332 lines
13 KiB
C++
332 lines
13 KiB
C++
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//===-- WebAssemblyFrameLowering.cpp - WebAssembly Frame Lowering ----------==//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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///
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/// \file
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/// This file contains the WebAssembly implementation of
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/// TargetFrameLowering class.
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///
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/// On WebAssembly, there aren't a lot of things to do here. There are no
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/// callee-saved registers to save, and no spill slots.
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///
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/// The stack grows downward.
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///
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//===----------------------------------------------------------------------===//
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#include "WebAssemblyFrameLowering.h"
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#include "MCTargetDesc/WebAssemblyMCTargetDesc.h"
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#include "WebAssembly.h"
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#include "WebAssemblyInstrInfo.h"
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#include "WebAssemblyMachineFunctionInfo.h"
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#include "WebAssemblySubtarget.h"
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#include "WebAssemblyTargetMachine.h"
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#include "WebAssemblyUtilities.h"
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#include "llvm/CodeGen/MachineFrameInfo.h"
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#include "llvm/CodeGen/MachineFunction.h"
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#include "llvm/CodeGen/MachineInstrBuilder.h"
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#include "llvm/CodeGen/MachineModuleInfoImpls.h"
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#include "llvm/CodeGen/MachineRegisterInfo.h"
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#include "llvm/MC/MCAsmInfo.h"
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#include "llvm/Support/Debug.h"
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using namespace llvm;
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#define DEBUG_TYPE "wasm-frame-info"
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// TODO: wasm64
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// TODO: Emit TargetOpcode::CFI_INSTRUCTION instructions
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/// We need a base pointer in the case of having items on the stack that
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/// require stricter alignment than the stack pointer itself. Because we need
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/// to shift the stack pointer by some unknown amount to force the alignment,
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/// we need to record the value of the stack pointer on entry to the function.
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bool WebAssemblyFrameLowering::hasBP(const MachineFunction &MF) const {
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const auto *RegInfo =
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MF.getSubtarget<WebAssemblySubtarget>().getRegisterInfo();
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return RegInfo->needsStackRealignment(MF);
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}
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/// Return true if the specified function should have a dedicated frame pointer
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/// register.
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bool WebAssemblyFrameLowering::hasFP(const MachineFunction &MF) const {
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const MachineFrameInfo &MFI = MF.getFrameInfo();
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// When we have var-sized objects, we move the stack pointer by an unknown
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// amount, and need to emit a frame pointer to restore the stack to where we
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// were on function entry.
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// If we already need a base pointer, we use that to fix up the stack pointer.
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// If there are no fixed-size objects, we would have no use of a frame
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// pointer, and thus should not emit one.
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bool HasFixedSizedObjects = MFI.getStackSize() > 0;
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bool NeedsFixedReference = !hasBP(MF) || HasFixedSizedObjects;
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return MFI.isFrameAddressTaken() ||
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(MFI.hasVarSizedObjects() && NeedsFixedReference) ||
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MFI.hasStackMap() || MFI.hasPatchPoint();
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}
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/// Under normal circumstances, when a frame pointer is not required, we reserve
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/// argument space for call sites in the function immediately on entry to the
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/// current function. This eliminates the need for add/sub sp brackets around
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/// call sites. Returns true if the call frame is included as part of the stack
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/// frame.
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bool WebAssemblyFrameLowering::hasReservedCallFrame(
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const MachineFunction &MF) const {
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return !MF.getFrameInfo().hasVarSizedObjects();
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}
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// Returns true if this function needs a local user-space stack pointer for its
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// local frame (not for exception handling).
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bool WebAssemblyFrameLowering::needsSPForLocalFrame(
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const MachineFunction &MF) const {
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auto &MFI = MF.getFrameInfo();
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return MFI.getStackSize() || MFI.adjustsStack() || hasFP(MF);
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}
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// In function with EH pads, we need to make a copy of the value of
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// __stack_pointer global in SP32/64 register, in order to use it when
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// restoring __stack_pointer after an exception is caught.
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bool WebAssemblyFrameLowering::needsPrologForEH(
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const MachineFunction &MF) const {
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auto EHType = MF.getTarget().getMCAsmInfo()->getExceptionHandlingType();
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return EHType == ExceptionHandling::Wasm &&
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MF.getFunction().hasPersonalityFn() && MF.getFrameInfo().hasCalls();
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}
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/// Returns true if this function needs a local user-space stack pointer.
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/// Unlike a machine stack pointer, the wasm user stack pointer is a global
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/// variable, so it is loaded into a register in the prolog.
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bool WebAssemblyFrameLowering::needsSP(const MachineFunction &MF) const {
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return needsSPForLocalFrame(MF) || needsPrologForEH(MF);
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}
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/// Returns true if the local user-space stack pointer needs to be written back
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/// to __stack_pointer global by this function (this is not meaningful if
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/// needsSP is false). If false, the stack red zone can be used and only a local
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/// SP is needed.
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bool WebAssemblyFrameLowering::needsSPWriteback(
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const MachineFunction &MF) const {
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auto &MFI = MF.getFrameInfo();
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assert(needsSP(MF));
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// When we don't need a local stack pointer for its local frame but only to
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// support EH, we don't need to write SP back in the epilog, because we don't
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// bump down the stack pointer in the prolog. We need to write SP back in the
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// epilog only if
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// 1. We need SP not only for EH support but also because we actually use
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// stack or we have a frame address taken.
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// 2. We cannot use the red zone.
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bool CanUseRedZone = MFI.getStackSize() <= RedZoneSize && !MFI.hasCalls() &&
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!MF.getFunction().hasFnAttribute(Attribute::NoRedZone);
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return needsSPForLocalFrame(MF) && !CanUseRedZone;
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}
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unsigned WebAssemblyFrameLowering::getSPReg(const MachineFunction &MF) {
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return MF.getSubtarget<WebAssemblySubtarget>().hasAddr64()
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? WebAssembly::SP64
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: WebAssembly::SP32;
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}
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unsigned WebAssemblyFrameLowering::getFPReg(const MachineFunction &MF) {
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return MF.getSubtarget<WebAssemblySubtarget>().hasAddr64()
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? WebAssembly::FP64
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: WebAssembly::FP32;
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}
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unsigned
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WebAssemblyFrameLowering::getOpcConst(const MachineFunction &MF) {
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return MF.getSubtarget<WebAssemblySubtarget>().hasAddr64()
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? WebAssembly::CONST_I64
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: WebAssembly::CONST_I32;
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}
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unsigned WebAssemblyFrameLowering::getOpcAdd(const MachineFunction &MF) {
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return MF.getSubtarget<WebAssemblySubtarget>().hasAddr64()
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? WebAssembly::ADD_I64
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: WebAssembly::ADD_I32;
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}
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unsigned WebAssemblyFrameLowering::getOpcSub(const MachineFunction &MF) {
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return MF.getSubtarget<WebAssemblySubtarget>().hasAddr64()
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? WebAssembly::SUB_I64
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: WebAssembly::SUB_I32;
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}
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unsigned WebAssemblyFrameLowering::getOpcAnd(const MachineFunction &MF) {
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return MF.getSubtarget<WebAssemblySubtarget>().hasAddr64()
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? WebAssembly::AND_I64
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: WebAssembly::AND_I32;
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}
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unsigned
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WebAssemblyFrameLowering::getOpcGlobGet(const MachineFunction &MF) {
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return MF.getSubtarget<WebAssemblySubtarget>().hasAddr64()
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? WebAssembly::GLOBAL_GET_I64
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: WebAssembly::GLOBAL_GET_I32;
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}
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unsigned
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WebAssemblyFrameLowering::getOpcGlobSet(const MachineFunction &MF) {
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return MF.getSubtarget<WebAssemblySubtarget>().hasAddr64()
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? WebAssembly::GLOBAL_SET_I64
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: WebAssembly::GLOBAL_SET_I32;
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}
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void WebAssemblyFrameLowering::writeSPToGlobal(
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unsigned SrcReg, MachineFunction &MF, MachineBasicBlock &MBB,
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MachineBasicBlock::iterator &InsertStore, const DebugLoc &DL) const {
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const auto *TII = MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
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const char *ES = "__stack_pointer";
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auto *SPSymbol = MF.createExternalSymbolName(ES);
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BuildMI(MBB, InsertStore, DL, TII->get(getOpcGlobSet(MF)))
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.addExternalSymbol(SPSymbol)
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.addReg(SrcReg);
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}
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MachineBasicBlock::iterator
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WebAssemblyFrameLowering::eliminateCallFramePseudoInstr(
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MachineFunction &MF, MachineBasicBlock &MBB,
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MachineBasicBlock::iterator I) const {
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assert(!I->getOperand(0).getImm() && (hasFP(MF) || hasBP(MF)) &&
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"Call frame pseudos should only be used for dynamic stack adjustment");
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auto &ST = MF.getSubtarget<WebAssemblySubtarget>();
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const auto *TII = ST.getInstrInfo();
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if (I->getOpcode() == TII->getCallFrameDestroyOpcode() &&
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needsSPWriteback(MF)) {
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DebugLoc DL = I->getDebugLoc();
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writeSPToGlobal(getSPReg(MF), MF, MBB, I, DL);
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}
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return MBB.erase(I);
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}
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void WebAssemblyFrameLowering::emitPrologue(MachineFunction &MF,
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MachineBasicBlock &MBB) const {
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// TODO: Do ".setMIFlag(MachineInstr::FrameSetup)" on emitted instructions
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auto &MFI = MF.getFrameInfo();
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assert(MFI.getCalleeSavedInfo().empty() &&
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"WebAssembly should not have callee-saved registers");
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if (!needsSP(MF))
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return;
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uint64_t StackSize = MFI.getStackSize();
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auto &ST = MF.getSubtarget<WebAssemblySubtarget>();
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const auto *TII = ST.getInstrInfo();
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auto &MRI = MF.getRegInfo();
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auto InsertPt = MBB.begin();
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while (InsertPt != MBB.end() &&
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WebAssembly::isArgument(InsertPt->getOpcode()))
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++InsertPt;
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DebugLoc DL;
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const TargetRegisterClass *PtrRC =
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MRI.getTargetRegisterInfo()->getPointerRegClass(MF);
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unsigned SPReg = getSPReg(MF);
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if (StackSize)
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SPReg = MRI.createVirtualRegister(PtrRC);
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const char *ES = "__stack_pointer";
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auto *SPSymbol = MF.createExternalSymbolName(ES);
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BuildMI(MBB, InsertPt, DL, TII->get(getOpcGlobGet(MF)), SPReg)
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.addExternalSymbol(SPSymbol);
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bool HasBP = hasBP(MF);
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if (HasBP) {
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auto FI = MF.getInfo<WebAssemblyFunctionInfo>();
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Register BasePtr = MRI.createVirtualRegister(PtrRC);
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FI->setBasePointerVreg(BasePtr);
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BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::COPY), BasePtr)
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.addReg(SPReg);
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}
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if (StackSize) {
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// Subtract the frame size
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Register OffsetReg = MRI.createVirtualRegister(PtrRC);
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BuildMI(MBB, InsertPt, DL, TII->get(getOpcConst(MF)), OffsetReg)
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.addImm(StackSize);
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BuildMI(MBB, InsertPt, DL, TII->get(getOpcSub(MF)), getSPReg(MF))
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.addReg(SPReg)
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.addReg(OffsetReg);
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}
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if (HasBP) {
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Register BitmaskReg = MRI.createVirtualRegister(PtrRC);
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Align Alignment = MFI.getMaxAlign();
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BuildMI(MBB, InsertPt, DL, TII->get(getOpcConst(MF)), BitmaskReg)
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.addImm((int64_t) ~(Alignment.value() - 1));
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BuildMI(MBB, InsertPt, DL, TII->get(getOpcAnd(MF)), getSPReg(MF))
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.addReg(getSPReg(MF))
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.addReg(BitmaskReg);
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}
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if (hasFP(MF)) {
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// Unlike most conventional targets (where FP points to the saved FP),
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// FP points to the bottom of the fixed-size locals, so we can use positive
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// offsets in load/store instructions.
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BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::COPY), getFPReg(MF))
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.addReg(getSPReg(MF));
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}
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if (StackSize && needsSPWriteback(MF)) {
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writeSPToGlobal(getSPReg(MF), MF, MBB, InsertPt, DL);
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}
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}
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void WebAssemblyFrameLowering::emitEpilogue(MachineFunction &MF,
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MachineBasicBlock &MBB) const {
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uint64_t StackSize = MF.getFrameInfo().getStackSize();
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if (!needsSP(MF) || !needsSPWriteback(MF))
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return;
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auto &ST = MF.getSubtarget<WebAssemblySubtarget>();
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const auto *TII = ST.getInstrInfo();
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auto &MRI = MF.getRegInfo();
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auto InsertPt = MBB.getFirstTerminator();
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DebugLoc DL;
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if (InsertPt != MBB.end())
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DL = InsertPt->getDebugLoc();
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// Restore the stack pointer. If we had fixed-size locals, add the offset
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// subtracted in the prolog.
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unsigned SPReg = 0;
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unsigned SPFPReg = hasFP(MF) ? getFPReg(MF) : getSPReg(MF);
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if (hasBP(MF)) {
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auto FI = MF.getInfo<WebAssemblyFunctionInfo>();
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SPReg = FI->getBasePointerVreg();
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} else if (StackSize) {
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const TargetRegisterClass *PtrRC =
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MRI.getTargetRegisterInfo()->getPointerRegClass(MF);
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Register OffsetReg = MRI.createVirtualRegister(PtrRC);
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BuildMI(MBB, InsertPt, DL, TII->get(getOpcConst(MF)), OffsetReg)
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.addImm(StackSize);
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// In the epilog we don't need to write the result back to the SP32/64
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// physreg because it won't be used again. We can use a stackified register
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// instead.
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SPReg = MRI.createVirtualRegister(PtrRC);
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BuildMI(MBB, InsertPt, DL, TII->get(getOpcAdd(MF)), SPReg)
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.addReg(SPFPReg)
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.addReg(OffsetReg);
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} else {
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SPReg = SPFPReg;
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}
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writeSPToGlobal(SPReg, MF, MBB, InsertPt, DL);
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}
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TargetFrameLowering::DwarfFrameBase
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WebAssemblyFrameLowering::getDwarfFrameBase(const MachineFunction &MF) const {
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DwarfFrameBase Loc;
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Loc.Kind = DwarfFrameBase::WasmFrameBase;
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const WebAssemblyFunctionInfo &MFI = *MF.getInfo<WebAssemblyFunctionInfo>();
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if (needsSP(MF) && MFI.isFrameBaseVirtual()) {
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unsigned LocalNum = MFI.getFrameBaseLocal();
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Loc.Location.WasmLoc = {WebAssembly::TI_LOCAL, LocalNum};
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} else {
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// TODO: This should work on a breakpoint at a function with no frame,
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// but probably won't work for traversing up the stack.
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Loc.Location.WasmLoc = {WebAssembly::TI_GLOBAL_RELOC, 0};
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}
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return Loc;
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}
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