420 lines
16 KiB
C++
420 lines
16 KiB
C++
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//===-- PPCBranchSelector.cpp - Emit long conditional branches ------------===//
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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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// This file contains a pass that scans a machine function to determine which
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// conditional branches need more than 16 bits of displacement to reach their
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// target basic block. It does this in two passes; a calculation of basic block
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// positions pass, and a branch pseudo op to machine branch opcode pass. This
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// pass should be run last, just before the assembly printer.
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//
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//===----------------------------------------------------------------------===//
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#include "MCTargetDesc/PPCPredicates.h"
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#include "PPC.h"
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#include "PPCInstrBuilder.h"
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#include "PPCInstrInfo.h"
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#include "PPCSubtarget.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/CodeGen/MachineFunctionPass.h"
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#include "llvm/CodeGen/MachineRegisterInfo.h"
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#include "llvm/CodeGen/TargetSubtargetInfo.h"
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#include "llvm/Support/MathExtras.h"
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#include "llvm/Target/TargetMachine.h"
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#include <algorithm>
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using namespace llvm;
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#define DEBUG_TYPE "ppc-branch-select"
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STATISTIC(NumExpanded, "Number of branches expanded to long format");
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STATISTIC(NumPrefixed, "Number of prefixed instructions");
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STATISTIC(NumPrefixedAligned,
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"Number of prefixed instructions that have been aligned");
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namespace {
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struct PPCBSel : public MachineFunctionPass {
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static char ID;
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PPCBSel() : MachineFunctionPass(ID) {
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initializePPCBSelPass(*PassRegistry::getPassRegistry());
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}
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// The sizes of the basic blocks in the function (the first
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// element of the pair); the second element of the pair is the amount of the
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// size that is due to potential padding.
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std::vector<std::pair<unsigned, unsigned>> BlockSizes;
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// The first block number which has imprecise instruction address.
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int FirstImpreciseBlock = -1;
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unsigned GetAlignmentAdjustment(MachineBasicBlock &MBB, unsigned Offset);
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unsigned ComputeBlockSizes(MachineFunction &Fn);
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void modifyAdjustment(MachineFunction &Fn);
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int computeBranchSize(MachineFunction &Fn,
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const MachineBasicBlock *Src,
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const MachineBasicBlock *Dest,
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unsigned BrOffset);
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bool runOnMachineFunction(MachineFunction &Fn) override;
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MachineFunctionProperties getRequiredProperties() const override {
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return MachineFunctionProperties().set(
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MachineFunctionProperties::Property::NoVRegs);
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}
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StringRef getPassName() const override { return "PowerPC Branch Selector"; }
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};
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char PPCBSel::ID = 0;
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}
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INITIALIZE_PASS(PPCBSel, "ppc-branch-select", "PowerPC Branch Selector",
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false, false)
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/// createPPCBranchSelectionPass - returns an instance of the Branch Selection
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/// Pass
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///
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FunctionPass *llvm::createPPCBranchSelectionPass() {
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return new PPCBSel();
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}
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/// In order to make MBB aligned, we need to add an adjustment value to the
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/// original Offset.
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unsigned PPCBSel::GetAlignmentAdjustment(MachineBasicBlock &MBB,
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unsigned Offset) {
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const Align Alignment = MBB.getAlignment();
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if (Alignment == Align(1))
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return 0;
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const Align ParentAlign = MBB.getParent()->getAlignment();
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if (Alignment <= ParentAlign)
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return offsetToAlignment(Offset, Alignment);
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// The alignment of this MBB is larger than the function's alignment, so we
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// can't tell whether or not it will insert nops. Assume that it will.
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if (FirstImpreciseBlock < 0)
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FirstImpreciseBlock = MBB.getNumber();
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return Alignment.value() + offsetToAlignment(Offset, Alignment);
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}
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/// We need to be careful about the offset of the first block in the function
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/// because it might not have the function's alignment. This happens because,
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/// under the ELFv2 ABI, for functions which require a TOC pointer, we add a
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/// two-instruction sequence to the start of the function.
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/// Note: This needs to be synchronized with the check in
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/// PPCLinuxAsmPrinter::EmitFunctionBodyStart.
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static inline unsigned GetInitialOffset(MachineFunction &Fn) {
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unsigned InitialOffset = 0;
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if (Fn.getSubtarget<PPCSubtarget>().isELFv2ABI() &&
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!Fn.getRegInfo().use_empty(PPC::X2))
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InitialOffset = 8;
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return InitialOffset;
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}
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/// Measure each MBB and compute a size for the entire function.
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unsigned PPCBSel::ComputeBlockSizes(MachineFunction &Fn) {
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const PPCInstrInfo *TII =
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static_cast<const PPCInstrInfo *>(Fn.getSubtarget().getInstrInfo());
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unsigned FuncSize = GetInitialOffset(Fn);
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for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E;
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++MFI) {
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MachineBasicBlock *MBB = &*MFI;
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// The end of the previous block may have extra nops if this block has an
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// alignment requirement.
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if (MBB->getNumber() > 0) {
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unsigned AlignExtra = GetAlignmentAdjustment(*MBB, FuncSize);
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auto &BS = BlockSizes[MBB->getNumber()-1];
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BS.first += AlignExtra;
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BS.second = AlignExtra;
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FuncSize += AlignExtra;
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}
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unsigned BlockSize = 0;
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unsigned UnalignedBytesRemaining = 0;
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for (MachineInstr &MI : *MBB) {
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unsigned MINumBytes = TII->getInstSizeInBytes(MI);
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if (MI.isInlineAsm() && (FirstImpreciseBlock < 0))
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FirstImpreciseBlock = MBB->getNumber();
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if (TII->isPrefixed(MI.getOpcode())) {
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NumPrefixed++;
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// All 8 byte instructions may require alignment. Each 8 byte
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// instruction may be aligned by another 4 bytes.
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// This means that an 8 byte instruction may require 12 bytes
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// (8 for the instruction itself and 4 for the alignment nop).
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// This will happen if an 8 byte instruction can be aligned to 64 bytes
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// by only adding a 4 byte nop.
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// We don't know the alignment at this point in the code so we have to
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// adopt a more pessimistic approach. If an instruction may need
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// alignment we assume that it does need alignment and add 4 bytes to
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// it. As a result we may end up with more long branches than before
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// but we are in the safe position where if we need a long branch we
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// have one.
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// The if statement checks to make sure that two 8 byte instructions
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// are at least 64 bytes away from each other. It is not possible for
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// two instructions that both need alignment to be within 64 bytes of
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// each other.
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if (!UnalignedBytesRemaining) {
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BlockSize += 4;
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UnalignedBytesRemaining = 60;
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NumPrefixedAligned++;
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}
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}
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UnalignedBytesRemaining -= std::min(UnalignedBytesRemaining, MINumBytes);
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BlockSize += MINumBytes;
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}
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BlockSizes[MBB->getNumber()].first = BlockSize;
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FuncSize += BlockSize;
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}
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return FuncSize;
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}
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/// Modify the basic block align adjustment.
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void PPCBSel::modifyAdjustment(MachineFunction &Fn) {
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unsigned Offset = GetInitialOffset(Fn);
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for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E;
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++MFI) {
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MachineBasicBlock *MBB = &*MFI;
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if (MBB->getNumber() > 0) {
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auto &BS = BlockSizes[MBB->getNumber()-1];
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BS.first -= BS.second;
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Offset -= BS.second;
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unsigned AlignExtra = GetAlignmentAdjustment(*MBB, Offset);
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BS.first += AlignExtra;
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BS.second = AlignExtra;
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Offset += AlignExtra;
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}
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Offset += BlockSizes[MBB->getNumber()].first;
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}
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}
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/// Determine the offset from the branch in Src block to the Dest block.
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/// BrOffset is the offset of the branch instruction inside Src block.
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int PPCBSel::computeBranchSize(MachineFunction &Fn,
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const MachineBasicBlock *Src,
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const MachineBasicBlock *Dest,
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unsigned BrOffset) {
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int BranchSize;
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Align MaxAlign = Align(4);
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bool NeedExtraAdjustment = false;
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if (Dest->getNumber() <= Src->getNumber()) {
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// If this is a backwards branch, the delta is the offset from the
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// start of this block to this branch, plus the sizes of all blocks
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// from this block to the dest.
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BranchSize = BrOffset;
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MaxAlign = std::max(MaxAlign, Src->getAlignment());
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int DestBlock = Dest->getNumber();
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BranchSize += BlockSizes[DestBlock].first;
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for (unsigned i = DestBlock+1, e = Src->getNumber(); i < e; ++i) {
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BranchSize += BlockSizes[i].first;
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MaxAlign = std::max(MaxAlign, Fn.getBlockNumbered(i)->getAlignment());
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}
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NeedExtraAdjustment = (FirstImpreciseBlock >= 0) &&
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(DestBlock >= FirstImpreciseBlock);
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} else {
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// Otherwise, add the size of the blocks between this block and the
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// dest to the number of bytes left in this block.
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unsigned StartBlock = Src->getNumber();
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BranchSize = BlockSizes[StartBlock].first - BrOffset;
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MaxAlign = std::max(MaxAlign, Dest->getAlignment());
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for (unsigned i = StartBlock+1, e = Dest->getNumber(); i != e; ++i) {
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BranchSize += BlockSizes[i].first;
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MaxAlign = std::max(MaxAlign, Fn.getBlockNumbered(i)->getAlignment());
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}
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NeedExtraAdjustment = (FirstImpreciseBlock >= 0) &&
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(Src->getNumber() >= FirstImpreciseBlock);
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}
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// We tend to over estimate code size due to large alignment and
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// inline assembly. Usually it causes larger computed branch offset.
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// But sometimes it may also causes smaller computed branch offset
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// than actual branch offset. If the offset is close to the limit of
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// encoding, it may cause problem at run time.
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// Following is a simplified example.
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//
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// actual estimated
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// address address
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// ...
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// bne Far 100 10c
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// .p2align 4
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// Near: 110 110
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// ...
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// Far: 8108 8108
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//
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// Actual offset: 0x8108 - 0x100 = 0x8008
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// Computed offset: 0x8108 - 0x10c = 0x7ffc
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//
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// This example also shows when we can get the largest gap between
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// estimated offset and actual offset. If there is an aligned block
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// ABB between branch and target, assume its alignment is <align>
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// bits. Now consider the accumulated function size FSIZE till the end
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// of previous block PBB. If the estimated FSIZE is multiple of
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// 2^<align>, we don't need any padding for the estimated address of
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// ABB. If actual FSIZE at the end of PBB is 4 bytes more than
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// multiple of 2^<align>, then we need (2^<align> - 4) bytes of
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// padding. It also means the actual branch offset is (2^<align> - 4)
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// larger than computed offset. Other actual FSIZE needs less padding
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// bytes, so causes smaller gap between actual and computed offset.
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//
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// On the other hand, if the inline asm or large alignment occurs
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// between the branch block and destination block, the estimated address
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// can be <delta> larger than actual address. If padding bytes are
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// needed for a later aligned block, the actual number of padding bytes
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// is at most <delta> more than estimated padding bytes. So the actual
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// aligned block address is less than or equal to the estimated aligned
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// block address. So the actual branch offset is less than or equal to
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// computed branch offset.
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//
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// The computed offset is at most ((1 << alignment) - 4) bytes smaller
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// than actual offset. So we add this number to the offset for safety.
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if (NeedExtraAdjustment)
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BranchSize += MaxAlign.value() - 4;
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return BranchSize;
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}
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bool PPCBSel::runOnMachineFunction(MachineFunction &Fn) {
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const PPCInstrInfo *TII =
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static_cast<const PPCInstrInfo *>(Fn.getSubtarget().getInstrInfo());
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// Give the blocks of the function a dense, in-order, numbering.
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Fn.RenumberBlocks();
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BlockSizes.resize(Fn.getNumBlockIDs());
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FirstImpreciseBlock = -1;
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// Measure each MBB and compute a size for the entire function.
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unsigned FuncSize = ComputeBlockSizes(Fn);
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// If the entire function is smaller than the displacement of a branch field,
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// we know we don't need to shrink any branches in this function. This is a
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// common case.
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if (FuncSize < (1 << 15)) {
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BlockSizes.clear();
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return false;
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}
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// For each conditional branch, if the offset to its destination is larger
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// than the offset field allows, transform it into a long branch sequence
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// like this:
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// short branch:
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// bCC MBB
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// long branch:
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// b!CC $PC+8
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// b MBB
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//
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bool MadeChange = true;
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bool EverMadeChange = false;
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while (MadeChange) {
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// Iteratively expand branches until we reach a fixed point.
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MadeChange = false;
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for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI != E;
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++MFI) {
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MachineBasicBlock &MBB = *MFI;
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unsigned MBBStartOffset = 0;
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for (MachineBasicBlock::iterator I = MBB.begin(), E = MBB.end();
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I != E; ++I) {
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MachineBasicBlock *Dest = nullptr;
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if (I->getOpcode() == PPC::BCC && !I->getOperand(2).isImm())
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Dest = I->getOperand(2).getMBB();
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else if ((I->getOpcode() == PPC::BC || I->getOpcode() == PPC::BCn) &&
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!I->getOperand(1).isImm())
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Dest = I->getOperand(1).getMBB();
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else if ((I->getOpcode() == PPC::BDNZ8 || I->getOpcode() == PPC::BDNZ ||
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I->getOpcode() == PPC::BDZ8 || I->getOpcode() == PPC::BDZ) &&
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!I->getOperand(0).isImm())
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Dest = I->getOperand(0).getMBB();
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if (!Dest) {
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MBBStartOffset += TII->getInstSizeInBytes(*I);
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continue;
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}
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// Determine the offset from the current branch to the destination
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// block.
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int BranchSize = computeBranchSize(Fn, &MBB, Dest, MBBStartOffset);
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|
|
// If this branch is in range, ignore it.
|
||
|
|
if (isInt<16>(BranchSize)) {
|
||
|
|
MBBStartOffset += 4;
|
||
|
|
continue;
|
||
|
|
}
|
||
|
|
|
||
|
|
// Otherwise, we have to expand it to a long branch.
|
||
|
|
MachineInstr &OldBranch = *I;
|
||
|
|
DebugLoc dl = OldBranch.getDebugLoc();
|
||
|
|
|
||
|
|
if (I->getOpcode() == PPC::BCC) {
|
||
|
|
// The BCC operands are:
|
||
|
|
// 0. PPC branch predicate
|
||
|
|
// 1. CR register
|
||
|
|
// 2. Target MBB
|
||
|
|
PPC::Predicate Pred = (PPC::Predicate)I->getOperand(0).getImm();
|
||
|
|
Register CRReg = I->getOperand(1).getReg();
|
||
|
|
|
||
|
|
// Jump over the uncond branch inst (i.e. $PC+8) on opposite condition.
|
||
|
|
BuildMI(MBB, I, dl, TII->get(PPC::BCC))
|
||
|
|
.addImm(PPC::InvertPredicate(Pred)).addReg(CRReg).addImm(2);
|
||
|
|
} else if (I->getOpcode() == PPC::BC) {
|
||
|
|
Register CRBit = I->getOperand(0).getReg();
|
||
|
|
BuildMI(MBB, I, dl, TII->get(PPC::BCn)).addReg(CRBit).addImm(2);
|
||
|
|
} else if (I->getOpcode() == PPC::BCn) {
|
||
|
|
Register CRBit = I->getOperand(0).getReg();
|
||
|
|
BuildMI(MBB, I, dl, TII->get(PPC::BC)).addReg(CRBit).addImm(2);
|
||
|
|
} else if (I->getOpcode() == PPC::BDNZ) {
|
||
|
|
BuildMI(MBB, I, dl, TII->get(PPC::BDZ)).addImm(2);
|
||
|
|
} else if (I->getOpcode() == PPC::BDNZ8) {
|
||
|
|
BuildMI(MBB, I, dl, TII->get(PPC::BDZ8)).addImm(2);
|
||
|
|
} else if (I->getOpcode() == PPC::BDZ) {
|
||
|
|
BuildMI(MBB, I, dl, TII->get(PPC::BDNZ)).addImm(2);
|
||
|
|
} else if (I->getOpcode() == PPC::BDZ8) {
|
||
|
|
BuildMI(MBB, I, dl, TII->get(PPC::BDNZ8)).addImm(2);
|
||
|
|
} else {
|
||
|
|
llvm_unreachable("Unhandled branch type!");
|
||
|
|
}
|
||
|
|
|
||
|
|
// Uncond branch to the real destination.
|
||
|
|
I = BuildMI(MBB, I, dl, TII->get(PPC::B)).addMBB(Dest);
|
||
|
|
|
||
|
|
// Remove the old branch from the function.
|
||
|
|
OldBranch.eraseFromParent();
|
||
|
|
|
||
|
|
// Remember that this instruction is 8-bytes, increase the size of the
|
||
|
|
// block by 4, remember to iterate.
|
||
|
|
BlockSizes[MBB.getNumber()].first += 4;
|
||
|
|
MBBStartOffset += 8;
|
||
|
|
++NumExpanded;
|
||
|
|
MadeChange = true;
|
||
|
|
}
|
||
|
|
}
|
||
|
|
|
||
|
|
if (MadeChange) {
|
||
|
|
// If we're going to iterate again, make sure we've updated our
|
||
|
|
// padding-based contributions to the block sizes.
|
||
|
|
modifyAdjustment(Fn);
|
||
|
|
}
|
||
|
|
|
||
|
|
EverMadeChange |= MadeChange;
|
||
|
|
}
|
||
|
|
|
||
|
|
BlockSizes.clear();
|
||
|
|
return true;
|
||
|
|
}
|