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[GlobalMerge][NFC] Skip sorting by profitability when it is not needed #124146

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Jan 24, 2025
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[GlobalMerge][NFC] Skip sorting by profitability when it is not needed #124146

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[GlobalMerge][NFC] Skip sorting by profitability when it is not needed
michaelmaitland Jan 23, 2025
f71bd49
fixup! clang-format
michaelmaitland Jan 23, 2025
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26 changes: 12 additions & 14 deletions llvm/lib/CodeGen/GlobalMerge.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -423,24 +423,12 @@ bool GlobalMergeImpl::doMerge(SmallVectorImpl<GlobalVariable *> &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 functions
// that use that exact set of globals).
//
// Multiply that by the size of the set to give us a crude profitability
// metric.
llvm::stable_sort(UsedGlobalSets,
[](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 (const UsedGlobalSet &UGS : llvm::reverse(UsedGlobalSets)) {
for (const UsedGlobalSet &UGS : UsedGlobalSets) {
if (UGS.UsageCount == 0)
continue;
if (UGS.Globals.count() > 1)
Expand All @@ -449,6 +437,16 @@ bool GlobalMergeImpl::doMerge(SmallVectorImpl<GlobalVariable *> &Globals,
return doMerge(Globals, AllGlobals, M, isConst, AddrSpace);
}

// 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 functions
// that use that exact set of globals). Multiply that by the size of the set
// to give us a crude profitability metric.
llvm::stable_sort(UsedGlobalSets,
[](const UsedGlobalSet &UGS1, const UsedGlobalSet &UGS2) {
return UGS1.Globals.count() * UGS1.UsageCount >=
UGS2.Globals.count() * UGS2.UsageCount;
});

// 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
Expand All @@ -458,7 +456,7 @@ bool GlobalMergeImpl::doMerge(SmallVectorImpl<GlobalVariable *> &Globals,
BitVector PickedGlobals(Globals.size());
bool Changed = false;

for (const UsedGlobalSet &UGS : llvm::reverse(UsedGlobalSets)) {
for (const UsedGlobalSet &UGS : UsedGlobalSets) {
if (UGS.UsageCount == 0)
continue;
if (PickedGlobals.anyCommon(UGS.Globals))
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