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[FunctionAttrs] deduce attr cold on functions if all CG paths call a cold function #101298

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[FunctionAttrs] deduce attr cold on functions if all CG paths call a cold function #101298

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[FunctionAttrs] Add tests for deducing attr `cold` on functions; NFC
goldsteinn Jul 31, 2024
85efc82
[FunctionAttrs] deduce attr `cold` on functions if all CG paths call …
goldsteinn Jul 31, 2024
3df7969
fixup fmt
goldsteinn Jul 31, 2024
7d22a7e
Fix fmt 2
goldsteinn Aug 11, 2024
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goldsteinn Aug 18, 2024
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69 changes: 69 additions & 0 deletions llvm/lib/Transforms/IPO/FunctionAttrs.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -82,6 +82,7 @@ STATISTIC(NumNoUnwind, "Number of functions marked as nounwind");
STATISTIC(NumNoFree, "Number of functions marked as nofree");
STATISTIC(NumWillReturn, "Number of functions marked as willreturn");
STATISTIC(NumNoSync, "Number of functions marked as nosync");
STATISTIC(NumCold, "Number of functions marked as cold");

STATISTIC(NumThinLinkNoRecurse,
"Number of functions marked as norecurse during thinlink");
Expand Down Expand Up @@ -1745,6 +1746,7 @@ static bool canReturn(Function &F) {
return false;
}


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// Set the noreturn function attribute if possible.
static void addNoReturnAttrs(const SCCNodeSet &SCCNodes,
SmallSet<Function *, 8> &Changed) {
Expand All @@ -1760,6 +1762,72 @@ static void addNoReturnAttrs(const SCCNodeSet &SCCNodes,
}
}

static bool
allBBPathsGoThroughCold(BasicBlock *BB,
SmallDenseMap<BasicBlock *, bool, 16> &Visited) {
// If BB contains a cold callsite this path through the CG is cold.
// Ignore whether the instructions actually are guranteed to transfer
// execution. Divergent behavior is considered unlikely.
if (any_of(*BB, [](Instruction &I) {
if (auto *CB = dyn_cast<CallBase>(&I))
return CB->hasFnAttr(Attribute::Cold);
return false;
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@nikic nikic Aug 18, 2024 •
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I'd add a note that this assumes throwing and diverging code paths are cold, which is why this does not check for guaranteed-to-transfer.

})) {
Visited[BB] = true;
return true;
}

auto Succs = successors(BB);
// We found a path that doesn't go through any cold callsite.
if (Succs.empty())
return false;

// We didn't find a cold callsite in this BB, so check that all successors
// contain a cold callsite (or that their successors do).
// Potential TODO: We could use static branch hints to assume certain
// successor paths are inherently cold, irrespective of if they contain a cold
// callsite.
for (auto *Succ : Succs) {
// Start with false, this is necessary to ensure we don't turn loops into
// cold.
auto R = Visited.try_emplace(Succ, false);
if (!R.second) {
if (R.first->second)
continue;
return false;
}
if (!allBBPathsGoThroughCold(Succ, Visited))
return false;
Visited[Succ] = true;
}

return true;
}

static bool allPathsGoThroughCold(Function &F) {
SmallDenseMap<BasicBlock *, bool, 16> Visited;
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It would be simpler to turn this into a BFS traversal?

SmallVector<BB> WorkList, Visited;
WorkList.push(entryBB);

while(!WorkList.empty()) {
  BB = WorkList.pop();
  if (BB contains cold calls)
     continue;

  if (BB contains return/calls without willreturn)
     return true;

   Visited.insert(BB);
  
   for (auto *SuccBB: successors(BB))
       if (!Visited.contains(SuccBB))
           WorkList.push_back(SuccBB);
}
return false;

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@goldsteinn goldsteinn Jul 31, 2024 •
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Thats what I originally had, but was running into issues with inf loops. (edit: not LLVM inf loops, but marking inf loops as cold).

Basically we need more information that just "did we visit this", we need "did we visit this and find a cold call site". I don't really know how to do that without depth-first.

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The basic calculus I had was we can either be marking:

entry:
  br label %loop
loop:
  br label %loop

as cold or we can miss the case:

entry:
br label %loop
loop:
%c = <some val>
br i1 %c, label %loop, label %done
done:
<cold call>

and think its better to err on under-attribution rather than over.

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@goldsteinn goldsteinn Jul 31, 2024 •
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(we could of course handle this with a dom tree, but I don't think its worth the compile time/complexity). I think most functions we will mark end up essentially being error wrappers which don't have very complex CGs.

Visited[&F.front()] = false;
return allBBPathsGoThroughCold(&F.front(), Visited);
}

// Set the cold function attribute if possible.
static void addColdAttrs(const SCCNodeSet &SCCNodes,
SmallSet<Function *, 8> &Changed) {
for (Function *F : SCCNodes) {
if (!F || !F->hasExactDefinition() || F->hasFnAttribute(Attribute::Naked) ||
F->hasFnAttribute(Attribute::Cold) || F->hasFnAttribute(Attribute::Hot))
continue;

// Potential TODO: We could add attribute `cold` on functions with `coldcc`.
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Please remove this todo. It has been implemented in GlobalOpt:

llvm-project/llvm/lib/Transforms/IPO/GlobalOpt.cpp

Lines 1997 to 2013 in 5286656

if (hasChangeableCC(&F, ChangeableCCCache)) {
NumInternalFunc++;
TargetTransformInfo &TTI = GetTTI(F);
// Change the calling convention to coldcc if either stress testing is
// enabled or the target would like to use coldcc on functions which are
// cold at all call sites and the callers contain no other non coldcc
// calls.
if (EnableColdCCStressTest ||
(TTI.useColdCCForColdCall(F) &&
isValidCandidateForColdCC(F, GetBFI, AllCallsCold))) {
ChangeableCCCache.erase(&F);
F.setCallingConv(CallingConv::Cold);
changeCallSitesToColdCC(&F);
Changed = true;
NumColdCC++;
}
}

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@goldsteinn goldsteinn Aug 11, 2024 •
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Hmm, this is the other way around where we are add coldcc to functions. The todo is saying add attribute cold on functions that have coldcc.

if (allPathsGoThroughCold(*F)) {
F->addFnAttr(Attribute::Cold);
++NumCold;
Changed.insert(F);
continue;
}
}
}

static bool functionWillReturn(const Function &F) {
// We can infer and propagate function attributes only when we know that the
// definition we'll get at link time is *exactly* the definition we see now.
Expand Down Expand Up @@ -1853,6 +1921,7 @@ deriveAttrsInPostOrder(ArrayRef<Function *> Functions, AARGetterT &&AARGetter,
addArgumentAttrs(Nodes.SCCNodes, Changed);
inferConvergent(Nodes.SCCNodes, Changed);
addNoReturnAttrs(Nodes.SCCNodes, Changed);
addColdAttrs(Nodes.SCCNodes, Changed);
addWillReturn(Nodes.SCCNodes, Changed);
addNoUndefAttrs(Nodes.SCCNodes, Changed);

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