[clang] [llvm] [transforms] Inline simple variadic functions (PR #81058)
Joseph Huber via llvm-commits
llvm-commits at lists.llvm.org
Wed Feb 7 17:59:58 PST 2024
================
@@ -0,0 +1,698 @@
+//===-- ExpandVariadicsPass.cpp --------------------------------*- C++ -*-=//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// This is an optimisation pass for variadic functions. If called from codegen,
+// it can serve as the implementation of variadic functions for a given target.
+//
+// The target-dependent parts are in namespace VariadicABIInfo. Enabling a new
+// target means adding a case to VariadicABIInfo::create() along with tests.
+//
+// The module pass using that information is class ExpandVariadics.
+//
+// The strategy is:
+// 1. Test whether a variadic function is sufficiently simple
+// 2. If it was, calls to it can be replaced with calls to a different function
+// 3. If it wasn't, try to split it into a simple function and a remainder
+// 4. Optionally rewrite the varadic function calling convention as well
+//
+// This pass considers "sufficiently simple" to mean a variadic function that
+// calls into a different function taking a va_list to do the real work. For
+// example, libc might implement fprintf as a single basic block calling into
+// vfprintf. This pass can then rewrite call to the variadic into some code
+// to construct a target-specific value to use for the va_list and a call
+// into the non-variadic implementation function. There's a test for that.
+//
+// Most other variadic functions whose definition is known can be converted into
+// that form. Create a new internal function taking a va_list where the original
+// took a ... parameter. Move the blocks across. Create a new block containing a
+// va_start that calls into the new function. This is nearly target independent.
+//
+// Where this transform is consistent with the ABI, e.g. AMDGPU or NVPTX, or
+// where the ABI can be chosen to align with this transform, the function
+// interface can be rewritten along with calls to unknown variadic functions.
+//
+// The aggregate effect is to unblock other transforms, most critically the
+// general purpose inliner. Known calls to variadic functions become zero cost.
+//
+// This pass does define some target specific information which is partially
+// redundant with other parts of the compiler. In particular, the call frame
+// it builds must be the exact complement of the va_arg lowering performed
+// by clang. The va_list construction is similar to work done by the backend
+// for targets that lower variadics there, though distinct in that this pass
+// constructs the pieces using alloca instead of relative to stack pointers.
+//
+// Consistency with clang is primarily tested by emitting va_arg using clang
+// then expanding the variadic functions using this pass, followed by trying
+// to constant fold the functions to no-ops.
+//
+// Target specific behaviour is tested in IR - mainly checking that values are
+// put into positions in call frames that make sense for that particular target.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Transforms/IPO/ExpandVariadics.h"
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/CodeGen/Passes.h"
+#include "llvm/IR/Constants.h"
+#include "llvm/IR/IRBuilder.h"
+#include "llvm/IR/IntrinsicInst.h"
+#include "llvm/IR/Module.h"
+#include "llvm/IR/PassManager.h"
+#include "llvm/InitializePasses.h"
+#include "llvm/Pass.h"
+#include "llvm/TargetParser/Triple.h"
+
+#define DEBUG_TYPE "expand-variadics"
+
+using namespace llvm;
+
+namespace {
+namespace VariadicABIInfo {
+
+// calling convention for passing as valist object, same as it would be in C
+// aarch64 uses byval
+enum class valistCC { value, pointer, /*byval*/ };
+
+struct Interface {
+protected:
+ Interface(uint32_t MinAlign, uint32_t MaxAlign)
+ : MinAlign(MinAlign), MaxAlign(MaxAlign) {}
+
+public:
+ virtual ~Interface() {}
+ const uint32_t MinAlign;
+ const uint32_t MaxAlign;
+
+ // Most ABIs use a void* or char* for va_list, others can specialise
+ virtual Type *vaListType(LLVMContext &Ctx) {
+ return PointerType::getUnqual(Ctx);
+ }
+
+ // How the vaListType is passed
+ virtual valistCC vaListCC() { return valistCC::value; }
+
+ // The valist might need to be stack allocated.
+ virtual bool valistOnStack() { return false; }
+
+ virtual void initializeVAList(LLVMContext &Ctx, IRBuilder<> &Builder,
+ AllocaInst * /*va_list*/, Value * /*buffer*/) {
+ // Function needs to be implemented if valist is on the stack
+ assert(!valistOnStack());
+ __builtin_unreachable();
+ }
+
+ // All targets currently implemented use a ptr for the valist parameter
+ Type *vaListParameterType(LLVMContext &Ctx) {
+ return PointerType::getUnqual(Ctx);
+ }
+
+ bool VAEndIsNop() { return true; }
+
+ bool VACopyIsMemcpy() { return true; }
+};
+
+struct X64SystemV final : public Interface {
+ // X64 documented behaviour:
+ // Slots are at least eight byte aligned and at most 16 byte aligned.
+ // If the type needs more than sixteen byte alignment, it still only gets
+ // that much alignment on the stack.
+ // X64 behaviour in clang:
+ // Slots are at least eight byte aligned and at most naturally aligned
+ // This matches clang, not the ABI docs.
+ X64SystemV() : Interface(8, 0) {}
+
+ Type *vaListType(LLVMContext &Ctx) override {
+ auto I32 = Type::getInt32Ty(Ctx);
+ auto Ptr = PointerType::getUnqual(Ctx);
+ return ArrayType::get(StructType::get(Ctx, {I32, I32, Ptr, Ptr}), 1);
+ }
+ valistCC vaListCC() override { return valistCC::pointer; }
+
+ bool valistOnStack() override { return true; }
+
+ void initializeVAList(LLVMContext &Ctx, IRBuilder<> &Builder,
+ AllocaInst *va_list, Value *voidBuffer) override {
+ assert(valistOnStack());
+ assert(va_list != nullptr);
+ assert(va_list->getAllocatedType() == vaListType(Ctx));
+
+ Type *va_list_ty = vaListType(Ctx);
+
+ Type *I32 = Type::getInt32Ty(Ctx);
+ Type *I64 = Type::getInt64Ty(Ctx);
+
+ Value *Idxs[3] = {
+ ConstantInt::get(I64, 0),
+ ConstantInt::get(I32, 0),
+ nullptr,
+ };
+
+ Idxs[2] = ConstantInt::get(I32, 0);
+ Builder.CreateStore(
+ ConstantInt::get(I32, 48),
+ Builder.CreateInBoundsGEP(va_list_ty, va_list, Idxs, "gp_offset"));
+
+ Idxs[2] = ConstantInt::get(I32, 1);
+ Builder.CreateStore(
+ ConstantInt::get(I32, 6 * 8 + 8 * 16),
+ Builder.CreateInBoundsGEP(va_list_ty, va_list, Idxs, "fp_offset"));
+
+ Idxs[2] = ConstantInt::get(I32, 2);
+ Builder.CreateStore(voidBuffer,
+ Builder.CreateInBoundsGEP(va_list_ty, va_list, Idxs,
+ "overfow_arg_area"));
+
+ Idxs[2] = ConstantInt::get(I32, 3);
+ Builder.CreateStore(
+ ConstantPointerNull::get(PointerType::getUnqual(Ctx)),
+ Builder.CreateInBoundsGEP(va_list_ty, va_list, Idxs, "reg_save_area"));
+ }
+};
+
+std::unique_ptr<Interface> create(Module &M) {
+ llvm::Triple Triple(M.getTargetTriple());
+ const bool IsLinuxABI = Triple.isOSLinux() || Triple.isOSCygMing();
+
+ switch (Triple.getArch()) {
+
+ case Triple::x86: {
+ // These seem to all fall out the same, despite getTypeStackAlign
+ // implying otherwise.
+ if (Triple.isOSDarwin()) {
+ struct X86Darwin final : public Interface {
+ // X86_32ABIInfo::getTypeStackAlignInBytes is misleading for this.
+ // The slotSize(4) implies a minimum alignment
+ // The AllowHigherAlign = true means there is no maximum alignment.
+ X86Darwin() : Interface(4, 0) {}
+ };
+
+ return std::make_unique<X86Darwin>();
+ }
+ if (Triple.getOS() == llvm::Triple::Win32) {
+ struct X86Windows final : public Interface {
+ X86Windows() : Interface(4, 0) {}
+ };
+ return std::make_unique<X86Windows>();
+ }
+
+ if (IsLinuxABI) {
+ struct X86Linux final : public Interface {
+ X86Linux() : Interface(4, 0) {}
+ };
+ return std::make_unique<X86Linux>();
+ }
+ break;
+ }
+
+ case Triple::x86_64: {
+ if (Triple.isWindowsMSVCEnvironment() || Triple.isOSWindows()) {
+ struct X64Windows final : public Interface {
+ X64Windows() : Interface(8, 8) {}
+ };
+ // x64 msvc emit vaarg passes > 8 byte values by pointer
+ // however the variadic call instruction created does not, e.g.
+ // a <4 x f32> will be passed as itself, not as a pointer or byval.
+ // Postponing resolution of that for now.
+ return nullptr;
+ }
+
+ if (Triple.isOSDarwin()) {
+ return std::make_unique<VariadicABIInfo::X64SystemV>();
+ }
+
+ if (IsLinuxABI) {
+ return std::make_unique<VariadicABIInfo::X64SystemV>();
+ }
+
+ break;
+ }
+
+ default:
+ return nullptr;
+ }
+
+ return nullptr;
+}
+
+} // namespace VariadicABIInfo
+
+class ExpandVariadics : public ModulePass {
+public:
+ static char ID;
+ std::unique_ptr<VariadicABIInfo::Interface> ABI;
+
+ ExpandVariadics() : ModulePass(ID) {}
+ StringRef getPassName() const override { return "Expand variadic functions"; }
+
+ // A predicate in that return nullptr means false
+ // Returns the function target to use when inlining on success
+ Function *isFunctionInlinable(Module &M, Function *F);
+
+ // Rewrite a call site.
+ void ExpandCall(Module &M, CallInst *CB, Function *VarargF, Function *NF);
+
+ // this could be partially target specific
+ bool expansionApplicableToFunction(Module &M, Function *F) {
+ if (F->isIntrinsic() || !F->isVarArg() ||
+ F->hasFnAttribute(Attribute::Naked))
+ return false;
+
+ if (F->getCallingConv() != CallingConv::C)
+ return false;
+
+ if (GlobalValue::isInterposableLinkage(F->getLinkage()))
+ return false;
+
+ for (const Use &U : F->uses()) {
+ const auto *CB = dyn_cast<CallBase>(U.getUser());
+
+ if (!CB)
+ return false;
+
+ if (CB->isMustTailCall()) {
+ return false;
+ }
+
+ if (!CB->isCallee(&U) || CB->getFunctionType() != F->getFunctionType()) {
+ return false;
+ }
+ }
+
+ // Branch funnels look like variadic functions but arent:
+ //
+ // define hidden void @__typeid_typeid1_0_branch_funnel(ptr nest %0, ...) {
+ // musttail call void (...) @llvm.icall.branch.funnel(ptr %0, ptr @vt1_1,
+ // ptr @vf1_1, ...) ret void
+ // }
+ //
+ // %1 = call i32 @__typeid_typeid1_0_branch_funnel(ptr nest %vtable, ptr
+ // %obj, i32 1)
+
+ // TODO: there should be a reasonable way to check for an intrinsic
+ // without inserting a prototype that then needs to be removed
+ Function *funnel =
+ Intrinsic::getDeclaration(&M, Intrinsic::icall_branch_funnel);
+ for (const User *U : funnel->users()) {
+ if (auto *I = dyn_cast<CallBase>(U)) {
+ if (F == I->getFunction()) {
+ return false;
+ }
+ }
+ }
+ if (funnel->use_empty())
+ funnel->eraseFromParent();
+
+ return true;
+ }
+
+ template <Intrinsic::ID ID>
+ static BasicBlock::iterator
+ skipIfInstructionIsSpecificIntrinsic(BasicBlock::iterator Iter) {
+ if (auto *Intrinsic = dyn_cast<IntrinsicInst>(&*Iter))
+ if (Intrinsic->getIntrinsicID() == ID)
+ Iter++;
+ return Iter;
+ }
+
+ bool callinstRewritable(CallBase *CB, Function *NF) {
+ if (CallInst *CI = dyn_cast<CallInst>(CB))
+ if (CI->isMustTailCall())
+ return false;
+
+ return true;
+ }
+
+ bool runOnFunction(Module &M, Function *F) {
+ bool changed = false;
+
+ if (!expansionApplicableToFunction(M, F))
+ return false;
+
+ Function *Equivalent = isFunctionInlinable(M, F);
+
+ if (!Equivalent)
+ return changed;
+
+ for (User *U : llvm::make_early_inc_range(F->users()))
+ if (CallInst *CB = dyn_cast<CallInst>(U)) {
+ Value *calledOperand = CB->getCalledOperand();
+ if (F == calledOperand) {
+ ExpandCall(M, CB, F, Equivalent);
+ changed = true;
+ }
+ }
+
+ return changed;
+ }
+
+ bool runOnModule(Module &M) override {
+ ABI = VariadicABIInfo::create(M);
+ if (!ABI)
+ return false;
+
+ bool Changed = false;
+ for (Function &F : llvm::make_early_inc_range(M)) {
+ Changed |= runOnFunction(M, &F);
+ }
+
+ return Changed;
+ }
+};
+
+Function *ExpandVariadics::isFunctionInlinable(Module &M, Function *F) {
+ assert(F->isVarArg());
+ assert(expansionApplicableToFunction(M, F));
+
+ if (F->isDeclaration())
+ return nullptr;
+
+ // A variadic function is inlinable if it is sufficiently simple.
+ // Specifically, if it is a single basic block which is functionally
+ // equivalent to packing the variadic arguments into a va_list which is
+ // passed to another function. The inlining strategy is to build a va_list
+ // in the caller and then call said inner function.
+
+ // Single basic block.
+ BasicBlock &BB = F->getEntryBlock();
+ if (!isa<ReturnInst>(BB.getTerminator())) {
+ return nullptr;
+ }
+
+ // Walk the block in order checking for specific instructions, some of them
+ // optional.
+ BasicBlock::iterator it = BB.begin();
+
+ AllocaInst *alloca = dyn_cast<AllocaInst>(&*it++);
+ if (!alloca)
+ return nullptr;
+
+ Value *valist_argument = alloca;
+
+ it = skipIfInstructionIsSpecificIntrinsic<Intrinsic::lifetime_start>(it);
+
+ VAStartInst *start = dyn_cast<VAStartInst>(&*it++);
+ if (!start || start->getArgList() != valist_argument) {
+ return nullptr;
+ }
+
+ // The va_list instance is stack allocated
+ // The ... replacement is a va_list passed "by value"
+ // That involves a load for some ABIs and passing the pointer for others
+ Value *valist_trailing_argument = nullptr;
+ switch (ABI->vaListCC()) {
+ case VariadicABIInfo::valistCC::value: {
+ // If it's being passed by value, need a load
+ // TODO: Check it's loading the right thing
+ auto *load = dyn_cast<LoadInst>(&*it);
+ if (!load)
+ return nullptr;
+ valist_trailing_argument = load;
+ it++;
+ break;
+ }
+ case VariadicABIInfo::valistCC::pointer: {
+ // If it's being passed by pointer, going to use the alloca directly
+ valist_trailing_argument = valist_argument;
+ break;
+ }
+ }
+
+ CallInst *call = dyn_cast<CallInst>(&*it++);
+ if (!call)
+ return nullptr;
+
+ if (auto *end = dyn_cast<VAEndInst>(&*it)) {
+ if (end->getArgList() != valist_argument)
+ return nullptr;
+ it++;
+ } else {
+ // Only fail on a missing va_end if it wasn't a no-op
+ if (!ABI->VAEndIsNop())
+ return nullptr;
+ }
+
+ it = skipIfInstructionIsSpecificIntrinsic<Intrinsic::lifetime_end>(it);
+
+ ReturnInst *ret = dyn_cast<ReturnInst>(&*it++);
+ if (!ret || it != BB.end())
+ return nullptr;
+
+ // The function call is expected to take the fixed arguments then the alloca
+ // TODO: Drop the vectors here, iterate over them both together instead.
+ SmallVector<Value *> FuncArgs;
+ for (Argument &A : F->args())
+ FuncArgs.push_back(&A);
+
+ SmallVector<Value *> CallArgs;
+ for (Use &A : call->args())
+ CallArgs.push_back(A);
+
+ size_t Fixed = FuncArgs.size();
+ if (Fixed + 1 != CallArgs.size())
+ return nullptr;
+
+ for (size_t i = 0; i < Fixed; i++)
+ if (FuncArgs[i] != CallArgs[i])
+ return nullptr;
+
+ if (CallArgs[Fixed] != valist_trailing_argument)
+ return nullptr;
+
+ // Check the varadic function returns the result of the inner call
+ Value *maybeReturnValue = ret->getReturnValue();
+ if (call->getType()->isVoidTy()) {
+ if (maybeReturnValue != nullptr)
+ return nullptr;
+ } else {
+ if (maybeReturnValue != call)
+ return nullptr;
+ }
+
+ // All checks passed. Found a va_list taking function we can use.
+ return call->getCalledFunction();
+}
+
+void ExpandVariadics::ExpandCall(Module &M, CallInst *CB, Function *VarargF,
+ Function *NF) {
+ const DataLayout &DL = M.getDataLayout();
+
+ if (!callinstRewritable(CB, NF)) {
+ return;
+ }
+
+ // This is something of a problem because the call instructions' idea of the
+ // function type doesn't necessarily match reality, before or after this
+ // pass
+ // Since the plan here is to build a new instruction there is no
+ // particular benefit to trying to preserve an incorrect initial type
+ // If the types don't match and we aren't changing ABI, leave it alone
+ // in case someone is deliberately doing dubious type punning through a
+ // varargs
+ FunctionType *FuncType = CB->getFunctionType();
+ if (FuncType != VarargF->getFunctionType()) {
+ return;
+ }
+
+ auto &Ctx = CB->getContext();
+
+ // Align the struct on ABI->MinAlign to start with
+ Align MaxFieldAlign(ABI->MinAlign ? ABI->MinAlign : 1);
+
+ // The strategy here is to allocate a call frame containing the variadic
+ // arguments laid out such that a target specific va_list can be initialised
+ // with it, such that target specific va_arg instructions will correctly
+ // iterate over it. Primarily this means getting the alignment right.
+
+ class {
+ // The awkward memory layout is to allow access to a contiguous array of
+ // types
+ enum { N = 4 };
+ SmallVector<Type *, N> fieldTypes;
+ SmallVector<std::pair<Value *, bool>, N> maybeValueIsByval;
+
+ public:
+ void append(Type *t, Value *v, bool isByval) {
+ fieldTypes.push_back(t);
+ maybeValueIsByval.push_back({v, isByval});
+ }
+
+ void padding(LLVMContext &Ctx, uint64_t by) {
+ append(ArrayType::get(Type::getInt8Ty(Ctx), by), nullptr, false);
+ }
+
+ size_t size() { return fieldTypes.size(); }
+ bool empty() { return fieldTypes.empty(); }
+
+ StructType *asStruct(LLVMContext &Ctx, StringRef Name) {
+ const bool isPacked = true;
+ return StructType::create(Ctx, fieldTypes,
+ (Twine(Name) + ".vararg").str(), isPacked);
+ }
+
+ void initialiseStructAlloca(const DataLayout &DL, IRBuilder<> &Builder,
+ AllocaInst *alloced) {
+
+ StructType *VarargsTy = cast<StructType>(alloced->getAllocatedType());
+
+ for (size_t i = 0; i < size(); i++) {
+ auto [v, isByval] = maybeValueIsByval[i];
+ if (!v)
+ continue;
+
+ auto r = Builder.CreateStructGEP(VarargsTy, alloced, i);
+ if (isByval) {
+ Type *ByValType = fieldTypes[i];
+ Builder.CreateMemCpy(r, {}, v, {},
+ DL.getTypeAllocSize(ByValType).getFixedValue());
+ } else {
+ Builder.CreateStore(v, r);
+ }
+ }
+ }
+ } Frame;
+
+ uint64_t CurrentOffset = 0;
+ for (unsigned I = FuncType->getNumParams(), E = CB->arg_size(); I < E; ++I) {
+ Value *ArgVal = CB->getArgOperand(I);
+ bool isByVal = CB->paramHasAttr(I, Attribute::ByVal);
+ Type *ArgType = isByVal ? CB->getParamByValType(I) : ArgVal->getType();
+ Align DataAlign = DL.getABITypeAlign(ArgType);
+
+ uint64_t DataAlignV = DataAlign.value();
+
+ // Currently using 0 as a sentinel to mean ignored
+ if (ABI->MinAlign && DataAlignV < ABI->MinAlign)
+ DataAlignV = ABI->MinAlign;
+ if (ABI->MaxAlign && DataAlignV > ABI->MaxAlign)
+ DataAlignV = ABI->MaxAlign;
+
+ DataAlign = Align(DataAlignV);
+ MaxFieldAlign = std::max(MaxFieldAlign, DataAlign);
+
+ if (uint64_t Rem = CurrentOffset % DataAlignV) {
+ // Inject explicit padding to deal with alignment requirements
+ uint64_t Padding = DataAlignV - Rem;
+ Frame.padding(Ctx, Padding);
+ CurrentOffset += Padding;
+ }
+
+ Frame.append(ArgType, ArgVal, isByVal);
+ CurrentOffset += DL.getTypeAllocSize(ArgType).getFixedValue();
+ }
+
+ if (Frame.empty()) {
+ // Not passing anything, hopefully va_arg won't try to dereference it
+ // Might be a target specific thing whether one can pass nullptr instead
+ // of undef i32
+ Frame.append(Type::getInt32Ty(Ctx), nullptr, false);
+ }
----------------
jhuber6 wrote:
```suggestion
// Not passing anything, hopefully va_arg won't try to dereference it
// Might be a target specific thing whether one can pass nullptr instead
// of undef i32
if (Frame.empty())
Frame.append(Type::getInt32Ty(Ctx), nullptr, false);
```
https://github.com/llvm/llvm-project/pull/81058
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