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[GlobalOpt] Cache whether CC is changeable #71381

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Nov 7, 2023
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[GlobalOpt] Cache whether CC is changeable #71381

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30 changes: 22 additions & 8 deletions llvm/lib/Transforms/IPO/GlobalOpt.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1680,13 +1680,16 @@ static void RemoveAttribute(Function *F, Attribute::AttrKind A) {
/// idea here is that we don't want to mess with the convention if the user
/// explicitly requested something with performance implications like coldcc,
/// GHC, or anyregcc.
static bool hasChangeableCC(Function *F) {
static bool hasChangeableCCImpl(Function *F) {
CallingConv::ID CC = F->getCallingConv();

// FIXME: Is it worth transforming x86_stdcallcc and x86_fastcallcc?
if (CC != CallingConv::C && CC != CallingConv::X86_ThisCall)
return false;

if (F->isVarArg())
return false;

// FIXME: Change CC for the whole chain of musttail calls when possible.
//
// Can't change CC of the function that either has musttail calls, or is a
Expand All @@ -1706,7 +1709,16 @@ static bool hasChangeableCC(Function *F) {
if (BB.getTerminatingMustTailCall())
return false;

return true;
return !F->hasAddressTaken();
}

using ChangeableCCCacheTy = SmallDenseMap<Function *, bool, 8>;
static bool hasChangeableCC(Function *F,
ChangeableCCCacheTy &ChangeableCCCache) {
auto Res = ChangeableCCCache.try_emplace(F, false);
if (Res.second)
Res.first->second = hasChangeableCCImpl(F);
return Res.first->second;
}

/// Return true if the block containing the call site has a BlockFrequency of
Expand Down Expand Up @@ -1760,7 +1772,8 @@ static void changeCallSitesToColdCC(Function *F) {
// coldcc calling convention.
static bool
hasOnlyColdCalls(Function &F,
function_ref<BlockFrequencyInfo &(Function &)> GetBFI) {
function_ref<BlockFrequencyInfo &(Function &)> GetBFI,
ChangeableCCCacheTy &ChangeableCCCache) {
for (BasicBlock &BB : F) {
for (Instruction &I : BB) {
if (CallInst *CI = dyn_cast<CallInst>(&I)) {
Expand All @@ -1779,8 +1792,7 @@ hasOnlyColdCalls(Function &F,
if (!CalledFn->hasLocalLinkage())
return false;
// Check if it's valid to use coldcc calling convention.
if (!hasChangeableCC(CalledFn) || CalledFn->isVarArg() ||
CalledFn->hasAddressTaken())
if (!hasChangeableCC(CalledFn, ChangeableCCCache))
return false;
BlockFrequencyInfo &CallerBFI = GetBFI(F);
if (!isColdCallSite(*CI, CallerBFI))
Expand Down Expand Up @@ -1905,9 +1917,10 @@ OptimizeFunctions(Module &M,

bool Changed = false;

ChangeableCCCacheTy ChangeableCCCache;
std::vector<Function *> AllCallsCold;
for (Function &F : llvm::make_early_inc_range(M))
if (hasOnlyColdCalls(F, GetBFI))
if (hasOnlyColdCalls(F, GetBFI, ChangeableCCCache))
AllCallsCold.push_back(&F);

// Optimize functions.
Expand Down Expand Up @@ -1969,7 +1982,7 @@ OptimizeFunctions(Module &M,
continue;
}

if (hasChangeableCC(&F) && !F.isVarArg() && !F.hasAddressTaken()) {
if (hasChangeableCC(&F, ChangeableCCCache)) {
NumInternalFunc++;
TargetTransformInfo &TTI = GetTTI(F);
// Change the calling convention to coldcc if either stress testing is
Expand All @@ -1979,14 +1992,15 @@ OptimizeFunctions(Module &M,
if (EnableColdCCStressTest ||
(TTI.useColdCCForColdCall(F) &&
isValidCandidateForColdCC(F, GetBFI, AllCallsCold))) {
ChangeableCCCache.erase(&F);
F.setCallingConv(CallingConv::Cold);
changeCallSitesToColdCC(&F);
Changed = true;
NumColdCC++;
}
}

if (hasChangeableCC(&F) && !F.isVarArg() && !F.hasAddressTaken()) {
if (hasChangeableCC(&F, ChangeableCCCache)) {
// If this function has a calling convention worth changing, is not a
// varargs function, and is only called directly, promote it to use the
// Fast calling convention.
Expand Down