llvm-6502/include/llvm/Transforms/IPO/LowerBitSets.h
Mehdi Amini 529919ff31 DataLayout is mandatory, update the API to reflect it with references.
Summary:
Now that the DataLayout is a mandatory part of the module, let's start
cleaning the codebase. This patch is a first attempt at doing that.

This patch is not exactly NFC as for instance some places were passing
a nullptr instead of the DataLayout, possibly just because there was a
default value on the DataLayout argument to many functions in the API.
Even though it is not purely NFC, there is no change in the
validation.

I turned as many pointer to DataLayout to references, this helped
figuring out all the places where a nullptr could come up.

I had initially a local version of this patch broken into over 30
independant, commits but some later commit were cleaning the API and
touching part of the code modified in the previous commits, so it
seemed cleaner without the intermediate state.

Test Plan:

Reviewers: echristo

Subscribers: llvm-commits

From: Mehdi Amini <mehdi.amini@apple.com>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@231740 91177308-0d34-0410-b5e6-96231b3b80d8
2015-03-10 02:37:25 +00:00

199 lines
7.1 KiB
C++

//===- LowerBitSets.h - Bitset lowering pass --------------------*- C++ -*-===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file defines parts of the bitset lowering pass implementation that may
// be usefully unit tested.
//
//===----------------------------------------------------------------------===//
#ifndef LLVM_TRANSFORMS_IPO_LOWERBITSETS_H
#define LLVM_TRANSFORMS_IPO_LOWERBITSETS_H
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/SmallVector.h"
#include <stdint.h>
#include <limits>
#include <set>
#include <vector>
namespace llvm {
class DataLayout;
class GlobalVariable;
class Value;
struct BitSetInfo {
// The indices of the set bits in the bitset.
std::set<uint64_t> Bits;
// The byte offset into the combined global represented by the bitset.
uint64_t ByteOffset;
// The size of the bitset in bits.
uint64_t BitSize;
// Log2 alignment of the bit set relative to the combined global.
// For example, a log2 alignment of 3 means that bits in the bitset
// represent addresses 8 bytes apart.
unsigned AlignLog2;
bool isSingleOffset() const {
return Bits.size() == 1;
}
bool isAllOnes() const {
return Bits.size() == BitSize;
}
bool containsGlobalOffset(uint64_t Offset) const;
bool containsValue(const DataLayout &DL,
const DenseMap<GlobalVariable *, uint64_t> &GlobalLayout,
Value *V, uint64_t COffset = 0) const;
};
struct BitSetBuilder {
SmallVector<uint64_t, 16> Offsets;
uint64_t Min, Max;
BitSetBuilder() : Min(std::numeric_limits<uint64_t>::max()), Max(0) {}
void addOffset(uint64_t Offset) {
if (Min > Offset)
Min = Offset;
if (Max < Offset)
Max = Offset;
Offsets.push_back(Offset);
}
BitSetInfo build();
};
/// This class implements a layout algorithm for globals referenced by bit sets
/// that tries to keep members of small bit sets together. This can
/// significantly reduce bit set sizes in many cases.
///
/// It works by assembling fragments of layout from sets of referenced globals.
/// Each set of referenced globals causes the algorithm to create a new
/// fragment, which is assembled by appending each referenced global in the set
/// into the fragment. If a referenced global has already been referenced by an
/// fragment created earlier, we instead delete that fragment and append its
/// contents into the fragment we are assembling.
///
/// By starting with the smallest fragments, we minimize the size of the
/// fragments that are copied into larger fragments. This is most intuitively
/// thought about when considering the case where the globals are virtual tables
/// and the bit sets represent their derived classes: in a single inheritance
/// hierarchy, the optimum layout would involve a depth-first search of the
/// class hierarchy (and in fact the computed layout ends up looking a lot like
/// a DFS), but a naive DFS would not work well in the presence of multiple
/// inheritance. This aspect of the algorithm ends up fitting smaller
/// hierarchies inside larger ones where that would be beneficial.
///
/// For example, consider this class hierarchy:
///
/// A B
/// \ / | \
/// C D E
///
/// We have five bit sets: bsA (A, C), bsB (B, C, D, E), bsC (C), bsD (D) and
/// bsE (E). If we laid out our objects by DFS traversing B followed by A, our
/// layout would be {B, C, D, E, A}. This is optimal for bsB as it needs to
/// cover the only 4 objects in its hierarchy, but not for bsA as it needs to
/// cover 5 objects, i.e. the entire layout. Our algorithm proceeds as follows:
///
/// Add bsC, fragments {{C}}
/// Add bsD, fragments {{C}, {D}}
/// Add bsE, fragments {{C}, {D}, {E}}
/// Add bsA, fragments {{A, C}, {D}, {E}}
/// Add bsB, fragments {{B, A, C, D, E}}
///
/// This layout is optimal for bsA, as it now only needs to cover two (i.e. 3
/// fewer) objects, at the cost of bsB needing to cover 1 more object.
///
/// The bit set lowering pass assigns an object index to each object that needs
/// to be laid out, and calls addFragment for each bit set passing the object
/// indices of its referenced globals. It then assembles a layout from the
/// computed layout in the Fragments field.
struct GlobalLayoutBuilder {
/// The computed layout. Each element of this vector contains a fragment of
/// layout (which may be empty) consisting of object indices.
std::vector<std::vector<uint64_t>> Fragments;
/// Mapping from object index to fragment index.
std::vector<uint64_t> FragmentMap;
GlobalLayoutBuilder(uint64_t NumObjects)
: Fragments(1), FragmentMap(NumObjects) {}
/// Add F to the layout while trying to keep its indices contiguous.
/// If a previously seen fragment uses any of F's indices, that
/// fragment will be laid out inside F.
void addFragment(const std::set<uint64_t> &F);
};
/// This class is used to build a byte array containing overlapping bit sets. By
/// loading from indexed offsets into the byte array and applying a mask, a
/// program can test bits from the bit set with a relatively short instruction
/// sequence. For example, suppose we have 15 bit sets to lay out:
///
/// A (16 bits), B (15 bits), C (14 bits), D (13 bits), E (12 bits),
/// F (11 bits), G (10 bits), H (9 bits), I (7 bits), J (6 bits), K (5 bits),
/// L (4 bits), M (3 bits), N (2 bits), O (1 bit)
///
/// These bits can be laid out in a 16-byte array like this:
///
/// Byte Offset
/// 0123456789ABCDEF
/// Bit
/// 7 HHHHHHHHHIIIIIII
/// 6 GGGGGGGGGGJJJJJJ
/// 5 FFFFFFFFFFFKKKKK
/// 4 EEEEEEEEEEEELLLL
/// 3 DDDDDDDDDDDDDMMM
/// 2 CCCCCCCCCCCCCCNN
/// 1 BBBBBBBBBBBBBBBO
/// 0 AAAAAAAAAAAAAAAA
///
/// For example, to test bit X of A, we evaluate ((bits[X] & 1) != 0), or to
/// test bit X of I, we evaluate ((bits[9 + X] & 0x80) != 0). This can be done
/// in 1-2 machine instructions on x86, or 4-6 instructions on ARM.
///
/// This is a byte array, rather than (say) a 2-byte array or a 4-byte array,
/// because for one thing it gives us better packing (the more bins there are,
/// the less evenly they will be filled), and for another, the instruction
/// sequences can be slightly shorter, both on x86 and ARM.
struct ByteArrayBuilder {
/// The byte array built so far.
std::vector<uint8_t> Bytes;
enum { BitsPerByte = 8 };
/// The number of bytes allocated so far for each of the bits.
uint64_t BitAllocs[BitsPerByte];
ByteArrayBuilder() {
memset(BitAllocs, 0, sizeof(BitAllocs));
}
/// Allocate BitSize bits in the byte array where Bits contains the bits to
/// set. AllocByteOffset is set to the offset within the byte array and
/// AllocMask is set to the bitmask for those bits. This uses the LPT (Longest
/// Processing Time) multiprocessor scheduling algorithm to lay out the bits
/// efficiently; the pass allocates bit sets in decreasing size order.
void allocate(const std::set<uint64_t> &Bits, uint64_t BitSize,
uint64_t &AllocByteOffset, uint8_t &AllocMask);
};
} // namespace llvm
#endif