required block's value is not already available on every path in.
//
for (FuncInfoDsc* const func : Funcs())
{
for (BasicBlock* const block : func->Blocks(this))
{
const PerBlockData& data = blockData[block->bbNum];
if (data.isRequired && (data.availableIn != data.requiredVip))
{
updateVirtualIPOnFrame(func, block, data.requiredVip);
}
}
}

#ifdef DEBUG
Expand Down
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