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[AArch64][SVE] Coalesce calls to the SVE ptrue intrinsic where possible
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It is possible to eliminate redundant calls to the SVE ptrue intrinsic.
For example: suppose that we have two SVE ptrue intrinsic calls P1 and
P2. If P1 is at least as wide as P2, then P2 can be written as a
reinterpret P1 using the SVE reinterpret intrinsics.

Coalescing ptrue intrinsics can result in fewer ptrue instructions in
the codegen, and is conducive to better analysis further down the line.

This commit extends the aarch64-sve-intrinsic-opts pass to support
coalescing ptrue intrisic calls.

Differential Revision: https://reviews.llvm.org/D94230
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Joe Ellis committed Feb 4, 2021
1 parent 5626adc commit c9439ca
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222 changes: 215 additions & 7 deletions llvm/lib/Target/AArch64/SVEIntrinsicOpts.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -9,14 +9,20 @@
//
// Performs general IR level optimizations on SVE intrinsics.
//
// The main goal of this pass is to remove unnecessary reinterpret
// intrinsics (llvm.aarch64.sve.convert.[to|from].svbool), e.g:
// This pass performs the following optimizations:
//
// %1 = @llvm.aarch64.sve.convert.to.svbool.nxv4i1(<vscale x 4 x i1> %a)
// %2 = @llvm.aarch64.sve.convert.from.svbool.nxv4i1(<vscale x 16 x i1> %1)
// - removes unnecessary reinterpret intrinsics
// (llvm.aarch64.sve.convert.[to|from].svbool), e.g:
// %1 = @llvm.aarch64.sve.convert.to.svbool.nxv4i1(<vscale x 4 x i1> %a)
// %2 = @llvm.aarch64.sve.convert.from.svbool.nxv4i1(<vscale x 16 x i1> %1)
//
// This pass also looks for ptest intrinsics & phi instructions where the
// operands are being needlessly converted to and from svbool_t.
// - removes unnecessary ptrue intrinsics (llvm.aarch64.sve.ptrue), e.g:
// %1 = @llvm.aarch64.sve.ptrue.nxv4i1(i32 31)
// %2 = @llvm.aarch64.sve.ptrue.nxv8i1(i32 31)
// ; (%1 can be replaced with a reinterpret of %2)
//
// - optimizes ptest intrinsics and phi instructions where the operands are
// being needlessly converted to and from svbool_t.
//
//===----------------------------------------------------------------------===//

Expand Down Expand Up @@ -56,8 +62,17 @@ struct SVEIntrinsicOpts : public ModulePass {
private:
static IntrinsicInst *isReinterpretToSVBool(Value *V);

bool coalescePTrueIntrinsicCalls(BasicBlock &BB,
SmallSetVector<IntrinsicInst *, 4> &PTrues);
bool optimizePTrueIntrinsicCalls(SmallSetVector<Function *, 4> &Functions);

/// Operates at the instruction-scope. I.e., optimizations are applied local
/// to individual instructions.
static bool optimizeIntrinsic(Instruction *I);
bool optimizeIntrinsicCalls(SmallSetVector<Function *, 4> &Functions);

/// Operates at the function-scope. I.e., optimizations are applied local to
/// the functions themselves.
bool optimizeFunctions(SmallSetVector<Function *, 4> &Functions);

static bool optimizeConvertFromSVBool(IntrinsicInst *I);
Expand Down Expand Up @@ -95,6 +110,188 @@ IntrinsicInst *SVEIntrinsicOpts::isReinterpretToSVBool(Value *V) {
return I;
}

/// Checks if a ptrue intrinsic call is promoted. The act of promoting a
/// ptrue will introduce zeroing. For example:
///
/// %1 = <vscale x 4 x i1> call @llvm.aarch64.sve.ptrue.nxv4i1(i32 31)
/// %2 = <vscale x 16 x i1> call @llvm.aarch64.sve.convert.to.svbool.nxv4i1(<vscale x 4 x i1> %1)
/// %3 = <vscale x 8 x i1> call @llvm.aarch64.sve.convert.from.svbool.nxv8i1(<vscale x 16 x i1> %2)
///
/// %1 is promoted, because it is converted:
///
/// <vscale x 4 x i1> => <vscale x 16 x i1> => <vscale x 8 x i1>
///
/// via a sequence of the SVE reinterpret intrinsics convert.{to,from}.svbool.
bool isPTruePromoted(IntrinsicInst *PTrue) {
// Find all users of this intrinsic that are calls to convert-to-svbool
// reinterpret intrinsics.
SmallVector<IntrinsicInst *, 4> ConvertToUses;
for (User *User : PTrue->users()) {
if (match(User, m_Intrinsic<Intrinsic::aarch64_sve_convert_to_svbool>())) {
ConvertToUses.push_back(cast<IntrinsicInst>(User));
}
}

// If no such calls were found, this is ptrue is not promoted.
if (ConvertToUses.empty())
return false;

// Otherwise, try to find users of the convert-to-svbool intrinsics that are
// calls to the convert-from-svbool intrinsic, and would result in some lanes
// being zeroed.
const auto *PTrueVTy = cast<ScalableVectorType>(PTrue->getType());
for (IntrinsicInst *ConvertToUse : ConvertToUses) {
for (User *User : ConvertToUse->users()) {
auto *IntrUser = dyn_cast<IntrinsicInst>(User);
if (IntrUser && IntrUser->getIntrinsicID() ==
Intrinsic::aarch64_sve_convert_from_svbool) {
const auto *IntrUserVTy = cast<ScalableVectorType>(IntrUser->getType());

// Would some lanes become zeroed by the conversion?
if (IntrUserVTy->getElementCount().getKnownMinValue() >
PTrueVTy->getElementCount().getKnownMinValue())
// This is a promoted ptrue.
return true;
}
}
}

// If no matching calls were found, this is not a promoted ptrue.
return false;
}

/// Attempts to coalesce ptrues in a basic block.
bool SVEIntrinsicOpts::coalescePTrueIntrinsicCalls(
BasicBlock &BB, SmallSetVector<IntrinsicInst *, 4> &PTrues) {
if (PTrues.size() <= 1)
return false;

// Find the ptrue with the most lanes.
auto *MostEncompassingPTrue = *std::max_element(
PTrues.begin(), PTrues.end(), [](auto *PTrue1, auto *PTrue2) {
auto *PTrue1VTy = cast<ScalableVectorType>(PTrue1->getType());
auto *PTrue2VTy = cast<ScalableVectorType>(PTrue2->getType());
return PTrue1VTy->getElementCount().getKnownMinValue() <
PTrue2VTy->getElementCount().getKnownMinValue();
});

// Remove the most encompassing ptrue, as well as any promoted ptrues, leaving
// behind only the ptrues to be coalesced.
PTrues.remove(MostEncompassingPTrue);
PTrues.remove_if([](auto *PTrue) { return isPTruePromoted(PTrue); });

// Hoist MostEncompassingPTrue to the start of the basic block. It is always
// safe to do this, since ptrue intrinsic calls are guaranteed to have no
// predecessors.
MostEncompassingPTrue->moveBefore(BB, BB.getFirstInsertionPt());

LLVMContext &Ctx = BB.getContext();
IRBuilder<> Builder(Ctx);
Builder.SetInsertPoint(&BB, ++MostEncompassingPTrue->getIterator());

auto *MostEncompassingPTrueVTy =
cast<VectorType>(MostEncompassingPTrue->getType());
auto *ConvertToSVBool = Builder.CreateIntrinsic(
Intrinsic::aarch64_sve_convert_to_svbool, {MostEncompassingPTrueVTy},
{MostEncompassingPTrue});

for (auto *PTrue : PTrues) {
auto *PTrueVTy = cast<VectorType>(PTrue->getType());

Builder.SetInsertPoint(&BB, ++ConvertToSVBool->getIterator());
auto *ConvertFromSVBool =
Builder.CreateIntrinsic(Intrinsic::aarch64_sve_convert_from_svbool,
{PTrueVTy}, {ConvertToSVBool});
PTrue->replaceAllUsesWith(ConvertFromSVBool);
PTrue->eraseFromParent();
}

return true;
}

/// The goal of this function is to remove redundant calls to the SVE ptrue
/// intrinsic in each basic block within the given functions.
///
/// SVE ptrues have two representations in LLVM IR:
/// - a logical representation -- an arbitrary-width scalable vector of i1s,
/// i.e. <vscale x N x i1>.
/// - a physical representation (svbool, <vscale x 16 x i1>) -- a 16-element
/// scalable vector of i1s, i.e. <vscale x 16 x i1>.
///
/// The SVE ptrue intrinsic is used to create a logical representation of an SVE
/// predicate. Suppose that we have two SVE ptrue intrinsic calls: P1 and P2. If
/// P1 creates a logical SVE predicate that is at least as wide as the logical
/// SVE predicate created by P2, then all of the bits that are true in the
/// physical representation of P2 are necessarily also true in the physical
/// representation of P1. P1 'encompasses' P2, therefore, the intrinsic call to
/// P2 is redundant and can be replaced by an SVE reinterpret of P1 via
/// convert.{to,from}.svbool.
///
/// Currently, this pass only coalesces calls to SVE ptrue intrinsics
/// if they match the following conditions:
///
/// - the call to the intrinsic uses either the SV_ALL or SV_POW2 patterns.
/// SV_ALL indicates that all bits of the predicate vector are to be set to
/// true. SV_POW2 indicates that all bits of the predicate vector up to the
/// largest power-of-two are to be set to true.
/// - the result of the call to the intrinsic is not promoted to a wider
/// predicate. In this case, keeping the extra ptrue leads to better codegen
/// -- coalescing here would create an irreducible chain of SVE reinterprets
/// via convert.{to,from}.svbool.
///
/// EXAMPLE:
///
/// %1 = <vscale x 8 x i1> ptrue(i32 SV_ALL)
/// ; Logical: <1, 1, 1, 1, 1, 1, 1, 1>
/// ; Physical: <1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0>
/// ...
///
/// %2 = <vscale x 4 x i1> ptrue(i32 SV_ALL)
/// ; Logical: <1, 1, 1, 1>
/// ; Physical: <1, 0, 0, 0, 1, 0, 0, 0, 1, 0, 0, 0, 1, 0, 0, 0>
/// ...
///
/// Here, %2 can be replaced by an SVE reinterpret of %1, giving, for instance:
///
/// %1 = <vscale x 8 x i1> ptrue(i32 i31)
/// %2 = <vscale x 16 x i1> convert.to.svbool(<vscale x 8 x i1> %1)
/// %3 = <vscale x 4 x i1> convert.from.svbool(<vscale x 16 x i1> %2)
///
bool SVEIntrinsicOpts::optimizePTrueIntrinsicCalls(
SmallSetVector<Function *, 4> &Functions) {
bool Changed = false;

for (auto *F : Functions) {
for (auto &BB : *F) {
SmallSetVector<IntrinsicInst *, 4> SVAllPTrues;
SmallSetVector<IntrinsicInst *, 4> SVPow2PTrues;

// For each basic block, collect the used ptrues and try to coalesce them.
for (Instruction &I : BB) {
if (I.use_empty())
continue;

auto *IntrI = dyn_cast<IntrinsicInst>(&I);
if (!IntrI || IntrI->getIntrinsicID() != Intrinsic::aarch64_sve_ptrue)
continue;

const auto PTruePattern =
cast<ConstantInt>(IntrI->getOperand(0))->getZExtValue();

if (PTruePattern == AArch64SVEPredPattern::all)
SVAllPTrues.insert(IntrI);
if (PTruePattern == AArch64SVEPredPattern::pow2)
SVPow2PTrues.insert(IntrI);
}

Changed |= coalescePTrueIntrinsicCalls(BB, SVAllPTrues);
Changed |= coalescePTrueIntrinsicCalls(BB, SVPow2PTrues);
}
}

return Changed;
}

/// The function will remove redundant reinterprets casting in the presence
/// of the control flow
bool SVEIntrinsicOpts::processPhiNode(IntrinsicInst *X) {
Expand Down Expand Up @@ -243,7 +440,7 @@ bool SVEIntrinsicOpts::optimizeIntrinsic(Instruction *I) {
return true;
}

bool SVEIntrinsicOpts::optimizeFunctions(
bool SVEIntrinsicOpts::optimizeIntrinsicCalls(
SmallSetVector<Function *, 4> &Functions) {
bool Changed = false;
for (auto *F : Functions) {
Expand All @@ -260,6 +457,16 @@ bool SVEIntrinsicOpts::optimizeFunctions(
return Changed;
}

bool SVEIntrinsicOpts::optimizeFunctions(
SmallSetVector<Function *, 4> &Functions) {
bool Changed = false;

Changed |= optimizePTrueIntrinsicCalls(Functions);
Changed |= optimizeIntrinsicCalls(Functions);

return Changed;
}

bool SVEIntrinsicOpts::runOnModule(Module &M) {
bool Changed = false;
SmallSetVector<Function *, 4> Functions;
Expand All @@ -276,6 +483,7 @@ bool SVEIntrinsicOpts::runOnModule(Module &M) {
case Intrinsic::aarch64_sve_ptest_any:
case Intrinsic::aarch64_sve_ptest_first:
case Intrinsic::aarch64_sve_ptest_last:
case Intrinsic::aarch64_sve_ptrue:
for (User *U : F.users())
Functions.insert(cast<Instruction>(U)->getFunction());
break;
Expand Down
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