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IRGen: Forward spare bits through aggregates.

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When doing struct layout for fixed-layout structs or tuples, combine the spare bit masks of their elements to form the spare bit mask of the aggregate, treating padding between elements as spare bits as well.

For now, disable using these spare bits to form extra inhabitants for structs and tuples; we would need additional runtime work to expose these extra inhabitants for correct generic runtime behavior. This puts us in a weird situation where 'enum { case A(Struct), B, C }' spills a bit but 'enum { case A(Struct), B(Struct), C }' doesn't, but the work to make the former happen isn't immediately critical for String optimization.

Swift SVN r12165
This commit is contained in:
Joe Groff
2014-01-10 23:15:34 +00:00
parent ee85fb341a
commit 9b38c4b5db
7 changed files with 198 additions and 18 deletions
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27
lib/IRGen/StructLayout.cpp
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@@ -80,12 +80,14 @@ StructLayout::StructLayout(IRGenModule &IGM, LayoutKind layoutKind,
assert(!builder.empty() == requiresHeapHeader(layoutKind));
MinimumAlign = Alignment(1);
MinimumSize = Size(0);
SpareBits.clear();
IsFixedLayout = true;
IsKnownPOD = IsPOD;
Ty = (typeToFill ? typeToFill : IGM.OpaquePtrTy->getElementType());
} else {
MinimumAlign = builder.getAlignment();
MinimumSize = builder.getSize();
SpareBits = getSpareBitsFromBuilder(builder);
IsFixedLayout = builder.isFixedLayout();
IsKnownPOD = builder.isKnownPOD();
if (typeToFill) {
@@ -211,20 +213,23 @@ void StructLayoutBuilder::addFixedSizeElement(ElementLayout &elt) {
= eltAlignment.getValue() - offsetFromAlignment.getValue();
assert(paddingRequired != 0);
// Regardless, the storage size goes up.
CurSize += Size(paddingRequired);
// Add the padding to the fixed layout.
if (isFixedLayout()) {
auto paddingTy = llvm::ArrayType::get(IGM.Int8Ty, paddingRequired);
StructFields.push_back(paddingTy);
// The padding can be used as spare bits by enum layout.
CurSpareBits.resize(CurSize.getValueInBits(), true);
}
// Regardless, the storage size goes up.
CurSize += Size(paddingRequired);
}
// If the overall structure so far has a fixed layout, then add
// this as a field to the layout.
if (isFixedLayout()) {
addElementAtFixedOffset(elt);
// Otherwise, just remember the next non-fixed offset index.
} else {
addElementAtNonFixedOffset(elt);
@@ -261,11 +266,21 @@ void StructLayoutBuilder::addEmptyElement(ElementLayout &elt) {
/// aggregate.
void StructLayoutBuilder::addElementAtFixedOffset(ElementLayout &elt) {
assert(isFixedLayout());
assert(isa<FixedTypeInfo>(elt.getType()));
auto &eltTI = cast<FixedTypeInfo>(elt.getType());
elt.completeFixed(elt.getType().isPOD(ResilienceScope::Local),
CurSize, StructFields.size());
StructFields.push_back(elt.getType().getStorageType());
// Carry over the spare bits from the element.
unsigned startBit = CurSize.getValueInBits();
unsigned eltBits = eltTI.getFixedSize().getValueInBits();
CurSpareBits.resize(startBit + eltBits);
if (!eltTI.getSpareBits().empty()) {
for (unsigned i = 0; i < eltBits; ++i) {
CurSpareBits[startBit + i] = eltTI.getSpareBits()[i];
}
}
}
/// Add an element at a non-fixed offset to the aggregate.
@@ -273,6 +288,7 @@ void StructLayoutBuilder::addElementAtNonFixedOffset(ElementLayout &elt) {
assert(!isFixedLayout());
elt.completeNonFixed(elt.getType().isPOD(ResilienceScope::Local),
NextNonFixedOffsetIndex);
CurSpareBits.clear();
}
/// Add a non-fixed-size element to the aggregate at offset zero.
@@ -281,6 +297,7 @@ void StructLayoutBuilder::addNonFixedSizeElementAtOffsetZero(ElementLayout &elt)
assert(!isa<FixedTypeInfo>(elt.getType()));
assert(CurSize.isZero());
elt.completeInitialNonFixedSize(elt.getType().isPOD(ResilienceScope::Local));
CurSpareBits.clear();
}
/// Produce the current fields as an anonymous structure.