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[CodeGen] Allow negative frame indicies in Register class. #164459

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Merged
merged 11 commits into from
Nov 5, 2025
+50 −9
Merged

[CodeGen] Allow negative frame indicies in Register class. #164459

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dc95302
Negative frame indicies as register.
Aug 23, 2025
7b2fcda
addressed review comments.
Oct 21, 2025
478c945
Renamed "FirstStackSlot" to "StackSlotZero".
Oct 22, 2025
695baef
addressed review comments.
Oct 22, 2025
29bf5cd
address review comments.
Oct 22, 2025
a871d1f
addressed review comments.
Oct 23, 2025
e6a016a
address review comments.
Oct 23, 2025
b30b8ea
Review feedback: more MaxFrameIndexBitwidth - 1
ppenzin Nov 4, 2025
00fa7fb
Review feedback - min and max stack values
ppenzin Nov 5, 2025
0d3cc18
Merge branch 'main' into neg_indices
ppenzin Nov 5, 2025
98162f1
Merge branch 'main' into neg_indices
ppenzin Nov 5, 2025
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12 changes: 8 additions & 4 deletions llvm/include/llvm/CodeGen/Register.h
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Original file line number Diff line number Diff line change
Expand Up @@ -10,6 +10,7 @@
#define LLVM_CODEGEN_REGISTER_H

#include "llvm/MC/MCRegister.h"
#include "llvm/Support/MathExtras.h"
#include <cassert>

namespace llvm {
Expand All @@ -35,7 +36,10 @@ class Register {
// DenseMapInfo<unsigned> uses -1u and -2u.
static_assert(std::numeric_limits<decltype(Reg)>::max() >= 0xFFFFFFFF,
"Reg isn't large enough to hold full range.");
static constexpr unsigned FirstStackSlot = 1u << 30;
static constexpr unsigned MaxFrameIndexBitwidth = 30;
static constexpr unsigned FirstStackSlot = 1u << MaxFrameIndexBitwidth;
static const unsigned StackSlotMask =
(unsigned)(-1) >> (CHAR_BIT * sizeof(unsigned) - MaxFrameIndexBitwidth);
static_assert(FirstStackSlot >= MCRegister::LastPhysicalReg);
static constexpr unsigned VirtualRegFlag = 1u << 31;

Expand All @@ -46,8 +50,8 @@ class Register {

/// Convert a non-negative frame index to a stack slot register value.
static Register index2StackSlot(int FI) {
assert(FI >= 0 && "Cannot hold a negative frame index.");
return Register(FI + Register::FirstStackSlot);
assert(isInt<30>(FI) && "Frame index must be at most 30 bit integer");
return Register((FI & Register::StackSlotMask) | Register::FirstStackSlot);
}

/// Return true if the specified register number is in
Expand Down Expand Up @@ -87,7 +91,7 @@ class Register {
/// Compute the frame index from a register value representing a stack slot.
int stackSlotIndex() const {
assert(isStack() && "Not a stack slot");
return static_cast<int>(Reg - Register::FirstStackSlot);
return static_cast<int>(SignExtend64(Reg & Register::StackSlotMask, 30));
}

constexpr operator unsigned() const { return Reg; }
Expand Down
3 changes: 0 additions & 3 deletions llvm/lib/CodeGen/ReachingDefAnalysis.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -193,7 +193,6 @@ void ReachingDefInfo::processDefs(MachineInstr *MI) {
for (auto &MO : MI->operands()) {
if (MO.isFI()) {
int FrameIndex = MO.getIndex();
assert(FrameIndex >= 0 && "Can't handle negative frame indicies yet!");
if (!isFIDef(*MI, FrameIndex, TII))
continue;
MBBFrameObjsReachingDefs[{MBBNumber, FrameIndex}].push_back(CurInstr);
Expand Down Expand Up @@ -302,8 +301,6 @@ void ReachingDefInfo::print(raw_ostream &OS) {
Register Reg;
if (MO.isFI()) {
int FrameIndex = MO.getIndex();
assert(FrameIndex >= 0 &&
"Can't handle negative frame indicies yet!");
Reg = Register::index2StackSlot(FrameIndex);
} else if (MO.isReg()) {
if (MO.isDef())
Expand Down