asmjit/core/rastack.cpp

// AsmJit - Machine code generation for C++
//
//  * Official AsmJit Home Page: https://asmjit.com
//  * Official Github Repository: https://github.com/asmjit/asmjit
//
// Copyright (c) 2008-2020 The AsmJit Authors
//
// This software is provided 'as-is', without any express or implied
// warranty. In no event will the authors be held liable for any damages
// arising from the use of this software.
//
// Permission is granted to anyone to use this software for any purpose,
// including commercial applications, and to alter it and redistribute it
// freely, subject to the following restrictions:
//
// 1. The origin of this software must not be misrepresented; you must not
//    claim that you wrote the original software. If you use this software
//    in a product, an acknowledgment in the product documentation would be
//    appreciated but is not required.
// 2. Altered source versions must be plainly marked as such, and must not be
//    misrepresented as being the original software.
// 3. This notice may not be removed or altered from any source distribution.

#include "../core/api-build_p.h"
#ifndef ASMJIT_NO_COMPILER

#include "../core/rastack_p.h"
#include "../core/support.h"

ASMJIT_BEGIN_NAMESPACE

// ============================================================================
// [asmjit::RAStackAllocator - Slots]
// ============================================================================

RAStackSlot* RAStackAllocator::newSlot(uint32_t baseRegId, uint32_t size, uint32_t alignment, uint32_t flags) noexcept {
  if (ASMJIT_UNLIKELY(_slots.willGrow(allocator(), 1) != kErrorOk))
    return nullptr;

  RAStackSlot* slot = allocator()->allocT<RAStackSlot>();
  if (ASMJIT_UNLIKELY(!slot))
    return nullptr;

  slot->_baseRegId = uint8_t(baseRegId);
  slot->_alignment = uint8_t(Support::max<uint32_t>(alignment, 1));
  slot->_flags = uint16_t(flags);
  slot->_useCount = 0;
  slot->_size = size;

  slot->_weight = 0;
  slot->_offset = 0;

  _alignment = Support::max<uint32_t>(_alignment, alignment);
  _slots.appendUnsafe(slot);
  return slot;
}

// ============================================================================
// [asmjit::RAStackAllocator - Utilities]
// ============================================================================

struct RAStackGap {
  inline RAStackGap() noexcept
    : offset(0),
      size(0) {}

  inline RAStackGap(uint32_t offset, uint32_t size) noexcept
    : offset(offset),
      size(size) {}

  inline RAStackGap(const RAStackGap& other) noexcept
    : offset(other.offset),
      size(other.size) {}

  uint32_t offset;
  uint32_t size;
};

Error RAStackAllocator::calculateStackFrame() noexcept {
  // Base weight added to all registers regardless of their size and alignment.
  uint32_t kBaseRegWeight = 16;

  // STEP 1:
  //
  // Update usage based on the size of the slot. We boost smaller slots in a way
  // that 32-bit register has higher priority than a 128-bit register, however,
  // if one 128-bit register is used 4 times more than some other 32-bit register
  // it will overweight it.
  for (RAStackSlot* slot : _slots) {
    uint32_t alignment = slot->alignment();
    ASMJIT_ASSERT(alignment > 0);

    uint32_t power = Support::min<uint32_t>(Support::ctz(alignment), 6);
    uint64_t weight;

    if (slot->isRegHome())
      weight = kBaseRegWeight + (uint64_t(slot->useCount()) * (7 - power));
    else
      weight = power;

    // If overflown, which has less chance of winning a lottery, just use max
    // possible weight. In such case it probably doesn't matter at all.
    if (weight > 0xFFFFFFFFu)
      weight = 0xFFFFFFFFu;

    slot->setWeight(uint32_t(weight));
  }

  // STEP 2:
  //
  // Sort stack slots based on their newly calculated weight (in descending order).
  _slots.sort([](const RAStackSlot* a, const RAStackSlot* b) noexcept {
    return a->weight() >  b->weight() ? 1 :
           a->weight() == b->weight() ? 0 : -1;
  });

  // STEP 3:
  //
  // Calculate offset of each slot. We start from the slot that has the highest
  // weight and advance to slots with lower weight. It could look that offsets
  // start from the first slot in our list and then simply increase, but it's
  // not always the case as we also try to fill all gaps introduced by the fact
  // that slots are sorted by weight and not by size & alignment, so when we need
  // to align some slot we distribute the gap caused by the alignment to `gaps`.
  uint32_t offset = 0;
  ZoneVector<RAStackGap> gaps[kSizeCount - 1];

  for (RAStackSlot* slot : _slots) {
    if (slot->isStackArg())
      continue;

    uint32_t slotAlignment = slot->alignment();
    uint32_t alignedOffset = Support::alignUp(offset, slotAlignment);

    // Try to find a slot within gaps first, before advancing the `offset`.
    bool foundGap = false;
    uint32_t gapSize = 0;
    uint32_t gapOffset = 0;

    {
      uint32_t slotSize = slot->size();
      if (slotSize < (1u << uint32_t(ASMJIT_ARRAY_SIZE(gaps)))) {
        // Iterate from the lowest to the highest possible.
        uint32_t index = Support::ctz(slotSize);
        do {
          if (!gaps[index].empty()) {
            RAStackGap gap = gaps[index].pop();

            ASMJIT_ASSERT(Support::isAligned(gap.offset, slotAlignment));
            slot->setOffset(int32_t(gap.offset));

            gapSize = gap.size - slotSize;
            gapOffset = gap.offset - slotSize;

            foundGap = true;
            break;
          }
        } while (++index < uint32_t(ASMJIT_ARRAY_SIZE(gaps)));
      }
    }

    // No gap found, we may create a new one(s) if the current offset is not aligned.
    if (!foundGap && offset != alignedOffset) {
      gapSize = alignedOffset - offset;
      gapOffset = alignedOffset;

      offset = alignedOffset;
    }

    // True if we have found a gap and not filled all of it or we aligned the current offset.
    if (gapSize) {
      uint32_t gapEnd = gapSize + gapOffset;
      while (gapOffset < gapEnd) {
        uint32_t index = Support::ctz(gapOffset);
        uint32_t slotSize = 1u << index;

        // Weird case, better to bail...
        if (gapEnd - gapOffset < slotSize)
          break;

        ASMJIT_PROPAGATE(gaps[index].append(allocator(), RAStackGap(gapOffset, slotSize)));
        gapOffset += slotSize;
      }
    }

    if (!foundGap) {
      ASMJIT_ASSERT(Support::isAligned(offset, slotAlignment));
      slot->setOffset(int32_t(offset));
      offset += slot->size();
    }
  }

  _stackSize = Support::alignUp(offset, _alignment);
  return kErrorOk;
}

Error RAStackAllocator::adjustSlotOffsets(int32_t offset) noexcept {
  for (RAStackSlot* slot : _slots)
    if (!slot->isStackArg())
      slot->_offset += offset;
  return kErrorOk;
}

ASMJIT_END_NAMESPACE

#endif // !ASMJIT_NO_COMPILER