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Learn IPConstProp to propagate arguments that are directly returned. Strictly 

speaking these are not constant values. However, when a function always returns
one of its arguments, then from the point of view of each caller the return
value is constant (or at least a known value) and can be replaced.

llvm-svn: 52397
This commit is contained in:
Matthijs Kooijman 2008-06-17 12:20:24 +00:00
parent f03c1ae407
commit 332836d68d
1 changed files with 11 additions and 2 deletions
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13
llvm/lib/Transforms/IPO/IPConstantPropagation.cpp
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@ -145,6 +145,10 @@ bool IPCP::PropagateConstantsIntoArguments(Function &F) {
// all callers that use those return values with the constant value. This will // all callers that use those return values with the constant value. This will
// leave in the actual return values and instructions, but deadargelim will // leave in the actual return values and instructions, but deadargelim will
// clean that up. // clean that up.
//
// Additionally if a function always returns one of its arguments directly,
// callers will be updated to use the value they pass in directly instead of
// using the return value.
bool IPCP::PropagateConstantReturn(Function &F) { bool IPCP::PropagateConstantReturn(Function &F) {
if (F.getReturnType() == Type::VoidTy) if (F.getReturnType() == Type::VoidTy)
return false; // No return value. return false; // No return value.
@ -188,8 +192,8 @@ bool IPCP::PropagateConstantReturn(Function &F) {
if (isa<UndefValue>(V)) if (isa<UndefValue>(V))
continue; continue;
// Try to see if all the rets return the same constant. // Try to see if all the rets return the same constant or argument.
if (isa<Constant>(V)) { if (isa<Constant>(V) || isa<Argument>(V)) {
if (isa<UndefValue>(RV)) { if (isa<UndefValue>(RV)) {
// No value found yet? Try the current one. // No value found yet? Try the current one.
RetVals[i] = V; RetVals[i] = V;
@ -255,6 +259,11 @@ bool IPCP::PropagateConstantReturn(Function &F) {
if (index != -1) { if (index != -1) {
Value *New = RetVals[index]; Value *New = RetVals[index];
if (New) { if (New) {
if (Argument *A = dyn_cast<Argument>(New))
// Was an argument returned? Then find the corresponding argument in
// the call instruction and use that. Add 1 to the argument number
// to skipp the first argument (the function itself).
New = Call->getOperand(A->getArgNo() + 1);
Ins->replaceAllUsesWith(New); Ins->replaceAllUsesWith(New);
Ins->eraseFromParent(); Ins->eraseFromParent();
} }