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//===-- WebAssemblyFrameLowering.cpp - WebAssembly Frame Lowering ----------==//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
///
/// \file
/// This file contains the WebAssembly implementation of
/// TargetFrameLowering class.
///
/// On WebAssembly, there aren't a lot of things to do here. There are no
/// callee-saved registers to save, and no spill slots.
///
/// The stack grows downward.
///
//===----------------------------------------------------------------------===//
#include "WebAssemblyFrameLowering.h"
#include "MCTargetDesc/WebAssemblyMCTargetDesc.h"
#include "WebAssemblyInstrInfo.h"
#include "WebAssemblyMachineFunctionInfo.h"
#include "WebAssemblySubtarget.h"
#include "WebAssemblyTargetMachine.h"
#include "WebAssemblyUtilities.h"
#include "llvm/CodeGen/MachineFrameInfo.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/MachineModuleInfoImpls.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/Support/Debug.h"
using namespace llvm;
#define DEBUG_TYPE "wasm-frame-info"
// TODO: wasm64
// TODO: Emit TargetOpcode::CFI_INSTRUCTION instructions
/// We need a base pointer in the case of having items on the stack that
/// require stricter alignment than the stack pointer itself. Because we need
/// to shift the stack pointer by some unknown amount to force the alignment,
/// we need to record the value of the stack pointer on entry to the function.
bool WebAssemblyFrameLowering::hasBP(
const MachineFunction &MF) const {
const auto *RegInfo =
MF.getSubtarget<WebAssemblySubtarget>().getRegisterInfo();
return RegInfo->needsStackRealignment(MF);
}
/// Return true if the specified function should have a dedicated frame pointer
/// register.
bool WebAssemblyFrameLowering::hasFP(const MachineFunction &MF) const {
const MachineFrameInfo &MFI = MF.getFrameInfo();
// When we have var-sized objects, we move the stack pointer by an unknown
// amount, and need to emit a frame pointer to restore the stack to where we
// were on function entry.
// If we already need a base pointer, we use that to fix up the stack pointer.
// If there are no fixed-size objects, we would have no use of a frame
// pointer, and thus should not emit one.
bool HasFixedSizedObjects = MFI.getStackSize() > 0;
bool NeedsFixedReference = !hasBP(MF) || HasFixedSizedObjects;
return MFI.isFrameAddressTaken() ||
(MFI.hasVarSizedObjects() && NeedsFixedReference) ||
MFI.hasStackMap() || MFI.hasPatchPoint();
}
/// Under normal circumstances, when a frame pointer is not required, we reserve
/// argument space for call sites in the function immediately on entry to the
/// current function. This eliminates the need for add/sub sp brackets around
/// call sites. Returns true if the call frame is included as part of the stack
/// frame.
bool WebAssemblyFrameLowering::hasReservedCallFrame(
const MachineFunction &MF) const {
return !MF.getFrameInfo().hasVarSizedObjects();
}
/// Returns true if this function needs a local user-space stack pointer.
/// Unlike a machine stack pointer, the wasm user stack pointer is a global
/// variable, so it is loaded into a register in the prolog.
bool WebAssemblyFrameLowering::needsSP(const MachineFunction &MF,
const MachineFrameInfo &MFI) const {
return MFI.getStackSize() || MFI.adjustsStack() || hasFP(MF);
}
/// Returns true if the local user-space stack pointer needs to be written back
/// to memory by this function (this is not meaningful if needsSP is false). If
/// false, the stack red zone can be used and only a local SP is needed.
bool WebAssemblyFrameLowering::needsSPWriteback(
const MachineFunction &MF, const MachineFrameInfo &MFI) const {
assert(needsSP(MF, MFI));
return MFI.getStackSize() > RedZoneSize || MFI.hasCalls() ||
MF.getFunction().hasFnAttribute(Attribute::NoRedZone);
}
static void writeSPToMemory(unsigned SrcReg, MachineFunction &MF,
MachineBasicBlock &MBB,
MachineBasicBlock::iterator &InsertAddr,
MachineBasicBlock::iterator &InsertStore,
const DebugLoc &DL) {
const auto *TII = MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
const char *ES = "__stack_pointer";
auto *SPSymbol = MF.createExternalSymbolName(ES);
BuildMI(MBB, InsertStore, DL, TII->get(WebAssembly::SET_GLOBAL_I32))
.addExternalSymbol(SPSymbol)
.addReg(SrcReg);
}
MachineBasicBlock::iterator
WebAssemblyFrameLowering::eliminateCallFramePseudoInstr(
MachineFunction &MF, MachineBasicBlock &MBB,
MachineBasicBlock::iterator I) const {
assert(!I->getOperand(0).getImm() && (hasFP(MF) || hasBP(MF)) &&
"Call frame pseudos should only be used for dynamic stack adjustment");
const auto *TII = MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
if (I->getOpcode() == TII->getCallFrameDestroyOpcode() &&
needsSPWriteback(MF, MF.getFrameInfo())) {
DebugLoc DL = I->getDebugLoc();
writeSPToMemory(WebAssembly::SP32, MF, MBB, I, I, DL);
}
return MBB.erase(I);
}
void WebAssemblyFrameLowering::emitPrologue(MachineFunction &MF,
MachineBasicBlock &MBB) const {
// TODO: Do ".setMIFlag(MachineInstr::FrameSetup)" on emitted instructions
auto &MFI = MF.getFrameInfo();
assert(MFI.getCalleeSavedInfo().empty() &&
"WebAssembly should not have callee-saved registers");
if (!needsSP(MF, MFI)) return;
uint64_t StackSize = MFI.getStackSize();
const auto *TII = MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
auto &MRI = MF.getRegInfo();
auto InsertPt = MBB.begin();
while (InsertPt != MBB.end() && WebAssembly::isArgument(*InsertPt))
++InsertPt;
DebugLoc DL;
const TargetRegisterClass *PtrRC =
MRI.getTargetRegisterInfo()->getPointerRegClass(MF);
unsigned SPReg = WebAssembly::SP32;
if (StackSize)
SPReg = MRI.createVirtualRegister(PtrRC);
const char *ES = "__stack_pointer";
auto *SPSymbol = MF.createExternalSymbolName(ES);
BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::GET_GLOBAL_I32), SPReg)
.addExternalSymbol(SPSymbol);
bool HasBP = hasBP(MF);
if (HasBP) {
auto FI = MF.getInfo<WebAssemblyFunctionInfo>();
unsigned BasePtr = MRI.createVirtualRegister(PtrRC);
FI->setBasePointerVreg(BasePtr);
BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::COPY), BasePtr)
.addReg(SPReg);
}
if (StackSize) {
// Subtract the frame size
unsigned OffsetReg = MRI.createVirtualRegister(PtrRC);
BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::CONST_I32), OffsetReg)
.addImm(StackSize);
BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::SUB_I32),
WebAssembly::SP32)
.addReg(SPReg)
.addReg(OffsetReg);
}
if (HasBP) {
unsigned BitmaskReg = MRI.createVirtualRegister(PtrRC);
unsigned Alignment = MFI.getMaxAlignment();
assert((1u << countTrailingZeros(Alignment)) == Alignment &&
"Alignment must be a power of 2");
BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::CONST_I32), BitmaskReg)
.addImm((int)~(Alignment - 1));
BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::AND_I32),
WebAssembly::SP32)
.addReg(WebAssembly::SP32)
.addReg(BitmaskReg);
}
if (hasFP(MF)) {
// Unlike most conventional targets (where FP points to the saved FP),
// FP points to the bottom of the fixed-size locals, so we can use positive
// offsets in load/store instructions.
BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::COPY),
WebAssembly::FP32)
.addReg(WebAssembly::SP32);
}
if (StackSize && needsSPWriteback(MF, MFI)) {
writeSPToMemory(WebAssembly::SP32, MF, MBB, InsertPt, InsertPt, DL);
}
}
void WebAssemblyFrameLowering::emitEpilogue(MachineFunction &MF,
MachineBasicBlock &MBB) const {
auto &MFI = MF.getFrameInfo();
uint64_t StackSize = MFI.getStackSize();
if (!needsSP(MF, MFI) || !needsSPWriteback(MF, MFI)) return;
const auto *TII = MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
auto &MRI = MF.getRegInfo();
auto InsertPt = MBB.getFirstTerminator();
DebugLoc DL;
if (InsertPt != MBB.end())
DL = InsertPt->getDebugLoc();
// Restore the stack pointer. If we had fixed-size locals, add the offset
// subtracted in the prolog.
unsigned SPReg = 0;
MachineBasicBlock::iterator InsertAddr = InsertPt;
if (hasBP(MF)) {
auto FI = MF.getInfo<WebAssemblyFunctionInfo>();
SPReg = FI->getBasePointerVreg();
} else if (StackSize) {
const TargetRegisterClass *PtrRC =
MRI.getTargetRegisterInfo()->getPointerRegClass(MF);
unsigned OffsetReg = MRI.createVirtualRegister(PtrRC);
InsertAddr =
BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::CONST_I32), OffsetReg)
.addImm(StackSize);
// In the epilog we don't need to write the result back to the SP32 physreg
// because it won't be used again. We can use a stackified register instead.
SPReg = MRI.createVirtualRegister(PtrRC);
BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::ADD_I32), SPReg)
.addReg(hasFP(MF) ? WebAssembly::FP32 : WebAssembly::SP32)
.addReg(OffsetReg);
} else {
SPReg = hasFP(MF) ? WebAssembly::FP32 : WebAssembly::SP32;
}
writeSPToMemory(SPReg, MF, MBB, InsertAddr, InsertPt, DL);
}