SMEABIPass.cpp

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//===--------- SMEABI - SME  ABI-------------------------------------------===//
//
// 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
//
//===----------------------------------------------------------------------===//
//
// This pass implements parts of the the SME ABI, such as:
// * Using the lazy-save mechanism before enabling the use of ZA.
// * Setting up the lazy-save mechanism around invokes.
//
//===----------------------------------------------------------------------===//
 
#include "AArch64.h"
#include "Utils/AArch64SMEAttributes.h"
#include "llvm/ADT/StringRef.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicsAArch64.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/Module.h"
#include "llvm/InitializePasses.h"
#include "llvm/Transforms/Utils/Cloning.h"
 
using namespace llvm;
 
#define DEBUG_TYPE "aarch64-sme-abi"
 
namespace {
struct SMEABI : public FunctionPass {
  static char ID; // Pass identification, replacement for typeid
  SMEABI() : FunctionPass(ID) {
    initializeSMEABIPass(*PassRegistry::getPassRegistry());
  }
 
  bool runOnFunction(Function &F) override;
 
private:
  bool updateNewStateFunctions(Module *M, Function *F, IRBuilder<> &Builder,
                               SMEAttrs FnAttrs);
};
} // end anonymous namespace
 
char SMEABI::ID = 0;
static const char *name = "SME ABI Pass";
INITIALIZE_PASS_BEGIN(SMEABI, DEBUG_TYPE, name, false, false)
INITIALIZE_PASS_END(SMEABI, DEBUG_TYPE, name, false, false)
 
FunctionPass *llvm::createSMEABIPass() { return new SMEABI(); }
 
//===----------------------------------------------------------------------===//
// Utility functions
//===----------------------------------------------------------------------===//
 
// Utility function to emit a call to __arm_tpidr2_save and clear TPIDR2_EL0.
void emitTPIDR2Save(Module *M, IRBuilder<> &Builder) {
  auto *TPIDR2SaveTy =
      FunctionType::get(Builder.getVoidTy(), {}, /*IsVarArgs=*/false);
  auto Attrs = AttributeList().addFnAttribute(M->getContext(),
                                              "aarch64_pstate_sm_compatible");
  FunctionCallee Callee =
      M->getOrInsertFunction("__arm_tpidr2_save", TPIDR2SaveTy, Attrs);
  CallInst *Call = Builder.CreateCall(Callee);
  Call->setCallingConv(
      CallingConv::AArch64_SME_ABI_Support_Routines_PreserveMost_From_X0);
 
  // A save to TPIDR2 should be followed by clearing TPIDR2_EL0.
  Function *WriteIntr =
      Intrinsic::getOrInsertDeclaration(M, Intrinsic::aarch64_sme_set_tpidr2);
  Builder.CreateCall(WriteIntr->getFunctionType(), WriteIntr,
                     Builder.getInt64(0));
}
 
/// This function generates code at the beginning and end of a function marked
/// with either `aarch64_new_za` or `aarch64_new_zt0`.
/// At the beginning of the function, the following code is generated:
///  - Commit lazy-save if active   [Private-ZA Interface*]
///  - Enable PSTATE.ZA             [Private-ZA Interface]
///  - Zero ZA                      [Has New ZA State]
///  - Zero ZT0                     [Has New ZT0 State]
///
/// * A function with new ZT0 state will not change ZA, so committing the
/// lazy-save is not strictly necessary. However, the lazy-save mechanism
/// may be active on entry to the function, with PSTATE.ZA set to 1. If
/// the new ZT0 function calls a function that does not share ZT0, we will
/// need to conditionally SMSTOP ZA before the call, setting PSTATE.ZA to 0.
/// For this reason, it's easier to always commit the lazy-save at the
/// beginning of the function regardless of whether it has ZA state.
///
/// At the end of the function, PSTATE.ZA is disabled if the function has a
/// Private-ZA Interface. A function is considered to have a Private-ZA
/// interface if it does not share ZA or ZT0.
///
bool SMEABI::updateNewStateFunctions(Module *M, Function *F,
                                     IRBuilder<> &Builder, SMEAttrs FnAttrs) {
  LLVMContext &Context = F->getContext();
  BasicBlock *OrigBB = &F->getEntryBlock();
  Builder.SetInsertPoint(&OrigBB->front());
 
  // Commit any active lazy-saves if this is a Private-ZA function. If the
  // value read from TPIDR2_EL0 is not null on entry to the function then
  // the lazy-saving scheme is active and we should call __arm_tpidr2_save
  // to commit the lazy save.
  if (FnAttrs.hasPrivateZAInterface()) {
    // Create the new blocks for reading TPIDR2_EL0 & enabling ZA state.
    auto *SaveBB = OrigBB->splitBasicBlock(OrigBB->begin(), "save.za", true);
    auto *PreludeBB = BasicBlock::Create(Context, "prelude", F, SaveBB);
 
    // Read TPIDR2_EL0 in PreludeBB & branch to SaveBB if not 0.
    Builder.SetInsertPoint(PreludeBB);
    Function *TPIDR2Intr =
        Intrinsic::getOrInsertDeclaration(M, Intrinsic::aarch64_sme_get_tpidr2);
    auto *TPIDR2 = Builder.CreateCall(TPIDR2Intr->getFunctionType(), TPIDR2Intr,
                                      {}, "tpidr2");
    auto *Cmp = Builder.CreateCmp(ICmpInst::ICMP_NE, TPIDR2,
                                  Builder.getInt64(0), "cmp");
    Builder.CreateCondBr(Cmp, SaveBB, OrigBB);
 
    // Create a call __arm_tpidr2_save, which commits the lazy save.
    Builder.SetInsertPoint(&SaveBB->back());
    emitTPIDR2Save(M, Builder);
 
    // Enable pstate.za at the start of the function.
    Builder.SetInsertPoint(&OrigBB->front());
    Function *EnableZAIntr =
        Intrinsic::getOrInsertDeclaration(M, Intrinsic::aarch64_sme_za_enable);
    Builder.CreateCall(EnableZAIntr->getFunctionType(), EnableZAIntr);
  }
 
  if (FnAttrs.isNewZA()) {
    Function *ZeroIntr =
        Intrinsic::getOrInsertDeclaration(M, Intrinsic::aarch64_sme_zero);
    Builder.CreateCall(ZeroIntr->getFunctionType(), ZeroIntr,
                       Builder.getInt32(0xff));
  }
 
  if (FnAttrs.isNewZT0()) {
    Function *ClearZT0Intr =
        Intrinsic::getOrInsertDeclaration(M, Intrinsic::aarch64_sme_zero_zt);
    Builder.CreateCall(ClearZT0Intr->getFunctionType(), ClearZT0Intr,
                       {Builder.getInt32(0)});
  }
 
  if (FnAttrs.hasPrivateZAInterface()) {
    // Before returning, disable pstate.za
    for (BasicBlock &BB : *F) {
      Instruction *T = BB.getTerminator();
      if (!T || !isa<ReturnInst>(T))
        continue;
      Builder.SetInsertPoint(T);
      Function *DisableZAIntr = Intrinsic::getOrInsertDeclaration(
          M, Intrinsic::aarch64_sme_za_disable);
      Builder.CreateCall(DisableZAIntr->getFunctionType(), DisableZAIntr);
    }
  }
 
  F->addFnAttr("aarch64_expanded_pstate_za");
  return true;
}
 
bool SMEABI::runOnFunction(Function &F) {
  Module *M = F.getParent();
  LLVMContext &Context = F.getContext();
  IRBuilder<> Builder(Context);
 
  if (F.isDeclaration() || F.hasFnAttribute("aarch64_expanded_pstate_za"))
    return false;
 
  bool Changed = false;
  SMEAttrs FnAttrs(F);
  if (FnAttrs.isNewZA() || FnAttrs.isNewZT0())
    Changed |= updateNewStateFunctions(M, &F, Builder, FnAttrs);
 
  return Changed;
}