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[Statepoint] Optimize Location structure size #78600

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Jan 19, 2024
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[Statepoint] Optimize Location structure size #78600

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13 changes: 7 additions & 6 deletions llvm/include/llvm/CodeGen/StackMaps.h
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Does it makes sense to use uint16_t instead of unsigned short?

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Just followed the types used in LiveOutReg struct below for consistency. Can change both to uint16_t.

Original file line number Diff line number Diff line change
Expand Up @@ -259,7 +259,7 @@ class StatepointOpers {
class StackMaps {
public:
struct Location {
enum LocationType {
enum LocationType : unsigned short {
Unprocessed,
Register,
Direct,
Expand All @@ -268,12 +268,13 @@ class StackMaps {
ConstantIndex
};
LocationType Type = Unprocessed;
unsigned Size = 0;
unsigned Reg = 0;
int64_t Offset = 0;
unsigned short Size = 0;
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This struct can be made even smaller (8 bytes) if we borrow some bits from size (or Reg), but that'd probably require changing StackMap format, since it allows for full 16 bits for size, even though it's not really useful in practice for most cases.

unsigned short Reg = 0;
int32_t Offset = 0;

Location() = default;
Location(LocationType Type, unsigned Size, unsigned Reg, int64_t Offset)
Location(LocationType Type, unsigned short Size, unsigned short Reg,
int32_t Offset)
: Type(Type), Size(Size), Reg(Reg), Offset(Offset) {}
};

Expand Down Expand Up @@ -369,7 +370,7 @@ class StackMaps {
MachineInstr::const_mop_iterator
parseOperand(MachineInstr::const_mop_iterator MOI,
MachineInstr::const_mop_iterator MOE, LocationVec &Locs,
LiveOutVec &LiveOuts) const;
LiveOutVec &LiveOuts);

/// Specialized parser of statepoint operands.
/// They do not directly correspond to StackMap record entries.
Expand Down
40 changes: 17 additions & 23 deletions llvm/lib/CodeGen/StackMaps.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -207,7 +207,7 @@ static unsigned getDwarfRegNum(unsigned Reg, const TargetRegisterInfo *TRI) {
MachineInstr::const_mop_iterator
StackMaps::parseOperand(MachineInstr::const_mop_iterator MOI,
MachineInstr::const_mop_iterator MOE, LocationVec &Locs,
LiveOutVec &LiveOuts) const {
LiveOutVec &LiveOuts) {
const TargetRegisterInfo *TRI = AP.MF->getSubtarget().getRegisterInfo();
if (MOI->isImm()) {
switch (MOI->getImm()) {
Expand Down Expand Up @@ -238,7 +238,22 @@ StackMaps::parseOperand(MachineInstr::const_mop_iterator MOI,
++MOI;
assert(MOI->isImm() && "Expected constant operand.");
int64_t Imm = MOI->getImm();
Locs.emplace_back(Location::Constant, sizeof(int64_t), 0, Imm);
if (isInt<32>(Imm)) {
Locs.emplace_back(Location::Constant, sizeof(int64_t), 0, Imm);
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Not sure about sizeof(int64_t). I think the size field is not really used for constants, since they are always 4 bytes, so I just kept it as 8 to reduce tests diff.

} else {
// ConstPool is intentionally a MapVector of 'uint64_t's (as
// opposed to 'int64_t's). We should never be in a situation
// where we have to insert either the tombstone or the empty
// keys into a map, and for a DenseMap<uint64_t, T> these are
// (uint64_t)0 and (uint64_t)-1. They can be and are
// represented using 32 bit integers.
assert((uint64_t)Imm != DenseMapInfo<uint64_t>::getEmptyKey() &&
(uint64_t)Imm != DenseMapInfo<uint64_t>::getTombstoneKey() &&
"empty and tombstone keys should fit in 32 bits!");
auto Result = ConstPool.insert(std::make_pair(Imm, Imm));
Locs.emplace_back(Location::ConstantIndex, sizeof(int64_t), 0,
Result.first - ConstPool.begin());
}
break;
}
}
Expand Down Expand Up @@ -497,27 +512,6 @@ void StackMaps::recordStackMapOpers(const MCSymbol &MILabel,
while (MOI != MOE)
MOI = parseOperand(MOI, MOE, Locations, LiveOuts);

// Move large constants into the constant pool.
for (auto &Loc : Locations) {
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Do this eagerly, that way we can reduce Offset size from 8 down to 4 bytes.

// Constants are encoded as sign-extended integers.
// -1 is directly encoded as .long 0xFFFFFFFF with no constant pool.
if (Loc.Type == Location::Constant && !isInt<32>(Loc.Offset)) {
Loc.Type = Location::ConstantIndex;
// ConstPool is intentionally a MapVector of 'uint64_t's (as
// opposed to 'int64_t's). We should never be in a situation
// where we have to insert either the tombstone or the empty
// keys into a map, and for a DenseMap<uint64_t, T> these are
// (uint64_t)0 and (uint64_t)-1. They can be and are
// represented using 32 bit integers.
assert((uint64_t)Loc.Offset != DenseMapInfo<uint64_t>::getEmptyKey() &&
(uint64_t)Loc.Offset !=
DenseMapInfo<uint64_t>::getTombstoneKey() &&
"empty and tombstone keys should fit in 32 bits!");
auto Result = ConstPool.insert(std::make_pair(Loc.Offset, Loc.Offset));
Loc.Offset = Result.first - ConstPool.begin();
}
}

// Create an expression to calculate the offset of the callsite from function
// entry.
const MCExpr *CSOffsetExpr = MCBinaryExpr::createSub(
Expand Down
9 changes: 4 additions & 5 deletions llvm/test/CodeGen/X86/statepoint-fixup-undef.mir
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Original file line number Diff line number Diff line change
Expand Up @@ -124,12 +124,11 @@ body: |
; STACKMAP-NEXT: .byte 0
; STACKMAP-NEXT: .short 0
; STACKMAP-NEXT: .long 1
; STACKMAP-NEXT: .long 1
; STACKMAP-NEXT: .long 0
; STACKMAP-NEXT: .long 1
; STACKMAP-NEXT: .quad test_undef
; STACKMAP-NEXT: .quad 88
; STACKMAP-NEXT: .quad 1
; STACKMAP-NEXT: .quad 4278124286
; STACKMAP-NEXT: .quad 2
; STACKMAP-NEXT: .long .Ltmp0-test_undef
; STACKMAP-NEXT: .short 0
Expand Down Expand Up @@ -182,13 +181,13 @@ body: |
; STACKMAP-NEXT: .short 3
; STACKMAP-NEXT: .short 0
; STACKMAP-NEXT: .long 0
; This is entry we're looking for, reference to constant pool entry 0xFEFEFEFE
; STACKMAP-NEXT: .byte 5
; This is a constant 0xFEFEFEFE we are looking for
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0xFEFEFEFE is no longer sign-extended to int64_t and as such doesn't require a constant pool entry. Since, as I understand, this was added mostly to not crash and it doesn't havy specific meaning, I think we are fine with this change.

@arsenm I saw that you had a stack that removed this special handling completely, although it didn't go anywhere, even though some patches were approved https://reviews.llvm.org/D122605

; STACKMAP-NEXT: .byte 4
; STACKMAP-NEXT: .byte 0
; STACKMAP-NEXT: .short 8
; STACKMAP-NEXT: .short 0
; STACKMAP-NEXT: .short 0
; STACKMAP-NEXT: .long 0
; STACKMAP-NEXT: .long -16843010
; STACKMAP-NEXT: .byte 4
; STACKMAP-NEXT: .byte 0
; STACKMAP-NEXT: .short 8
Expand Down