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llvm-project/llvm/lib/CodeGen/StackColoring.cpp

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//===- StackColoring.cpp --------------------------------------------------===//
//
// 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 the stack-coloring optimization that looks for
// lifetime markers machine instructions (LIFESTART_BEGIN and LIFESTART_END),
// which represent the possible lifetime of stack slots. It attempts to
// merge disjoint stack slots and reduce the used stack space.
// NOTE: This pass is not StackSlotColoring, which optimizes spill slots.
//
// TODO: In the future we plan to improve stack coloring in the following ways:
// 1. Allow merging multiple small slots into a single larger slot at different
// offsets.
// 2. Merge this pass with StackSlotColoring and allow merging of allocas with
// spill slots.
//
//===----------------------------------------------------------------------===//
#include "llvm/ADT/BitVector.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/DepthFirstIterator.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/CodeGen/LiveInterval.h"
#include "llvm/CodeGen/MachineBasicBlock.h"
#include "llvm/CodeGen/MachineFrameInfo.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/MachineMemOperand.h"
#include "llvm/CodeGen/MachineOperand.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/CodeGen/SelectionDAGNodes.h"
#include "llvm/CodeGen/SlotIndexes.h"
#include "llvm/CodeGen/TargetOpcodes.h"
#include "llvm/CodeGen/WinEHFuncInfo.h"
#include "llvm/Config/llvm-config.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DebugInfoMetadata.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Metadata.h"
#include "llvm/IR/Use.h"
#include "llvm/IR/Value.h"
#include "llvm/InitializePasses.h"
#include "llvm/Pass.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include <algorithm>
#include <cassert>
#include <limits>
#include <memory>
#include <utility>
using namespace llvm;
#define DEBUG_TYPE "stack-coloring"
static cl::opt<bool>
DisableColoring("no-stack-coloring",
cl::init(false), cl::Hidden,
cl::desc("Disable stack coloring"));
/// The user may write code that uses allocas outside of the declared lifetime
/// zone. This can happen when the user returns a reference to a local
/// data-structure. We can detect these cases and decide not to optimize the
/// code. If this flag is enabled, we try to save the user. This option
/// is treated as overriding LifetimeStartOnFirstUse below.
static cl::opt<bool>
ProtectFromEscapedAllocas("protect-from-escaped-allocas",
cl::init(false), cl::Hidden,
cl::desc("Do not optimize lifetime zones that "
"are broken"));
/// Enable enhanced dataflow scheme for lifetime analysis (treat first
/// use of stack slot as start of slot lifetime, as opposed to looking
/// for LIFETIME_START marker). See "Implementation notes" below for
/// more info.
static cl::opt<bool>
LifetimeStartOnFirstUse("stackcoloring-lifetime-start-on-first-use",
cl::init(true), cl::Hidden,
cl::desc("Treat stack lifetimes as starting on first use, not on START marker."));
STATISTIC(NumMarkerSeen, "Number of lifetime markers found.");
STATISTIC(StackSpaceSaved, "Number of bytes saved due to merging slots.");
STATISTIC(StackSlotMerged, "Number of stack slot merged.");
STATISTIC(EscapedAllocas, "Number of allocas that escaped the lifetime region");
//===----------------------------------------------------------------------===//
// StackColoring Pass
//===----------------------------------------------------------------------===//
//
// Stack Coloring reduces stack usage by merging stack slots when they
// can't be used together. For example, consider the following C program:
//
// void bar(char *, int);
// void foo(bool var) {
// A: {
// char z[4096];
// bar(z, 0);
// }
//
// char *p;
// char x[4096];
// char y[4096];
// if (var) {
// p = x;
// } else {
// bar(y, 1);
// p = y + 1024;
// }
// B:
// bar(p, 2);
// }
//
// Naively-compiled, this program would use 12k of stack space. However, the
// stack slot corresponding to `z` is always destroyed before either of the
// stack slots for `x` or `y` are used, and then `x` is only used if `var`
// is true, while `y` is only used if `var` is false. So in no time are 2
// of the stack slots used together, and therefore we can merge them,
// compiling the function using only a single 4k alloca:
//
// void foo(bool var) { // equivalent
// char x[4096];
// char *p;
// bar(x, 0);
// if (var) {
// p = x;
// } else {
// bar(x, 1);
// p = x + 1024;
// }
// bar(p, 2);
// }
//
// This is an important optimization if we want stack space to be under
// control in large functions, both open-coded ones and ones created by
// inlining.
//
// Implementation Notes:
// ---------------------
//
// An important part of the above reasoning is that `z` can't be accessed
// while the latter 2 calls to `bar` are running. This is justified because
// `z`'s lifetime is over after we exit from block `A:`, so any further
// accesses to it would be UB. The way we represent this information
// in LLVM is by having frontends delimit blocks with `lifetime.start`
// and `lifetime.end` intrinsics.
//
// The effect of these intrinsics seems to be as follows (maybe I should
// specify this in the reference?):
//
// L1) at start, each stack-slot is marked as *out-of-scope*, unless no
// lifetime intrinsic refers to that stack slot, in which case
// it is marked as *in-scope*.
// L2) on a `lifetime.start`, a stack slot is marked as *in-scope* and
// the stack slot is overwritten with `undef`.
// L3) on a `lifetime.end`, a stack slot is marked as *out-of-scope*.
// L4) on function exit, all stack slots are marked as *out-of-scope*.
// L5) `lifetime.end` is a no-op when called on a slot that is already
// *out-of-scope*.
// L6) memory accesses to *out-of-scope* stack slots are UB.
// L7) when a stack-slot is marked as *out-of-scope*, all pointers to it
// are invalidated, unless the slot is "degenerate". This is used to
// justify not marking slots as in-use until the pointer to them is
// used, but feels a bit hacky in the presence of things like LICM. See
// the "Degenerate Slots" section for more details.
//
// Now, let's ground stack coloring on these rules. We'll define a slot
// as *in-use* at a (dynamic) point in execution if it either can be
// written to at that point, or if it has a live and non-undef content
// at that point.
//
// Obviously, slots that are never *in-use* together can be merged, and
// in our example `foo`, the slots for `x`, `y` and `z` are never
// in-use together (of course, sometimes slots that *are* in-use together
// might still be mergable, but we don't care about that here).
//
// In this implementation, we successively merge pairs of slots that are
// not *in-use* together. We could be smarter - for example, we could merge
// a single large slot with 2 small slots, or we could construct the
// interference graph and run a "smart" graph coloring algorithm, but with
// that aside, how do we find out whether a pair of slots might be *in-use*
// together?
//
// From our rules, we see that *out-of-scope* slots are never *in-use*,
// and from (L7) we see that "non-degenerate" slots remain non-*in-use*
// until their address is taken. Therefore, we can approximate slot activity
// using dataflow.
//
// A subtle point: naively, we might try to figure out which pairs of
// stack-slots interfere by propagating `S in-use` through the CFG for every
// stack-slot `S`, and having `S` and `T` interfere if there is a CFG point in
// which they are both *in-use*.
//
// That is sound, but overly conservative in some cases: in our (artificial)
// example `foo`, either `x` or `y` might be in use at the label `B:`, but
// as `x` is only in use if we came in from the `var` edge and `y` only
// if we came from the `!var` edge, they still can't be in use together.
// See PR32488 for an important real-life case.
//
// If we wanted to find all points of interference precisely, we could
// propagate `S in-use` and `S&T in-use` predicates through the CFG. That
// would be precise, but requires propagating `O(n^2)` dataflow facts.
//
// However, we aren't interested in the *set* of points of interference
// between 2 stack slots, only *whether* there *is* such a point. So we
// can rely on a little trick: for `S` and `T` to be in-use together,
// one of them needs to become in-use while the other is in-use (or
// they might both become in use simultaneously). We can check this
// by also keeping track of the points at which a stack slot might *start*
// being in-use.
//
// Exact first use:
// ----------------
//
// Consider the following motivating example:
//
// int foo() {
// char b1[1024], b2[1024];
// if (...) {
// char b3[1024];
// <uses of b1, b3>;
// return x;
// } else {
// char b4[1024], b5[1024];
// <uses of b2, b4, b5>;
// return y;
// }
// }
//
// In the code above, "b3" and "b4" are declared in distinct lexical
// scopes, meaning that it is easy to prove that they can share the
// same stack slot. Variables "b1" and "b2" are declared in the same
// scope, meaning that from a lexical point of view, their lifetimes
// overlap. From a control flow pointer of view, however, the two
// variables are accessed in disjoint regions of the CFG, thus it
// should be possible for them to share the same stack slot. An ideal
// stack allocation for the function above would look like:
//
// slot 0: b1, b2
// slot 1: b3, b4
// slot 2: b5
//
// Achieving this allocation is tricky, however, due to the way
// lifetime markers are inserted. Here is a simplified view of the
// control flow graph for the code above:
//
// +------ block 0 -------+
// 0| LIFETIME_START b1, b2 |
// 1| <test 'if' condition> |
// +-----------------------+
// ./ \.
// +------ block 1 -------+ +------ block 2 -------+
// 2| LIFETIME_START b3 | 5| LIFETIME_START b4, b5 |
// 3| <uses of b1, b3> | 6| <uses of b2, b4, b5> |
// 4| LIFETIME_END b3 | 7| LIFETIME_END b4, b5 |
// +-----------------------+ +-----------------------+
// \. /.
// +------ block 3 -------+
// 8| <cleanupcode> |
// 9| LIFETIME_END b1, b2 |
// 10| return |
// +-----------------------+
//
// If we create live intervals for the variables above strictly based
// on the lifetime markers, we'll get the set of intervals on the
// left. If we ignore the lifetime start markers and instead treat a
// variable's lifetime as beginning with the first reference to the
// var, then we get the intervals on the right.
//
// LIFETIME_START First Use
// b1: [0,9] [3,4] [8,9]
// b2: [0,9] [6,9]
// b3: [2,4] [3,4]
// b4: [5,7] [6,7]
// b5: [5,7] [6,7]
//
// For the intervals on the left, the best we can do is overlap two
// variables (b3 and b4, for example); this gives us a stack size of
// 4*1024 bytes, not ideal. When treating first-use as the start of a
// lifetime, we can additionally overlap b1 and b5, giving us a 3*1024
// byte stack (better).
//
// Degenerate Slots:
// -----------------
//
// Relying entirely on first-use of stack slots is problematic,
// however, due to the fact that optimizations can sometimes migrate
// uses of a variable outside of its lifetime start/end region. Here
// is an example:
//
// int bar() {
// char b1[1024], b2[1024];
// if (...) {
// <uses of b2>
// return y;
// } else {
// <uses of b1>
// while (...) {
// char b3[1024];
// <uses of b3>
// }
// }
// }
//
// Before optimization, the control flow graph for the code above
// might look like the following:
//
// +------ block 0 -------+
// 0| LIFETIME_START b1, b2 |
// 1| <test 'if' condition> |
// +-----------------------+
// ./ \.
// +------ block 1 -------+ +------- block 2 -------+
// 2| <uses of b2> | 3| <uses of b1> |
// +-----------------------+ +-----------------------+
// | |
// | +------- block 3 -------+ <-\.
// | 4| <while condition> | |
// | +-----------------------+ |
// | / | |
// | / +------- block 4 -------+
// \ / 5| LIFETIME_START b3 | |
// \ / 6| <uses of b3> | |
// \ / 7| LIFETIME_END b3 | |
// \ | +------------------------+ |
// \ | \ /
// +------ block 5 -----+ \---------------
// 8| <cleanupcode> |
// 9| LIFETIME_END b1, b2 |
// 10| return |
// +---------------------+
//
// During optimization, however, it can happen that an instruction
// computing an address in "b3" (for example, a loop-invariant GEP) is
// hoisted up out of the loop from block 4 to block 2. [Note that
// this is not an actual load from the stack, only an instruction that
// computes the address to be loaded]. If this happens, there is now a
// path leading from the first use of b3 to the return instruction
// that does not encounter the b3 LIFETIME_END, hence b3's lifetime is
// now larger than if we were computing live intervals strictly based
// on lifetime markers. In the example above, this lengthened lifetime
// would mean that it would appear illegal to overlap b3 with b2.
//
// To deal with this such cases, the code in ::collectMarkers() below
// tries to identify "degenerate" slots -- those slots where on a single
// forward pass through the CFG we encounter a first reference to slot
// K before we hit the slot K lifetime start marker. For such slots,
// we fall back on using the lifetime start marker as the beginning of
// the variable's lifetime. NB: with this implementation, slots can
// appear degenerate in cases where there is unstructured control flow:
//
// if (q) goto mid;
// if (x > 9) {
// int b[100];
// memcpy(&b[0], ...);
// mid: b[k] = ...;
// abc(&b);
// }
//
// If in RPO ordering chosen to walk the CFG we happen to visit the b[k]
// before visiting the memcpy block (which will contain the lifetime start
// for "b" then it will appear that 'b' has a degenerate lifetime.
//
namespace {
/// StackColoring - A machine pass for merging disjoint stack allocations,
/// marked by the LIFETIME_START and LIFETIME_END pseudo instructions.
class StackColoring : public MachineFunctionPass {
MachineFrameInfo *MFI;
MachineFunction *MF;
/// A class representing liveness information for a single basic block.
/// Each bit in the BitVector represents the liveness property
/// for a different stack slot.
struct BlockLifetimeInfo {
/// Which slots BEGINs in each basic block.
BitVector Begin;
/// Which slots ENDs in each basic block.
BitVector End;
/// Which slots are marked as LIVE_IN, coming into each basic block.
BitVector LiveIn;
/// Which slots are marked as LIVE_OUT, coming out of each basic block.
BitVector LiveOut;
};
/// Maps active slots (per bit) for each basic block.
using LivenessMap = DenseMap<const MachineBasicBlock *, BlockLifetimeInfo>;
LivenessMap BlockLiveness;
/// Maps serial numbers to basic blocks.
DenseMap<const MachineBasicBlock *, int> BasicBlocks;
/// Maps basic blocks to a serial number.
SmallVector<const MachineBasicBlock *, 8> BasicBlockNumbering;
/// Maps slots to their use interval. Outside of this interval, slots
/// values are either dead or `undef` and they will not be written to.
SmallVector<std::unique_ptr<LiveInterval>, 16> Intervals;
/// Maps slots to the points where they can become in-use.
SmallVector<SmallVector<SlotIndex, 4>, 16> LiveStarts;
/// VNInfo is used for the construction of LiveIntervals.
VNInfo::Allocator VNInfoAllocator;
/// SlotIndex analysis object.
SlotIndexes *Indexes;
/// The list of lifetime markers found. These markers are to be removed
/// once the coloring is done.
SmallVector<MachineInstr*, 8> Markers;
/// Record the FI slots for which we have seen some sort of
/// lifetime marker (either start or end).
BitVector InterestingSlots;
/// FI slots that need to be handled conservatively (for these
/// slots lifetime-start-on-first-use is disabled).
BitVector ConservativeSlots;
/// Number of iterations taken during data flow analysis.
unsigned NumIterations;
public:
static char ID;
StackColoring() : MachineFunctionPass(ID) {
initializeStackColoringPass(*PassRegistry::getPassRegistry());
}
void getAnalysisUsage(AnalysisUsage &AU) const override;
bool runOnMachineFunction(MachineFunction &Func) override;
private:
/// Used in collectMarkers
using BlockBitVecMap = DenseMap<const MachineBasicBlock *, BitVector>;
/// Debug.
void dump() const;
void dumpIntervals() const;
void dumpBB(MachineBasicBlock *MBB) const;
void dumpBV(const char *tag, const BitVector &BV) const;
/// Removes all of the lifetime marker instructions from the function.
/// \returns true if any markers were removed.
bool removeAllMarkers();
/// Scan the machine function and find all of the lifetime markers.
/// Record the findings in the BEGIN and END vectors.
/// \returns the number of markers found.
unsigned collectMarkers(unsigned NumSlot);
/// Perform the dataflow calculation and calculate the lifetime for each of
/// the slots, based on the BEGIN/END vectors. Set the LifetimeLIVE_IN and
/// LifetimeLIVE_OUT maps that represent which stack slots are live coming
/// in and out blocks.
void calculateLocalLiveness();
/// Returns TRUE if we're using the first-use-begins-lifetime method for
/// this slot (if FALSE, then the start marker is treated as start of lifetime).
bool applyFirstUse(int Slot) {
if (!LifetimeStartOnFirstUse || ProtectFromEscapedAllocas)
return false;
if (ConservativeSlots.test(Slot))
return false;
return true;
}
/// Examines the specified instruction and returns TRUE if the instruction
/// represents the start or end of an interesting lifetime. The slot or slots
/// starting or ending are added to the vector "slots" and "isStart" is set
/// accordingly.
/// \returns True if inst contains a lifetime start or end
bool isLifetimeStartOrEnd(const MachineInstr &MI,
SmallVector<int, 4> &slots,
bool &isStart);
/// Construct the LiveIntervals for the slots.
void calculateLiveIntervals(unsigned NumSlots);
/// Go over the machine function and change instructions which use stack
/// slots to use the joint slots.
void remapInstructions(DenseMap<int, int> &SlotRemap);
/// The input program may contain instructions which are not inside lifetime
/// markers. This can happen due to a bug in the compiler or due to a bug in
/// user code (for example, returning a reference to a local variable).
/// This procedure checks all of the instructions in the function and
/// invalidates lifetime ranges which do not contain all of the instructions
/// which access that frame slot.
void removeInvalidSlotRanges();
/// Map entries which point to other entries to their destination.
/// A->B->C becomes A->C.
void expungeSlotMap(DenseMap<int, int> &SlotRemap, unsigned NumSlots);
};
} // end anonymous namespace
char StackColoring::ID = 0;
char &llvm::StackColoringID = StackColoring::ID;
INITIALIZE_PASS_BEGIN(StackColoring, DEBUG_TYPE,
"Merge disjoint stack slots", false, false)
INITIALIZE_PASS_DEPENDENCY(SlotIndexes)
INITIALIZE_PASS_END(StackColoring, DEBUG_TYPE,
"Merge disjoint stack slots", false, false)
void StackColoring::getAnalysisUsage(AnalysisUsage &AU) const {
AU.addRequired<SlotIndexes>();
MachineFunctionPass::getAnalysisUsage(AU);
}
#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
LLVM_DUMP_METHOD void StackColoring::dumpBV(const char *tag,
const BitVector &BV) const {
dbgs() << tag << " : { ";
for (unsigned I = 0, E = BV.size(); I != E; ++I)
dbgs() << BV.test(I) << " ";
dbgs() << "}\n";
}
LLVM_DUMP_METHOD void StackColoring::dumpBB(MachineBasicBlock *MBB) const {
LivenessMap::const_iterator BI = BlockLiveness.find(MBB);
assert(BI != BlockLiveness.end() && "Block not found");
const BlockLifetimeInfo &BlockInfo = BI->second;
dumpBV("BEGIN", BlockInfo.Begin);
dumpBV("END", BlockInfo.End);
dumpBV("LIVE_IN", BlockInfo.LiveIn);
dumpBV("LIVE_OUT", BlockInfo.LiveOut);
}
LLVM_DUMP_METHOD void StackColoring::dump() const {
for (MachineBasicBlock *MBB : depth_first(MF)) {
dbgs() << "Inspecting block #" << MBB->getNumber() << " ["
<< MBB->getName() << "]\n";
dumpBB(MBB);
}
}
LLVM_DUMP_METHOD void StackColoring::dumpIntervals() const {
for (unsigned I = 0, E = Intervals.size(); I != E; ++I) {
dbgs() << "Interval[" << I << "]:\n";
Intervals[I]->dump();
}
}
#endif
static inline int getStartOrEndSlot(const MachineInstr &MI)
{
assert((MI.getOpcode() == TargetOpcode::LIFETIME_START ||
MI.getOpcode() == TargetOpcode::LIFETIME_END) &&
"Expected LIFETIME_START or LIFETIME_END op");
const MachineOperand &MO = MI.getOperand(0);
int Slot = MO.getIndex();
if (Slot >= 0)
return Slot;
return -1;
}
// At the moment the only way to end a variable lifetime is with
// a VARIABLE_LIFETIME op (which can't contain a start). If things
// change and the IR allows for a single inst that both begins
// and ends lifetime(s), this interface will need to be reworked.
bool StackColoring::isLifetimeStartOrEnd(const MachineInstr &MI,
SmallVector<int, 4> &slots,
bool &isStart) {
if (MI.getOpcode() == TargetOpcode::LIFETIME_START ||
MI.getOpcode() == TargetOpcode::LIFETIME_END) {
int Slot = getStartOrEndSlot(MI);
if (Slot < 0)
return false;
if (!InterestingSlots.test(Slot))
return false;
slots.push_back(Slot);
if (MI.getOpcode() == TargetOpcode::LIFETIME_END) {
isStart = false;
return true;
}
if (!applyFirstUse(Slot)) {
isStart = true;
return true;
}
} else if (LifetimeStartOnFirstUse && !ProtectFromEscapedAllocas) {
if (!MI.isDebugInstr()) {
bool found = false;
for (const MachineOperand &MO : MI.operands()) {
if (!MO.isFI())
continue;
int Slot = MO.getIndex();
if (Slot<0)
continue;
if (InterestingSlots.test(Slot) && applyFirstUse(Slot)) {
slots.push_back(Slot);
found = true;
}
}
if (found) {
isStart = true;
return true;
}
}
}
return false;
}
unsigned StackColoring::collectMarkers(unsigned NumSlot) {
unsigned MarkersFound = 0;
BlockBitVecMap SeenStartMap;
InterestingSlots.clear();
InterestingSlots.resize(NumSlot);
ConservativeSlots.clear();
ConservativeSlots.resize(NumSlot);
// number of start and end lifetime ops for each slot
SmallVector<int, 8> NumStartLifetimes(NumSlot, 0);
SmallVector<int, 8> NumEndLifetimes(NumSlot, 0);
// Step 1: collect markers and populate the "InterestingSlots"
// and "ConservativeSlots" sets.
for (MachineBasicBlock *MBB : depth_first(MF)) {
// Compute the set of slots for which we've seen a START marker but have
// not yet seen an END marker at this point in the walk (e.g. on entry
// to this bb).
BitVector BetweenStartEnd;
BetweenStartEnd.resize(NumSlot);
for (MachineBasicBlock::const_pred_iterator PI = MBB->pred_begin(),
PE = MBB->pred_end(); PI != PE; ++PI) {
BlockBitVecMap::const_iterator I = SeenStartMap.find(*PI);
if (I != SeenStartMap.end()) {
BetweenStartEnd |= I->second;
}
}
// Walk the instructions in the block to look for start/end ops.
for (MachineInstr &MI : *MBB) {
if (MI.getOpcode() == TargetOpcode::LIFETIME_START ||
MI.getOpcode() == TargetOpcode::LIFETIME_END) {
int Slot = getStartOrEndSlot(MI);
if (Slot < 0)
continue;
InterestingSlots.set(Slot);
if (MI.getOpcode() == TargetOpcode::LIFETIME_START) {
BetweenStartEnd.set(Slot);
NumStartLifetimes[Slot] += 1;
} else {
BetweenStartEnd.reset(Slot);
NumEndLifetimes[Slot] += 1;
}
const AllocaInst *Allocation = MFI->getObjectAllocation(Slot);
if (Allocation) {
LLVM_DEBUG(dbgs() << "Found a lifetime ");
LLVM_DEBUG(dbgs() << (MI.getOpcode() == TargetOpcode::LIFETIME_START
? "start"
: "end"));
LLVM_DEBUG(dbgs() << " marker for slot #" << Slot);
LLVM_DEBUG(dbgs()
<< " with allocation: " << Allocation->getName() << "\n");
}
Markers.push_back(&MI);
MarkersFound += 1;
} else {
for (const MachineOperand &MO : MI.operands()) {
if (!MO.isFI())
continue;
int Slot = MO.getIndex();
if (Slot < 0)
continue;
if (! BetweenStartEnd.test(Slot)) {
ConservativeSlots.set(Slot);
}
}
}
}
BitVector &SeenStart = SeenStartMap[MBB];
SeenStart |= BetweenStartEnd;
}
if (!MarkersFound) {
return 0;
}
// PR27903: slots with multiple start or end lifetime ops are not
// safe to enable for "lifetime-start-on-first-use".
for (unsigned slot = 0; slot < NumSlot; ++slot)
if (NumStartLifetimes[slot] > 1 || NumEndLifetimes[slot] > 1)
ConservativeSlots.set(slot);
LLVM_DEBUG(dumpBV("Conservative slots", ConservativeSlots));
// Step 2: compute begin/end sets for each block
// NOTE: We use a depth-first iteration to ensure that we obtain a
// deterministic numbering.
for (MachineBasicBlock *MBB : depth_first(MF)) {
// Assign a serial number to this basic block.
BasicBlocks[MBB] = BasicBlockNumbering.size();
BasicBlockNumbering.push_back(MBB);
// Keep a reference to avoid repeated lookups.
BlockLifetimeInfo &BlockInfo = BlockLiveness[MBB];
BlockInfo.Begin.resize(NumSlot);
BlockInfo.End.resize(NumSlot);
SmallVector<int, 4> slots;
for (MachineInstr &MI : *MBB) {
bool isStart = false;
slots.clear();
if (isLifetimeStartOrEnd(MI, slots, isStart)) {
if (!isStart) {
assert(slots.size() == 1 && "unexpected: MI ends multiple slots");
int Slot = slots[0];
if (BlockInfo.Begin.test(Slot)) {
BlockInfo.Begin.reset(Slot);
}
BlockInfo.End.set(Slot);
} else {
for (auto Slot : slots) {
LLVM_DEBUG(dbgs() << "Found a use of slot #" << Slot);
LLVM_DEBUG(dbgs()
<< " at " << printMBBReference(*MBB) << " index ");
LLVM_DEBUG(Indexes->getInstructionIndex(MI).print(dbgs()));
const AllocaInst *Allocation = MFI->getObjectAllocation(Slot);
if (Allocation) {
LLVM_DEBUG(dbgs()
<< " with allocation: " << Allocation->getName());
}
LLVM_DEBUG(dbgs() << "\n");
if (BlockInfo.End.test(Slot)) {
BlockInfo.End.reset(Slot);
}
BlockInfo.Begin.set(Slot);
}
}
}
}
}
// Update statistics.
NumMarkerSeen += MarkersFound;
return MarkersFound;
}
void StackColoring::calculateLocalLiveness() {
unsigned NumIters = 0;
bool changed = true;
while (changed) {
changed = false;
++NumIters;
for (const MachineBasicBlock *BB : BasicBlockNumbering) {
// Use an iterator to avoid repeated lookups.
LivenessMap::iterator BI = BlockLiveness.find(BB);
assert(BI != BlockLiveness.end() && "Block not found");
BlockLifetimeInfo &BlockInfo = BI->second;
// Compute LiveIn by unioning together the LiveOut sets of all preds.
BitVector LocalLiveIn;
for (MachineBasicBlock::const_pred_iterator PI = BB->pred_begin(),
PE = BB->pred_end(); PI != PE; ++PI) {
LivenessMap::const_iterator I = BlockLiveness.find(*PI);
// PR37130: transformations prior to stack coloring can
// sometimes leave behind statically unreachable blocks; these
// can be safely skipped here.
if (I != BlockLiveness.end())
LocalLiveIn |= I->second.LiveOut;
}
// Compute LiveOut by subtracting out lifetimes that end in this
// block, then adding in lifetimes that begin in this block. If
// we have both BEGIN and END markers in the same basic block
// then we know that the BEGIN marker comes after the END,
// because we already handle the case where the BEGIN comes
// before the END when collecting the markers (and building the
// BEGIN/END vectors).
BitVector LocalLiveOut = LocalLiveIn;
LocalLiveOut.reset(BlockInfo.End);
LocalLiveOut |= BlockInfo.Begin;
// Update block LiveIn set, noting whether it has changed.
if (LocalLiveIn.test(BlockInfo.LiveIn)) {
changed = true;
BlockInfo.LiveIn |= LocalLiveIn;
}
// Update block LiveOut set, noting whether it has changed.
if (LocalLiveOut.test(BlockInfo.LiveOut)) {
changed = true;
BlockInfo.LiveOut |= LocalLiveOut;
}
}
} // while changed.
NumIterations = NumIters;
}
void StackColoring::calculateLiveIntervals(unsigned NumSlots) {
SmallVector<SlotIndex, 16> Starts;
SmallVector<bool, 16> DefinitelyInUse;
// For each block, find which slots are active within this block
// and update the live intervals.
for (const MachineBasicBlock &MBB : *MF) {
Starts.clear();
Starts.resize(NumSlots);
DefinitelyInUse.clear();
DefinitelyInUse.resize(NumSlots);
// Start the interval of the slots that we previously found to be 'in-use'.
BlockLifetimeInfo &MBBLiveness = BlockLiveness[&MBB];
for (int pos = MBBLiveness.LiveIn.find_first(); pos != -1;
pos = MBBLiveness.LiveIn.find_next(pos)) {
Starts[pos] = Indexes->getMBBStartIdx(&MBB);
}
// Create the interval for the basic blocks containing lifetime begin/end.
for (const MachineInstr &MI : MBB) {
SmallVector<int, 4> slots;
bool IsStart = false;
if (!isLifetimeStartOrEnd(MI, slots, IsStart))
continue;
SlotIndex ThisIndex = Indexes->getInstructionIndex(MI);
for (auto Slot : slots) {
if (IsStart) {
// If a slot is already definitely in use, we don't have to emit
// a new start marker because there is already a pre-existing
// one.
if (!DefinitelyInUse[Slot]) {
LiveStarts[Slot].push_back(ThisIndex);
DefinitelyInUse[Slot] = true;
}
if (!Starts[Slot].isValid())
Starts[Slot] = ThisIndex;
} else {
if (Starts[Slot].isValid()) {
VNInfo *VNI = Intervals[Slot]->getValNumInfo(0);
Intervals[Slot]->addSegment(
LiveInterval::Segment(Starts[Slot], ThisIndex, VNI));
Starts[Slot] = SlotIndex(); // Invalidate the start index
DefinitelyInUse[Slot] = false;
}
}
}
}
// Finish up started segments
for (unsigned i = 0; i < NumSlots; ++i) {
if (!Starts[i].isValid())
continue;
SlotIndex EndIdx = Indexes->getMBBEndIdx(&MBB);
VNInfo *VNI = Intervals[i]->getValNumInfo(0);
Intervals[i]->addSegment(LiveInterval::Segment(Starts[i], EndIdx, VNI));
}
}
}
bool StackColoring::removeAllMarkers() {
unsigned Count = 0;
for (MachineInstr *MI : Markers) {
MI->eraseFromParent();
Count++;
}
Markers.clear();
LLVM_DEBUG(dbgs() << "Removed " << Count << " markers.\n");
return Count;
}
void StackColoring::remapInstructions(DenseMap<int, int> &SlotRemap) {
unsigned FixedInstr = 0;
unsigned FixedMemOp = 0;
unsigned FixedDbg = 0;
// Remap debug information that refers to stack slots.
for (auto &VI : MF->getVariableDbgInfo()) {
if (!VI.Var)
continue;
if (SlotRemap.count(VI.Slot)) {
LLVM_DEBUG(dbgs() << "Remapping debug info for ["
<< cast<DILocalVariable>(VI.Var)->getName() << "].\n");
VI.Slot = SlotRemap[VI.Slot];
FixedDbg++;
}
}
// Keep a list of *allocas* which need to be remapped.
DenseMap<const AllocaInst*, const AllocaInst*> Allocas;
// Keep a list of allocas which has been affected by the remap.
SmallPtrSet<const AllocaInst*, 32> MergedAllocas;
for (const std::pair<int, int> &SI : SlotRemap) {
const AllocaInst *From = MFI->getObjectAllocation(SI.first);
const AllocaInst *To = MFI->getObjectAllocation(SI.second);
assert(To && From && "Invalid allocation object");
Allocas[From] = To;
// If From is before wo, its possible that there is a use of From between
// them.
if (From->comesBefore(To))
const_cast<AllocaInst*>(To)->moveBefore(const_cast<AllocaInst*>(From));
// AA might be used later for instruction scheduling, and we need it to be
// able to deduce the correct aliasing releationships between pointers
// derived from the alloca being remapped and the target of that remapping.
// The only safe way, without directly informing AA about the remapping
// somehow, is to directly update the IR to reflect the change being made
// here.
Instruction *Inst = const_cast<AllocaInst *>(To);
if (From->getType() != To->getType()) {
BitCastInst *Cast = new BitCastInst(Inst, From->getType());
Cast->insertAfter(Inst);
Inst = Cast;
}
// We keep both slots to maintain AliasAnalysis metadata later.
MergedAllocas.insert(From);
MergedAllocas.insert(To);
// Transfer the stack protector layout tag, but make sure that SSPLK_AddrOf
// does not overwrite SSPLK_SmallArray or SSPLK_LargeArray, and make sure
// that SSPLK_SmallArray does not overwrite SSPLK_LargeArray.
MachineFrameInfo::SSPLayoutKind FromKind
= MFI->getObjectSSPLayout(SI.first);
MachineFrameInfo::SSPLayoutKind ToKind = MFI->getObjectSSPLayout(SI.second);
if (FromKind != MachineFrameInfo::SSPLK_None &&
(ToKind == MachineFrameInfo::SSPLK_None ||
(ToKind != MachineFrameInfo::SSPLK_LargeArray &&
FromKind != MachineFrameInfo::SSPLK_AddrOf)))
MFI->setObjectSSPLayout(SI.second, FromKind);
// The new alloca might not be valid in a llvm.dbg.declare for this
// variable, so undef out the use to make the verifier happy.
AllocaInst *FromAI = const_cast<AllocaInst *>(From);
if (FromAI->isUsedByMetadata())
ValueAsMetadata::handleRAUW(FromAI, UndefValue::get(FromAI->getType()));
for (auto &Use : FromAI->uses()) {
if (BitCastInst *BCI = dyn_cast<BitCastInst>(Use.get()))
if (BCI->isUsedByMetadata())
ValueAsMetadata::handleRAUW(BCI, UndefValue::get(BCI->getType()));
}
// Note that this will not replace uses in MMOs (which we'll update below),
// or anywhere else (which is why we won't delete the original
// instruction).
FromAI->replaceAllUsesWith(Inst);
}
// Remap all instructions to the new stack slots.
std::vector<std::vector<MachineMemOperand *>> SSRefs(
MFI->getObjectIndexEnd());
for (MachineBasicBlock &BB : *MF)
for (MachineInstr &I : BB) {
// Skip lifetime markers. We'll remove them soon.
if (I.getOpcode() == TargetOpcode::LIFETIME_START ||
I.getOpcode() == TargetOpcode::LIFETIME_END)
continue;
// Update the MachineMemOperand to use the new alloca.
for (MachineMemOperand *MMO : I.memoperands()) {
// We've replaced IR-level uses of the remapped allocas, so we only
// need to replace direct uses here.
const AllocaInst *AI = dyn_cast_or_null<AllocaInst>(MMO->getValue());
if (!AI)
continue;
if (!Allocas.count(AI))
continue;
MMO->setValue(Allocas[AI]);
FixedMemOp++;
}
// Update all of the machine instruction operands.
for (MachineOperand &MO : I.operands()) {
if (!MO.isFI())
continue;
int FromSlot = MO.getIndex();
// Don't touch arguments.
if (FromSlot<0)
continue;
// Only look at mapped slots.
if (!SlotRemap.count(FromSlot))
continue;
// In a debug build, check that the instruction that we are modifying is
// inside the expected live range. If the instruction is not inside
// the calculated range then it means that the alloca usage moved
// outside of the lifetime markers, or that the user has a bug.
// NOTE: Alloca address calculations which happen outside the lifetime
// zone are okay, despite the fact that we don't have a good way
// for validating all of the usages of the calculation.
#ifndef NDEBUG
bool TouchesMemory = I.mayLoadOrStore();
// If we *don't* protect the user from escaped allocas, don't bother
// validating the instructions.
if (!I.isDebugInstr() && TouchesMemory && ProtectFromEscapedAllocas) {
SlotIndex Index = Indexes->getInstructionIndex(I);
const LiveInterval *Interval = &*Intervals[FromSlot];
assert(Interval->find(Index) != Interval->end() &&
"Found instruction usage outside of live range.");
}
#endif
// Fix the machine instructions.
int ToSlot = SlotRemap[FromSlot];
MO.setIndex(ToSlot);
FixedInstr++;
}
// We adjust AliasAnalysis information for merged stack slots.
SmallVector<MachineMemOperand *, 2> NewMMOs;
bool ReplaceMemOps = false;
for (MachineMemOperand *MMO : I.memoperands()) {
// Collect MachineMemOperands which reference
// FixedStackPseudoSourceValues with old frame indices.
if (const auto *FSV = dyn_cast_or_null<FixedStackPseudoSourceValue>(
MMO->getPseudoValue())) {
int FI = FSV->getFrameIndex();
auto To = SlotRemap.find(FI);
if (To != SlotRemap.end())
SSRefs[FI].push_back(MMO);
}
// If this memory location can be a slot remapped here,
// we remove AA information.
bool MayHaveConflictingAAMD = false;
if (MMO->getAAInfo()) {
if (const Value *MMOV = MMO->getValue()) {
SmallVector<Value *, 4> Objs;
getUnderlyingObjectsForCodeGen(MMOV, Objs, MF->getDataLayout());
if (Objs.empty())
MayHaveConflictingAAMD = true;
else
for (Value *V : Objs) {
// If this memory location comes from a known stack slot
// that is not remapped, we continue checking.
// Otherwise, we need to invalidate AA infomation.
const AllocaInst *AI = dyn_cast_or_null<AllocaInst>(V);
if (AI && MergedAllocas.count(AI)) {
MayHaveConflictingAAMD = true;
break;
}
}
}
}
if (MayHaveConflictingAAMD) {
NewMMOs.push_back(MF->getMachineMemOperand(MMO, AAMDNodes()));
ReplaceMemOps = true;
} else {
NewMMOs.push_back(MMO);
}
}
// If any memory operand is updated, set memory references of
// this instruction.
if (ReplaceMemOps)
I.setMemRefs(*MF, NewMMOs);
}
// Rewrite MachineMemOperands that reference old frame indices.
for (auto E : enumerate(SSRefs))
if (!E.value().empty()) {
const PseudoSourceValue *NewSV =
MF->getPSVManager().getFixedStack(SlotRemap.find(E.index())->second);
for (MachineMemOperand *Ref : E.value())
Ref->setValue(NewSV);
}
// Update the location of C++ catch objects for the MSVC personality routine.
if (WinEHFuncInfo *EHInfo = MF->getWinEHFuncInfo())
for (WinEHTryBlockMapEntry &TBME : EHInfo->TryBlockMap)
for (WinEHHandlerType &H : TBME.HandlerArray)
if (H.CatchObj.FrameIndex != std::numeric_limits<int>::max() &&
SlotRemap.count(H.CatchObj.FrameIndex))
H.CatchObj.FrameIndex = SlotRemap[H.CatchObj.FrameIndex];
LLVM_DEBUG(dbgs() << "Fixed " << FixedMemOp << " machine memory operands.\n");
LLVM_DEBUG(dbgs() << "Fixed " << FixedDbg << " debug locations.\n");
LLVM_DEBUG(dbgs() << "Fixed " << FixedInstr << " machine instructions.\n");
}
void StackColoring::removeInvalidSlotRanges() {
for (MachineBasicBlock &BB : *MF)
for (MachineInstr &I : BB) {
if (I.getOpcode() == TargetOpcode::LIFETIME_START ||
I.getOpcode() == TargetOpcode::LIFETIME_END || I.isDebugInstr())
continue;
// Some intervals are suspicious! In some cases we find address
// calculations outside of the lifetime zone, but not actual memory
// read or write. Memory accesses outside of the lifetime zone are a clear
// violation, but address calculations are okay. This can happen when
// GEPs are hoisted outside of the lifetime zone.
// So, in here we only check instructions which can read or write memory.
if (!I.mayLoad() && !I.mayStore())
continue;
// Check all of the machine operands.
for (const MachineOperand &MO : I.operands()) {
if (!MO.isFI())
continue;
int Slot = MO.getIndex();
if (Slot<0)
continue;
if (Intervals[Slot]->empty())
continue;
// Check that the used slot is inside the calculated lifetime range.
// If it is not, warn about it and invalidate the range.
LiveInterval *Interval = &*Intervals[Slot];
SlotIndex Index = Indexes->getInstructionIndex(I);
if (Interval->find(Index) == Interval->end()) {
Interval->clear();
LLVM_DEBUG(dbgs() << "Invalidating range #" << Slot << "\n");
EscapedAllocas++;
}
}
}
}
void StackColoring::expungeSlotMap(DenseMap<int, int> &SlotRemap,
unsigned NumSlots) {
// Expunge slot remap map.
for (unsigned i=0; i < NumSlots; ++i) {
// If we are remapping i
if (SlotRemap.count(i)) {
int Target = SlotRemap[i];
// As long as our target is mapped to something else, follow it.
while (SlotRemap.count(Target)) {
Target = SlotRemap[Target];
SlotRemap[i] = Target;
}
}
}
}
bool StackColoring::runOnMachineFunction(MachineFunction &Func) {
LLVM_DEBUG(dbgs() << "********** Stack Coloring **********\n"
<< "********** Function: " << Func.getName() << '\n');
MF = &Func;
MFI = &MF->getFrameInfo();
Indexes = &getAnalysis<SlotIndexes>();
BlockLiveness.clear();
BasicBlocks.clear();
BasicBlockNumbering.clear();
Markers.clear();
Intervals.clear();
LiveStarts.clear();
VNInfoAllocator.Reset();
unsigned NumSlots = MFI->getObjectIndexEnd();
// If there are no stack slots then there are no markers to remove.
if (!NumSlots)
return false;
SmallVector<int, 8> SortedSlots;
SortedSlots.reserve(NumSlots);
Intervals.reserve(NumSlots);
LiveStarts.resize(NumSlots);
unsigned NumMarkers = collectMarkers(NumSlots);
unsigned TotalSize = 0;
LLVM_DEBUG(dbgs() << "Found " << NumMarkers << " markers and " << NumSlots
<< " slots\n");
LLVM_DEBUG(dbgs() << "Slot structure:\n");
for (int i=0; i < MFI->getObjectIndexEnd(); ++i) {
LLVM_DEBUG(dbgs() << "Slot #" << i << " - " << MFI->getObjectSize(i)
<< " bytes.\n");
TotalSize += MFI->getObjectSize(i);
}
LLVM_DEBUG(dbgs() << "Total Stack size: " << TotalSize << " bytes\n\n");
// Don't continue because there are not enough lifetime markers, or the
// stack is too small, or we are told not to optimize the slots.
if (NumMarkers < 2 || TotalSize < 16 || DisableColoring ||
skipFunction(Func.getFunction())) {
LLVM_DEBUG(dbgs() << "Will not try to merge slots.\n");
return removeAllMarkers();
}
for (unsigned i=0; i < NumSlots; ++i) {
std::unique_ptr<LiveInterval> LI(new LiveInterval(i, 0));
LI->getNextValue(Indexes->getZeroIndex(), VNInfoAllocator);
Intervals.push_back(std::move(LI));
SortedSlots.push_back(i);
}
// Calculate the liveness of each block.
calculateLocalLiveness();
LLVM_DEBUG(dbgs() << "Dataflow iterations: " << NumIterations << "\n");
LLVM_DEBUG(dump());
// Propagate the liveness information.
calculateLiveIntervals(NumSlots);
LLVM_DEBUG(dumpIntervals());
// Search for allocas which are used outside of the declared lifetime
// markers.
if (ProtectFromEscapedAllocas)
removeInvalidSlotRanges();
// Maps old slots to new slots.
DenseMap<int, int> SlotRemap;
unsigned RemovedSlots = 0;
unsigned ReducedSize = 0;
// Do not bother looking at empty intervals.
for (unsigned I = 0; I < NumSlots; ++I) {
if (Intervals[SortedSlots[I]]->empty())
SortedSlots[I] = -1;
}
// This is a simple greedy algorithm for merging allocas. First, sort the
// slots, placing the largest slots first. Next, perform an n^2 scan and look
// for disjoint slots. When you find disjoint slots, merge the samller one
// into the bigger one and update the live interval. Remove the small alloca
// and continue.
// Sort the slots according to their size. Place unused slots at the end.
// Use stable sort to guarantee deterministic code generation.
llvm::stable_sort(SortedSlots, [this](int LHS, int RHS) {
// We use -1 to denote a uninteresting slot. Place these slots at the end.
if (LHS == -1)
return false;
if (RHS == -1)
return true;
// Sort according to size.
return MFI->getObjectSize(LHS) > MFI->getObjectSize(RHS);
});
for (auto &s : LiveStarts)
llvm::sort(s);
bool Changed = true;
while (Changed) {
Changed = false;
for (unsigned I = 0; I < NumSlots; ++I) {
if (SortedSlots[I] == -1)
continue;
for (unsigned J=I+1; J < NumSlots; ++J) {
if (SortedSlots[J] == -1)
continue;
int FirstSlot = SortedSlots[I];
int SecondSlot = SortedSlots[J];
LiveInterval *First = &*Intervals[FirstSlot];
LiveInterval *Second = &*Intervals[SecondSlot];
auto &FirstS = LiveStarts[FirstSlot];
auto &SecondS = LiveStarts[SecondSlot];
assert(!First->empty() && !Second->empty() && "Found an empty range");
// Merge disjoint slots. This is a little bit tricky - see the
// Implementation Notes section for an explanation.
if (!First->isLiveAtIndexes(SecondS) &&
!Second->isLiveAtIndexes(FirstS)) {
Changed = true;
First->MergeSegmentsInAsValue(*Second, First->getValNumInfo(0));
int OldSize = FirstS.size();
FirstS.append(SecondS.begin(), SecondS.end());
auto Mid = FirstS.begin() + OldSize;
std::inplace_merge(FirstS.begin(), Mid, FirstS.end());
SlotRemap[SecondSlot] = FirstSlot;
SortedSlots[J] = -1;
LLVM_DEBUG(dbgs() << "Merging #" << FirstSlot << " and slots #"
<< SecondSlot << " together.\n");
Align MaxAlignment = std::max(MFI->getObjectAlign(FirstSlot),
MFI->getObjectAlign(SecondSlot));
assert(MFI->getObjectSize(FirstSlot) >=
MFI->getObjectSize(SecondSlot) &&
"Merging a small object into a larger one");
RemovedSlots+=1;
ReducedSize += MFI->getObjectSize(SecondSlot);
MFI->setObjectAlignment(FirstSlot, MaxAlignment);
MFI->RemoveStackObject(SecondSlot);
}
}
}
}// While changed.
// Record statistics.
StackSpaceSaved += ReducedSize;
StackSlotMerged += RemovedSlots;
LLVM_DEBUG(dbgs() << "Merge " << RemovedSlots << " slots. Saved "
<< ReducedSize << " bytes\n");
// Scan the entire function and update all machine operands that use frame
// indices to use the remapped frame index.
expungeSlotMap(SlotRemap, NumSlots);
remapInstructions(SlotRemap);
return removeAllMarkers();
}
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