bool GlobalMerge::doMerge(SmallVectorImpl<GlobalVariable*> &Globals,
Module &M, bool isConst, unsigned AddrSpace) const {
auto &DL = M.getDataLayout();
// FIXME: Find better heuristics
std::stable_sort(Globals.begin(), Globals.end(),
[&DL](const GlobalVariable *GV1, const GlobalVariable *GV2) {
return DL.getTypeAllocSize(GV1->getValueType()) <
DL.getTypeAllocSize(GV2->getValueType());
});
// If we want to just blindly group all globals together, do so.
if (!GlobalMergeGroupByUse) {
BitVector AllGlobals(Globals.size());
AllGlobals.set();
return doMerge(Globals, AllGlobals, M, isConst, AddrSpace);
}
// If we want to be smarter, look at all uses of each global, to try to
// discover all sets of globals used together, and how many times each of
// these sets occurred.
//
// Keep this reasonably efficient, by having an append-only list of all sets
// discovered so far (UsedGlobalSet), and mapping each "together-ness" unit of
// code (currently, a Function) to the set of globals seen so far that are
// used together in that unit (GlobalUsesByFunction).
//
// When we look at the Nth global, we know that any new set is either:
// - the singleton set {N}, containing this global only, or
// - the union of {N} and a previously-discovered set, containing some
// combination of the previous N-1 globals.
// Using that knowledge, when looking at the Nth global, we can keep:
// - a reference to the singleton set {N} (CurGVOnlySetIdx)
// - a list mapping each previous set to its union with {N} (EncounteredUGS),
// if it actually occurs.
// We keep track of the sets of globals used together "close enough".
struct UsedGlobalSet {
BitVector Globals;
unsigned UsageCount = 1;
UsedGlobalSet(size_t Size) : Globals(Size) {}
};
// Each set is unique in UsedGlobalSets.
std::vector<UsedGlobalSet> UsedGlobalSets;
// Avoid repeating the create-global-set pattern.
auto CreateGlobalSet = [&]() -> UsedGlobalSet & {
UsedGlobalSets.emplace_back(Globals.size());
return UsedGlobalSets.back();
};
// The first set is the empty set.
CreateGlobalSet().UsageCount = 0;
// We define "close enough" to be "in the same function".
// FIXME: Grouping uses by function is way too aggressive, so we should have
// a better metric for distance between uses.
// The obvious alternative would be to group by BasicBlock, but that's in
// turn too conservative..
// Anything in between wouldn't be trivial to compute, so just stick with
// per-function grouping.
// The value type is an index into UsedGlobalSets.
// The default (0) conveniently points to the empty set.
DenseMap<Function *, size_t /*UsedGlobalSetIdx*/> GlobalUsesByFunction;
// Now, look at each merge-eligible global in turn.
// Keep track of the sets we already encountered to which we added the
// current global.
// Each element matches the same-index element in UsedGlobalSets.
// This lets us efficiently tell whether a set has already been expanded to
// include the current global.
std::vector<size_t> EncounteredUGS;
for (size_t GI = 0, GE = Globals.size(); GI != GE; ++GI) {
GlobalVariable *GV = Globals[GI];
// Reset the encountered sets for this global...
std::fill(EncounteredUGS.begin(), EncounteredUGS.end(), 0);
// ...and grow it in case we created new sets for the previous global.
EncounteredUGS.resize(UsedGlobalSets.size());
// We might need to create a set that only consists of the current global.
// Keep track of its index into UsedGlobalSets.
size_t CurGVOnlySetIdx = 0;
// For each global, look at all its Uses.
for (auto &U : GV->uses()) {
// This Use might be a ConstantExpr. We're interested in Instruction
// users, so look through ConstantExpr...
Use *UI, *UE;
if (ConstantExpr *CE = dyn_cast<ConstantExpr>(U.getUser())) {
if (CE->use_empty())
continue;
UI = &*CE->use_begin();
UE = nullptr;
} else if (isa<Instruction>(U.getUser())) {
UI = &U;
UE = UI->getNext();
} else {
continue;
}
// ...to iterate on all the instruction users of the global.
// Note that we iterate on Uses and not on Users to be able to getNext().
for (; UI != UE; UI = UI->getNext()) {
Instruction *I = dyn_cast<Instruction>(UI->getUser());
if (!I)
continue;
Function *ParentFn = I->getParent()->getParent();
// If we're only optimizing for size, ignore non-minsize functions.
if (OnlyOptimizeForSize && !ParentFn->optForMinSize())
continue;
size_t UGSIdx = GlobalUsesByFunction[ParentFn];
// If this is the first global the basic block uses, map it to the set
// consisting of this global only.
if (!UGSIdx) {
// If that set doesn't exist yet, create it.
if (!CurGVOnlySetIdx) {
CurGVOnlySetIdx = UsedGlobalSets.size();
CreateGlobalSet().Globals.set(GI);
} else {
++UsedGlobalSets[CurGVOnlySetIdx].UsageCount;
}
GlobalUsesByFunction[ParentFn] = CurGVOnlySetIdx;
continue;
}
// If we already encountered this BB, just increment the counter.
if (UsedGlobalSets[UGSIdx].Globals.test(GI)) {
++UsedGlobalSets[UGSIdx].UsageCount;
continue;
}
// If not, the previous set wasn't actually used in this function.
--UsedGlobalSets[UGSIdx].UsageCount;
// If we already expanded the previous set to include this global, just
// reuse that expanded set.
if (size_t ExpandedIdx = EncounteredUGS[UGSIdx]) {
++UsedGlobalSets[ExpandedIdx].UsageCount;
GlobalUsesByFunction[ParentFn] = ExpandedIdx;
continue;
}
// If not, create a new set consisting of the union of the previous set
// and this global. Mark it as encountered, so we can reuse it later.
GlobalUsesByFunction[ParentFn] = EncounteredUGS[UGSIdx] =
UsedGlobalSets.size();
UsedGlobalSet &NewUGS = CreateGlobalSet();
NewUGS.Globals.set(GI);
NewUGS.Globals |= UsedGlobalSets[UGSIdx].Globals;
}
}
}
// Now we found a bunch of sets of globals used together. We accumulated
// the number of times we encountered the sets (i.e., the number of blocks
// that use that exact set of globals).
//
// Multiply that by the size of the set to give us a crude profitability
// metric.
std::stable_sort(UsedGlobalSets.begin(), UsedGlobalSets.end(),
[](const UsedGlobalSet &UGS1, const UsedGlobalSet &UGS2) {
return UGS1.Globals.count() * UGS1.UsageCount <
UGS2.Globals.count() * UGS2.UsageCount;
});
// We can choose to merge all globals together, but ignore globals never used
// with another global. This catches the obviously non-profitable cases of
// having a single global, but is aggressive enough for any other case.
if (GlobalMergeIgnoreSingleUse) {
BitVector AllGlobals(Globals.size());
for (size_t i = 0, e = UsedGlobalSets.size(); i != e; ++i) {
const UsedGlobalSet &UGS = UsedGlobalSets[e - i - 1];
if (UGS.UsageCount == 0)
continue;
if (UGS.Globals.count() > 1)
AllGlobals |= UGS.Globals;
}
return doMerge(Globals, AllGlobals, M, isConst, AddrSpace);
}
// Starting from the sets with the best (=biggest) profitability, find a
// good combination.
// The ideal (and expensive) solution can only be found by trying all
// combinations, looking for the one with the best profitability.
// Don't be smart about it, and just pick the first compatible combination,
// starting with the sets with the best profitability.
BitVector PickedGlobals(Globals.size());
bool Changed = false;
for (size_t i = 0, e = UsedGlobalSets.size(); i != e; ++i) {
const UsedGlobalSet &UGS = UsedGlobalSets[e - i - 1];
if (UGS.UsageCount == 0)
continue;
if (PickedGlobals.anyCommon(UGS.Globals))
continue;
PickedGlobals |= UGS.Globals;
// If the set only contains one global, there's no point in merging.
// Ignore the global for inclusion in other sets though, so keep it in
// PickedGlobals.
if (UGS.Globals.count() < 2)
continue;
Changed |= doMerge(Globals, UGS.Globals, M, isConst, AddrSpace);
}
return Changed;
}
