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JIT: enhance escape analysis to understand Span<T> capture #112543

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JIT: enhance escape analysis to understand Span<T> capture #112543

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2a5c220
JIT: enhance escape analysis to understand `Span<T>` capture
AndyAyersMS Feb 13, 2025
0b4a754
model calls to methods that can return span arg fields
AndyAyersMS Feb 14, 2025
cf45f6b
restrict to span captured arrays
AndyAyersMS Feb 14, 2025
7f33222
remove printf
AndyAyersMS Feb 15, 2025
2e4e81d
align up array layouts; avoid tail calls
AndyAyersMS Feb 17, 2025
6f81a23
JIT: array allocation fixes
AndyAyersMS Feb 19, 2025
67d0ae5
Merge branch 'main' into SpanCapture
AndyAyersMS Feb 19, 2025
079e48b
update test case
AndyAyersMS Feb 19, 2025
62cb6fa
more tests; handle out/ref arg cases
AndyAyersMS Feb 20, 2025
de9657b
Merge remote-tracking branch 'origin/SpanCapture' into SpanCapture
AndyAyersMS Feb 20, 2025
7f6f055
Merge branch 'main' into SpanCapture
AndyAyersMS Feb 20, 2025
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7 changes: 7 additions & 0 deletions src/coreclr/jit/compiler.h
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Original file line number Diff line number Diff line change
Expand Up @@ -1208,6 +1208,13 @@ class LclVarDsc
m_layout = layout;
}

// Change the layout to one that may not be compatible.
void ChangeLayout(ClassLayout* layout)
{
assert(varTypeIsStruct(lvType));
m_layout = layout;
}

// Grow the size of a block layout local.
void GrowBlockLayout(ClassLayout* layout)
{
Expand Down
238 changes: 211 additions & 27 deletions src/coreclr/jit/objectalloc.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -233,7 +233,7 @@ void ObjectAllocator::MarkEscapingVarsAndBuildConnGraph()
lclEscapes = false;
m_allocator->CheckForGuardedAllocationOrCopy(m_block, m_stmt, use, lclNum);
}
else if (tree->OperIs(GT_LCL_VAR) && tree->TypeIs(TYP_REF, TYP_BYREF, TYP_I_IMPL))
else if (tree->OperIs(GT_LCL_VAR, GT_LCL_ADDR) && tree->TypeIs(TYP_REF, TYP_BYREF, TYP_I_IMPL, TYP_STRUCT))
{
assert(tree == m_ancestors.Top());
if (!m_allocator->CanLclVarEscapeViaParentStack(&m_ancestors, lclNum, m_block))
Expand Down Expand Up @@ -265,7 +265,7 @@ void ObjectAllocator::MarkEscapingVarsAndBuildConnGraph()
{
var_types type = comp->lvaTable[lclNum].TypeGet();

if (type == TYP_REF || genActualType(type) == TYP_I_IMPL || type == TYP_BYREF)
if (type == TYP_REF || genActualType(type) == TYP_I_IMPL || type == TYP_BYREF || type == TYP_STRUCT)
{
m_ConnGraphAdjacencyMatrix[lclNum] = BitVecOps::MakeEmpty(&m_bitVecTraits);

Expand Down Expand Up @@ -1023,10 +1023,13 @@ bool ObjectAllocator::CanLclVarEscapeViaParentStack(ArrayStack<GenTree*>* parent
assert(parentStack != nullptr);
int parentIndex = 1;

bool keepChecking = true;
bool canLclVarEscapeViaParentStack = true;
bool isCopy = true;
bool isEnumeratorLocal = comp->lvaGetDesc(lclNum)->lvIsEnumerator;
LclVarDsc* const lclDsc = comp->lvaGetDesc(lclNum);

bool keepChecking = true;
bool canLclVarEscapeViaParentStack = true;
bool isCopy = true;
bool const isEnumeratorLocal = lclDsc->lvIsEnumerator;
bool const isSpanLocal = lclDsc->IsSpan();

while (keepChecking)
{
Expand Down Expand Up @@ -1093,11 +1096,21 @@ bool ObjectAllocator::CanLclVarEscapeViaParentStack(ArrayStack<GenTree*>* parent
case GT_ADD:
case GT_SUB:
case GT_FIELD_ADDR:
// Check whether the local escapes via its grandparent.
// Check whether the local escapes higher up
++parentIndex;
keepChecking = true;
break;

case GT_LCL_ADDR:
if (isSpanLocal)
{
// Check whether the local escapes higher up
++parentIndex;
keepChecking = true;
JITDUMP("... i'm good thanks\n");
}
break;

case GT_BOX:
isCopy = wasCopy;
++parentIndex;
Expand All @@ -1119,11 +1132,43 @@ bool ObjectAllocator::CanLclVarEscapeViaParentStack(ArrayStack<GenTree*>* parent
case GT_STOREIND:
case GT_STORE_BLK:
case GT_BLK:
if (tree != parent->AsIndir()->Addr())
{
GenTree* const addr = parent->AsIndir()->Addr();
if (tree != addr)
{
// TODO-ObjectStackAllocation: track stores to fields.
JITDUMP("... tree != addr\n");

// Is this an array element address store to (the pointer) field of a span?
// (note we can't yet handle cases where a span captures an object)
//
if (parent->OperIs(GT_STOREIND) && addr->OperIs(GT_FIELD_ADDR) && tree->OperIs(GT_INDEX_ADDR))
{
// Todo: mark the span pointer field addr like we mark IsSpanLength?
// (for now we don't worry which field we store to)
//
GenTree* const base = addr->AsOp()->gtGetOp1();

if (base->OperIs(GT_LCL_ADDR))
{
unsigned const dstLclNum = base->AsLclVarCommon()->GetLclNum();
LclVarDsc* const dstLclDsc = comp->lvaGetDesc(dstLclNum);

if (dstLclDsc->IsSpan())
{
JITDUMP("... span ptr store\n");
// Add an edge to the connection graph.
AddConnGraphEdge(dstLclNum, lclNum);
canLclVarEscapeViaParentStack = false;
}
}
}
break;
}
else
{
JITDUMP("... tree == addr\n");
}
}
FALLTHROUGH;
case GT_IND:
// Address of the field/ind is not taken so the local doesn't escape.
Expand All @@ -1132,12 +1177,12 @@ bool ObjectAllocator::CanLclVarEscapeViaParentStack(ArrayStack<GenTree*>* parent

case GT_CALL:
{
GenTreeCall* const asCall = parent->AsCall();
GenTreeCall* const call = parent->AsCall();

if (asCall->IsHelperCall())
if (call->IsHelperCall())
{
canLclVarEscapeViaParentStack =
!Compiler::s_helperCallProperties.IsNoEscape(comp->eeGetHelperNum(asCall->gtCallMethHnd));
!Compiler::s_helperCallProperties.IsNoEscape(comp->eeGetHelperNum(call->gtCallMethHnd));
}

// Note there is nothing special here about the parent being a call. We could move all this processing
Expand All @@ -1156,6 +1201,125 @@ bool ObjectAllocator::CanLclVarEscapeViaParentStack(ArrayStack<GenTree*>* parent
JITDUMP("Enumerator V%02u passed to call...\n", lclNum);
canLclVarEscapeViaParentStack = !CheckForGuardedUse(block, parent, lclNum);
}
else if (isSpanLocal && !call->CanTailCall())
{
// A span argument can't escape, but the callee can pass its fields back via
// return value and the return value may escape. (Todo: model this more precisely
// -- see if we're assigning to a byref gc field)
//
JITDUMP("Span V%02u parent is call...\n", lclNum);

CallArg* const retBufArg = call->gtArgs.GetRetBufferArg();
if (retBufArg != nullptr)
{
GenTree* const retBuf = retBufArg->GetNode();

// Is it possible to assign directly to a struct's struct field,
// or is the retbuf always a standalone struct? Seems like it has
// to be the latter to give proper no-alias guarantees.
//
if (retBuf->OperIs(GT_LCL_ADDR))
{
GenTreeLclVarCommon* const dstLocal = retBuf->AsLclVarCommon();

if (dstLocal == tree)
{
JITDUMP(" ... span is local retbuf\n");
}
else
{
const unsigned dstLclNum = dstLocal->GetLclNum();
LclVarDsc* const dstLclDsc = comp->lvaGetDesc(dstLclNum);

if (dstLclDsc->HasGCPtr())
{
// Model as assignment of the argument to the return value local
//
JITDUMP(" ... (possible) byref return buffer. Modelling as V%02u <- V%02u\n",
dstLclNum, lclNum);
AddConnGraphEdge(dstLclNum, lclNum);
}
else
{
JITDUMP(" ... non-byref return buffer\n");
}
}
canLclVarEscapeViaParentStack = false;
}
else
{
JITDUMP(" ... retbuf is not a local\n");
}
}
else if (call->TypeGet() == TYP_STRUCT)
{
CORINFO_CLASS_HANDLE retClsHnd = call->gtRetClsHnd;
assert(retClsHnd != NO_CLASS_HANDLE);
DWORD attrs = comp->info.compCompHnd->getClassAttribs(retClsHnd);

if ((attrs & CORINFO_FLG_BYREF_LIKE) != 0)
{
JITDUMP(" ... byref return\n");
// defer to parent.
++parentIndex;
keepChecking = true;
}
else
{
JITDUMP(" ... non-byref return\n");
canLclVarEscapeViaParentStack = false;
}
}
else
{
JITDUMP(" ... non-struct return\n");
canLclVarEscapeViaParentStack = false;
}

// The callee may also assign to any ref or out params.
// Try and model this as an assignment, if possible.
//
for (CallArg& arg : call->gtArgs.Args())
{
if (!arg.IsUserArg())
{
continue;
}

GenTree* const argNode = arg.GetNode();

if ((argNode != tree) && (argNode->TypeIs(TYP_BYREF)))
{
// Try and resolve to a local and model as an assignment.
// (todo: could also be a field of a local)
//
if (argNode->OperIs(GT_LCL_ADDR))
{
GenTreeLclVarCommon* const argLocal = argNode->AsLclVarCommon();
const unsigned argLclNum = argLocal->GetLclNum();
LclVarDsc* const argLclDsc = comp->lvaGetDesc(argLclNum);

if (argLclDsc->HasGCPtr())
{
// Model as assignment of the argument to the byref local
//
JITDUMP(" ... (possible) byref out param. Modelling as V%02u <- V%02u\n", argLclNum,
lclNum);
AddConnGraphEdge(argLclNum, lclNum);
}
}
else
{
// Can't easily tell what this byref refers to, assume the worst.
// (points-to analysis anyone)?
// Would be nice to track byref referent types too.
//
JITDUMP(" ... byref arg [%06u]; assuming escape\n", comp->dspTreeID(argNode));
canLclVarEscapeViaParentStack = true;
}
}
}
}
break;
}

Expand Down Expand Up @@ -1227,6 +1391,7 @@ void ObjectAllocator::UpdateAncestorTypes(GenTree* tree, ArrayStack<GenTree*>* p
case GT_SUB:
case GT_FIELD_ADDR:
case GT_INDEX_ADDR:
case GT_LCL_ADDR:
if (parent->TypeGet() == TYP_REF)
{
parent->ChangeType(newType);
Expand Down Expand Up @@ -1264,17 +1429,18 @@ void ObjectAllocator::UpdateAncestorTypes(GenTree* tree, ArrayStack<GenTree*>* p
case GT_STOREIND:
case GT_STORE_BLK:
case GT_BLK:
assert(tree == parent->AsIndir()->Addr());

// The new target could be *not* on the heap.
parent->gtFlags &= ~GTF_IND_TGT_HEAP;

if (newType != TYP_BYREF)
if (tree == parent->AsIndir()->Addr())
{
// This indicates that a write barrier is not needed when writing
// to this field/indirection since the address is not pointing to the heap.
// It's either null or points to inside a stack-allocated object.
parent->gtFlags |= GTF_IND_TGT_NOT_HEAP;
// The new target could be *not* on the heap.
parent->gtFlags &= ~GTF_IND_TGT_HEAP;

if (newType != TYP_BYREF)
{
// This indicates that a write barrier is not needed when writing
// to this field/indirection since the address is not pointing to the heap.
// It's either null or points to inside a stack-allocated object.
parent->gtFlags |= GTF_IND_TGT_NOT_HEAP;
}
}
break;

Expand Down Expand Up @@ -1335,10 +1501,7 @@ void ObjectAllocator::RewriteUses()
if ((lclNum < BitVecTraits::GetSize(&m_allocator->m_bitVecTraits)) &&
m_allocator->MayLclVarPointToStack(lclNum))
{
// Analysis does not handle indirect access to pointer locals.
assert(tree->OperIsScalarLocal());

var_types newType;
var_types newType = TYP_UNDEF;
if (m_allocator->m_HeapLocalToStackLocalMap.TryGetValue(lclNum, &newLclNum))
{
assert(tree->OperIs(GT_LCL_VAR)); // Must be a use.
Expand All @@ -1355,12 +1518,33 @@ void ObjectAllocator::RewriteUses()
}
}

if (lclVarDsc->lvType != newType)
// For local structs, retype the GC fields.
//
if (lclVarDsc->lvType == TYP_STRUCT)
{
// We should only see spans here.
//
assert(lclVarDsc->IsSpan());
ClassLayout* const layout = lclVarDsc->GetLayout();
ClassLayout* newLayout = nullptr;

if (newType == TYP_I_IMPL)
{
// No GC refs remain, so now a block layout
newLayout = m_compiler->typGetBlkLayout(layout->GetSize());
JITDUMP("Changing layout of span V%02u to block\n", lclNum);
lclVarDsc->ChangeLayout(newLayout);
}
}
// For locals, retype the local
//
else if (lclVarDsc->lvType != newType)
{
JITDUMP("Changing the type of V%02u from %s to %s\n", lclNum, varTypeName(lclVarDsc->lvType),
varTypeName(newType));
lclVarDsc->lvType = newType;
}

m_allocator->UpdateAncestorTypes(tree, &m_ancestors, newType);

if (newLclNum != BAD_VAR_NUM)
Expand Down
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