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//===- ExpandByVal.cpp - Expand out use of "byval" and "sret" attributes---===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass expands out by-value passing of structs as arguments and
// return values. In LLVM IR terms, it expands out the "byval" and
// "sret" function argument attributes.
//
// The semantics of the "byval" attribute are that the callee function
// gets a private copy of the pointed-to argument that it is allowed
// to modify. In implementing this, we have a choice between making
// the caller responsible for making the copy or making the callee
// responsible for making the copy. We choose the former, because
// this matches how the normal native calling conventions work, and
// because it often allows the caller to write struct contents
// directly into the stack slot that it passes the callee, without an
// additional copy.
//
// Note that this pass does not attempt to modify functions that pass
// structs by value without using "byval" or "sret", such as:
//
// define %struct.X @func() ; struct return
// define void @func(%struct.X %arg) ; struct arg
//
// The pass only handles functions such as:
//
// define void @func(%struct.X* sret %result_buffer) ; struct return
// define void @func(%struct.X* byval %ptr_to_arg) ; struct arg
//
// This is because PNaCl Clang generates the latter and not the former.
//
//===----------------------------------------------------------------------===//
#include "llvm/IR/Attributes.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Module.h"
#include "llvm/Pass.h"
#include "llvm/Transforms/NaCl.h"
using namespace llvm;
namespace {
// This is a ModulePass so that it can strip attributes from
// declared functions as well as defined functions.
class ExpandByVal : public ModulePass {
public:
static char ID; // Pass identification, replacement for typeid
ExpandByVal() : ModulePass(ID) {
initializeExpandByValPass(*PassRegistry::getPassRegistry());
}
virtual bool runOnModule(Module &M);
};
}
char ExpandByVal::ID = 0;
INITIALIZE_PASS(ExpandByVal, "expand-byval",
"Expand out by-value passing of structs",
false, false)
// removeAttribute() currently does not work on Attribute::Alignment
// (it fails with an assertion error), so we have to take a more
// convoluted route to removing this attribute by recreating the
// AttributeSet.
AttributeSet RemoveAttrs(LLVMContext &Context, AttributeSet Attrs) {
SmallVector<AttributeSet, 8> AttrList;
for (unsigned Slot = 0; Slot < Attrs.getNumSlots(); ++Slot) {
unsigned Index = Attrs.getSlotIndex(Slot);
AttrBuilder AB;
for (AttributeSet::iterator Attr = Attrs.begin(Slot), E = Attrs.end(Slot);
Attr != E; ++Attr) {
if (!Attr->isAlignAttribute() &&
Attr->isEnumAttribute() &&
Attr->getKindAsEnum() != Attribute::ByVal &&
Attr->getKindAsEnum() != Attribute::StructRet) {
AB.addAttribute(*Attr);
}
// IR semantics require that ByVal implies NoAlias. However, IR
// semantics do not require StructRet to imply NoAlias. For
// example, a global variable address can be passed as a
// StructRet argument, although Clang does not do so and Clang
// explicitly adds NoAlias to StructRet arguments.
if (Attr->isEnumAttribute() &&
Attr->getKindAsEnum() == Attribute::ByVal) {
AB.addAttribute(Attribute::get(Context, Attribute::NoAlias));
}
}
AttrList.push_back(AttributeSet::get(Context, Index, AB));
}
return AttributeSet::get(Context, AttrList);
}
// ExpandCall() can take a CallInst or an InvokeInst. It returns
// whether the instruction was modified.
template <class InstType>
static bool ExpandCall(DataLayout *DL, InstType *Call) {
bool Modify = false;
AttributeSet Attrs = Call->getAttributes();
for (unsigned ArgIdx = 0; ArgIdx < Call->getNumArgOperands(); ++ArgIdx) {
unsigned AttrIdx = ArgIdx + 1;
if (Attrs.hasAttribute(AttrIdx, Attribute::StructRet))
Modify = true;
if (Attrs.hasAttribute(AttrIdx, Attribute::ByVal)) {
Modify = true;
Value *ArgPtr = Call->getArgOperand(ArgIdx);
Type *ArgType = ArgPtr->getType()->getPointerElementType();
ConstantInt *ArgSize = ConstantInt::get(
Call->getContext(), APInt(64, DL->getTypeStoreSize(ArgType)));
unsigned Alignment = Attrs.getParamAlignment(AttrIdx);
// In principle, using the alignment from the argument attribute
// should be enough. However, Clang is not emitting this
// attribute for PNaCl. LLVM alloca instructions do not use the
// ABI alignment of the type, so this must be specified
// explicitly.
// See https://code.google.com/p/nativeclient/issues/detail?id=3403
unsigned AllocAlignment =
std::max(Alignment, DL->getABITypeAlignment(ArgType));
// Make a copy of the byval argument.
Instruction *CopyBuf = new AllocaInst(ArgType, 0, AllocAlignment,
ArgPtr->getName() + ".byval_copy");
Function *Func = Call->getParent()->getParent();
Func->getEntryBlock().getInstList().push_front(CopyBuf);
IRBuilder<> Builder(Call);
Builder.CreateLifetimeStart(CopyBuf, ArgSize);
// Using the argument's alignment attribute for the memcpy
// should be OK because the LLVM Language Reference says that
// the alignment attribute specifies "the alignment of the stack
// slot to form and the known alignment of the pointer specified
// to the call site".
// XXX EMSCRIPTEN: we also need to take into account the "intrinsic"
// alignment of the structure being copied. The
// arg align may be "0" but the struct could only be
// 2-byte aligned for example. This seems to contradict
// the quote above, so this might be a bug elsewhere
// in LLVM. FIXME
// https://code.google.com/p/nativeclient/issues/detail?id=3798
Alignment = AllocAlignment;
Instruction *MemCpy = Builder.CreateMemCpy(CopyBuf, ArgPtr, ArgSize,
Alignment);
MemCpy->setDebugLoc(Call->getDebugLoc());
Call->setArgOperand(ArgIdx, CopyBuf);
// Mark the argument copy as unused using llvm.lifetime.end.
if (isa<CallInst>(Call)) {
BasicBlock::iterator It = BasicBlock::iterator(Call);
Builder.SetInsertPoint(++It);
Builder.CreateLifetimeEnd(CopyBuf, ArgSize);
} else if (InvokeInst *Invoke = dyn_cast<InvokeInst>(Call)) {
Builder.SetInsertPoint(Invoke->getNormalDest()->getFirstInsertionPt());
Builder.CreateLifetimeEnd(CopyBuf, ArgSize);
Builder.SetInsertPoint(Invoke->getUnwindDest()->getFirstInsertionPt());
Builder.CreateLifetimeEnd(CopyBuf, ArgSize);
}
}
}
if (Modify) {
Call->setAttributes(RemoveAttrs(Call->getContext(), Attrs));
if (CallInst *CI = dyn_cast<CallInst>(Call)) {
// This is no longer a tail call because the callee references
// memory alloca'd by the caller.
CI->setTailCall(false);
}
}
return Modify;
}
bool ExpandByVal::runOnModule(Module &M) {
bool Modified = false;
DataLayout DL(&M);
for (Module::iterator Func = M.begin(), E = M.end(); Func != E; ++Func) {
AttributeSet NewAttrs = RemoveAttrs(Func->getContext(),
Func->getAttributes());
Modified |= (NewAttrs != Func->getAttributes());
Func->setAttributes(NewAttrs);
for (Function::iterator BB = Func->begin(), E = Func->end();
BB != E; ++BB) {
for (BasicBlock::iterator Inst = BB->begin(), E = BB->end();
Inst != E; ++Inst) {
if (CallInst *Call = dyn_cast<CallInst>(Inst)) {
Modified |= ExpandCall(&DL, Call);
} else if (InvokeInst *Call = dyn_cast<InvokeInst>(Inst)) {
Modified |= ExpandCall(&DL, Call);
}
}
}
}
return Modified;
}
ModulePass *llvm::createExpandByValPass() {
return new ExpandByVal();
}
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