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Improve comments and reorganize a bit - no functionality

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change.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@61569 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Duncan Sands 2009-01-02 11:46:24 +00:00
parent 71df0d6008
commit b2f2279056
1 changed files with 44 additions and 56 deletions
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100
lib/Transforms/IPO/FunctionAttrs.cpp
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@ -9,10 +9,12 @@
//
// This file implements a simple interprocedural pass which walks the
// call-graph, looking for functions which do not access or only read
// non-local memory, and marking them readnone/readonly. It addition,
// it deduces which function arguments (of pointer type) do not escape,
// and marks them nocapture. It implements this as a bottom-up traversal
// of the call-graph.
// non-local memory, and marking them readnone/readonly. In addition,
// it marks function arguments (of pointer type) 'nocapture' if a call
// to the function does not create any copies of the pointer value that
// outlive the call. This more or less means that the pointer is only
// dereferenced, and not returned from the function or stored in a global.
// This pass is implemented as a bottom-up traversal of the call-graph.
//
//===----------------------------------------------------------------------===//
@ -46,7 +48,7 @@ namespace {
// AddNoCaptureAttrs - Deduce nocapture attributes for the SCC.
bool AddNoCaptureAttrs(const std::vector<CallGraphNode *> &SCC);
// isCaptured - Returns true if this pointer value escapes.
// isCaptured - Return true if this pointer value may be captured.
bool isCaptured(Function &F, Value *V);
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
@ -170,15 +172,15 @@ bool FunctionAttrs::AddReadAttrs(const std::vector<CallGraphNode *> &SCC) {
F->addAttribute(~0, ReadsMemory? Attribute::ReadOnly : Attribute::ReadNone);
if (ReadsMemory)
NumReadOnly++;
++NumReadOnly;
else
NumReadNone++;
++NumReadNone;
}
return MadeChange;
}
/// isCaptured - Returns whether this pointer value is captured.
/// isCaptured - Return true if this pointer value may be captured.
bool FunctionAttrs::isCaptured(Function &F, Value *V) {
SmallVector<Use*, 16> Worklist;
SmallPtrSet<Use*, 16> Visited;
@ -196,68 +198,54 @@ bool FunctionAttrs::isCaptured(Function &F, Value *V) {
V = U->get();
if (isa<LoadInst>(I)) {
// Loading a pointer does not cause it to escape.
continue;
}
if (isa<StoreInst>(I)) {
// Loading a pointer does not cause it to be captured. Note that the
// loaded value might be the pointer itself (think of self-referential
// objects), but that's ok as long as it's not this function that stored
// the pointer there.
} else if (isa<StoreInst>(I)) {
if (V == I->getOperand(0))
// Stored the pointer - escapes. TODO: improve this.
// Stored the pointer - it is captured. TODO: improve this.
return true;
// Storing to the pointee does not cause the pointer to escape.
continue;
}
if (isa<FreeInst>(I)) {
// Freeing a pointer does not cause it to escape.
continue;
}
CallSite CS = CallSite::get(I);
if (CS.getInstruction()) {
// Does not escape if the callee is readonly and doesn't return a
// copy through its own return value.
// Storing to the pointee does not cause the pointer to be captured.
} else if (isa<FreeInst>(I)) {
// Freeing a pointer does not cause it to be captured.
} else if (isa<CallInst>(I) || isa<InvokeInst>(I)) {
CallSite CS = CallSite::get(I);
// Not captured if the callee is readonly and doesn't return a copy
// through its return value.
if (CS.onlyReadsMemory() && I->getType() == Type::VoidTy)
continue;
// Does not escape if passed via 'nocapture' arguments. Note that
// calling a function pointer does not in itself cause that
// function pointer to escape. This is a subtle point considering
// that (for example) the callee might return its own address. It
// is analogous to saying that loading a value from a pointer does
// not cause the pointer to escape, even though the loaded value
// might be the pointer itself (think of self-referential objects).
// Not captured if only passed via 'nocapture' arguments. Note that
// calling a function pointer does not in itself cause the pointer to
// be captured. This is a subtle point considering that (for example)
// the callee might return its own address. It is analogous to saying
// that loading a value from a pointer does not cause the pointer to be
// captured, even though the loaded value might be the pointer itself
// (think of self-referential objects).
CallSite::arg_iterator B = CS.arg_begin(), E = CS.arg_end();
for (CallSite::arg_iterator A = B; A != E; ++A)
if (A->get() == V && !CS.paramHasAttr(A-B+1, Attribute::NoCapture))
// The parameter is not marked 'nocapture' - escapes.
if (A->get() == V && !CS.paramHasAttr(A - B + 1, Attribute::NoCapture))
// The parameter is not marked 'nocapture' - captured.
return true;
// Only passed via 'nocapture' arguments, or is the called function.
// Does not escape.
continue;
}
if (isa<BitCastInst>(I) || isa<GetElementPtrInst>(I) ||
isa<PHINode>(I) || isa<SelectInst>(I)) {
// Type conversion, calculating an offset, or merging values.
// The original value does not escape via this if the new value doesn't.
// Note that in the case of a select instruction it is important that
// the value not be used as the condition, since otherwise one bit of
// information might escape. It cannot be the condition because it has
// the wrong type.
// Only passed via 'nocapture' arguments, or is the called function - not
// captured.
} else if (isa<BitCastInst>(I) || isa<PHINode>(I) ||
isa<GetElementPtrInst>(I) || isa<SelectInst>(I)) {
// The original value is not captured via this if the instruction isn't.
for (Instruction::use_iterator UI = I->use_begin(), UE = I->use_end();
UI != UE; ++UI) {
Use *U = &UI.getUse();
if (Visited.insert(U))
Worklist.push_back(U);
}
continue;
} else {
// Something else - be conservative and say it is captured.
return true;
}
// Something else - be conservative and say it escapes.
return true;
}
// All uses examined - not captured.
return false;
}
@ -274,9 +262,9 @@ bool FunctionAttrs::AddNoCaptureAttrs(const std::vector<CallGraphNode *> &SCC) {
// External node - skip it;
continue;
// If the function is readonly and doesn't return any value, we
// know that the pointer value can't escape. Mark all of its pointer
// arguments nocapture.
// If the function is readonly and doesn't return any value, we know that
// the pointer value is not captured. Mark all of its pointer arguments
// nocapture.
if (F->onlyReadsMemory() && F->getReturnType() == Type::VoidTy) {
for (Function::arg_iterator A = F->arg_begin(), E = F->arg_end();
A != E; ++A)