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+//===- AArch64SRLTDefineSuperRegs.cpp
-------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM
Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// When SubRegister Liveness Tracking (SRLT) is enabled, this pass adds
+// extra implicit-def's to instructions that define the low N bits of
+// a GPR/FPR register to also define the top bits, because all AArch64
+// instructions that write the low bits of a GPR/FPR also implicitly zero
+// the top bits. For example, 'mov w0, w1' writes zeroes to the top 32-bits of
+// x0, so this pass adds a `implicit-def $x0` after register allocation.
+//
+// These semantics are originally represented in the MIR using `SUBREG_TO_REG`
+// which expresses that the top bits have been defined by the preceding
+// instructions, but during register coalescing this information is lost and in
+// contrast to when SRTL is disabled, when rewriting virtual -> physical
+// registers the implicit-defs are not added to the instruction.
+//
+// There have been several attempts to fix this in the coalescer [1], but each
+// iteration has exposed new bugs and the patch had to be reverted.
+// Additionally, the concept of adding 'implicit-def' of a virtual register is
+// particularly fragile and many places don't expect it (for example in
+// `X86::commuteInstructionImpl` the code only looks at specific operands and
+// does not consider implicit-defs. Similar in `SplitEditor::addDeadDef` where
+// it traverses operand 'defs' rather than 'all_defs').
+//
+// We want a temporary solution that doesn't impact other targets and is
simpler
+// and less intrusive than the patch proposed for the register coalescer [1],
so
+// that we can enable SRLT for AArch64.
+//
+// The approach here is to just add the 'implicit-def' manually after rewriting
+// virtual regs -> phsyical regs. This still means that during the register
+// allocation process the dependences are not accurately represented in the MIR
+// and LiveIntervals, but there are several reasons why we believe this isn't a
+// problem in practice:
+// (A) The register allocator only spills entire virtual registers.
+// This is additionally guarded by code in
+// AArch64InstrInfo::storeRegToStackSlot/loadRegFromStackSlot
+// where it checks if a register matches the expected register class.
+// (B) Rematerialization only happens when the instruction writes the full
+// register.
+// (C) The high bits of the AArch64 register cannot be written independently.
+// (D) Instructions that write only part of a register always take that same
+// register as a tied input operand, to indicate it's a merging operation.
+//
+// (A) means that for two virtual registers of regclass GPR32 and GPR64, if the
+// GPR32 register is coalesced into the GPR64 vreg then the full GPR64 would
+// be spilled/filled even if only the low 32-bits would be required for the
+// given liverange. (B) means that the top bits of a GPR64 would never be
+// overwritten by rematerialising a GPR32 sub-register for a given liverange.
+// (C-D) means that we can assume that the MIR as input to the register
+// allocator correctly expresses the instruction behaviour and dependences
+// between values, so unless the register allocator would violate (A) or (B),
+// the MIR is otherwise sound.
+//
+// Alternative approaches have also been considered, such as:
+// (1) Changing the AArch64 instruction definitions to write all bits and
+// extract the low N bits for the result.
+// (2) Disabling coalescing of SUBREG_TO_REG and using regalloc hints to tell
+// the register allocator to favour the same register for the input/output.
+// (3) Adding a new coalescer guard node with a tied-operand constraint, such
+// that when the SUBREG_TO_REG is removed, something still represents that
+// the top bits are defined. The node would get removed before rewriting
+// virtregs.
+// (4) Using an explicit INSERT_SUBREG into a zero value and try to optimize
+// away the INSERT_SUBREG (this is a more explicit variant of (2) and (3))
+// (5) Adding a new MachineOperand flag that represents the top bits would be
+// defined, but are not read nor undef.
+//
+// (1) would be the best approach but would be a significant effort as it
+// requires rewriting most/all instruction definitions and fixing MIR passes
+// that rely on the current definitions, whereas (2-4) result in sub-optimal
+// code that can't really be avoided because the explicit nodes would stop
+// rematerialization. (5) might be a way to mitigate the
+// fragility of implicit-def's of virtual registers if we want to pursue
+// landing [1], but then we'd rather choose approach (1) to avoid using
+// SUBREG_TO_REG entirely.
+//
+// [1] https://github.com/llvm/llvm-project/pull/168353
+//===----------------------------------------------------------------------===//
+
+#include "AArch64InstrInfo.h"
+#include "AArch64MachineFunctionInfo.h"
+#include "AArch64Subtarget.h"
+#include "MCTargetDesc/AArch64AddressingModes.h"
+#include "llvm/ADT/BitVector.h"
+#include "llvm/ADT/SmallSet.h"
+#include "llvm/CodeGen/MachineBasicBlock.h"
+#include "llvm/CodeGen/MachineFunctionPass.h"
+#include "llvm/CodeGen/MachineRegisterInfo.h"
+#include "llvm/CodeGen/TargetRegisterInfo.h"
+#include "llvm/Support/Debug.h"
+
+using namespace llvm;
+
+#define DEBUG_TYPE "aarch64-srlt-define-superregs"
+#define PASS_NAME "AArch64 SRLT Define Super-Regs Pass"
+
+namespace {
+
+struct AArch64SRLTDefineSuperRegs : public MachineFunctionPass {
+ inline static char ID = 0;
+
+ AArch64SRLTDefineSuperRegs() : MachineFunctionPass(ID) {}
+
+ bool runOnMachineFunction(MachineFunction &MF) override;
+
+ void collectWidestSuperReg(Register R, const BitVector &RequiredBaseRegUnits,
+ const BitVector &QHiRegUnits,
+ SmallSet<Register, 8> &SuperRegs);
+
+ StringRef getPassName() const override { return PASS_NAME; }
+
+ void getAnalysisUsage(AnalysisUsage &AU) const override {
+ AU.setPreservesCFG();
+ AU.addPreservedID(MachineLoopInfoID);
+ AU.addPreservedID(MachineDominatorsID);
+ MachineFunctionPass::getAnalysisUsage(AU);
+ }
+
+private:
+ MachineFunction *MF = nullptr;
+ const AArch64Subtarget *Subtarget = nullptr;
+ const AArch64RegisterInfo *TRI = nullptr;
+ const AArch64FunctionInfo *AFI = nullptr;
+ const TargetInstrInfo *TII = nullptr;
+ MachineRegisterInfo *MRI = nullptr;
+};
+
+} // end anonymous namespace
+
+INITIALIZE_PASS(AArch64SRLTDefineSuperRegs, DEBUG_TYPE, PASS_NAME, false,
+ false)
+
+SmallVector<MCRegister> SupportedAliasRegs = {
----------------
MacDue wrote:
This does not need to be a (mutable) vector:
```suggestion
constexpr MCRegister SupportedAliasRegs[] = {
```
https://github.com/llvm/llvm-project/pull/174188
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