void Compiler::optDumpCopyPropStack(LclNumToGenTreePtrStack* curSsaName)
{
JITDUMP("{ ");
for (LclNumToGenTreePtrStack::KeyIterator iter = curSsaName->Begin(); !iter.Equal(curSsaName->End()); ++iter)
{
GenTree* node = iter.GetValue()->Index(0);
JITDUMP("%d-[%06d]:V%02u ", iter.Get(), dspTreeID(node), node->AsLclVarCommon()->gtLclNum);
}
JITDUMP("}\n\n");
}
/**************************************************************************************
*
* Perform copy propagation on a given tree as we walk the graph and if it is a local
* variable, then look up all currently live definitions and check if any of those
* definitions share the same value number. If so, then we can make the replacement.
*
*/
void Compiler::optCopyProp(BasicBlock* block, GenTreePtr stmt, GenTreePtr tree, LclNumToGenTreePtrStack* curSsaName)
{
// TODO-Review: EH successor/predecessor iteration seems broken.
if (block->bbCatchTyp == BBCT_FINALLY || block->bbCatchTyp == BBCT_FAULT)
{
return;
}
// If not local nothing to do.
if (!tree->IsLocal())
{
return;
}
if (tree->OperGet() == GT_PHI_ARG || tree->OperGet() == GT_LCL_FLD)
{
return;
}
// Propagate only on uses.
if (tree->gtFlags & GTF_VAR_DEF)
{
return;
}
unsigned lclNum = tree->AsLclVarCommon()->GetLclNum();
// Skip address exposed variables.
if (fgExcludeFromSsa(lclNum))
{
return;
}
assert(tree->gtVNPair.GetConservative() != ValueNumStore::NoVN);
for (LclNumToGenTreePtrStack::KeyIterator iter = curSsaName->Begin(); !iter.Equal(curSsaName->End()); ++iter)
{
unsigned newLclNum = iter.Get();
GenTreePtr op = iter.GetValue()->Index(0);
// Nothing to do if same.
if (lclNum == newLclNum)
{
continue;
}
// Skip variables with assignments embedded in the statement (i.e., with a comma). Because we
// are not currently updating their SSA names as live in the copy-prop pass of the stmt.
if (VarSetOps::IsMember(this, optCopyPropKillSet, lvaTable[newLclNum].lvVarIndex))
{
continue;
}
if (op->gtFlags & GTF_VAR_CAST)
{
continue;
}
if (gsShadowVarInfo != nullptr && lvaTable[newLclNum].lvIsParam &&
gsShadowVarInfo[newLclNum].shadowCopy == lclNum)
{
continue;
}
ValueNum opVN = GetUseAsgDefVNOrTreeVN(op);
if (opVN == ValueNumStore::NoVN)
{
continue;
}
if (op->TypeGet() != tree->TypeGet())
{
continue;
}
if (opVN != tree->gtVNPair.GetConservative())
{
continue;
}
if (optCopyProp_LclVarScore(&lvaTable[lclNum], &lvaTable[newLclNum], true) <= 0)
{
continue;
}
// Check whether the newLclNum is live before being substituted. Otherwise, we could end
// up in a situation where there must've been a phi node that got pruned because the variable
// is not live anymore. For example,
// if
// x0 = 1
// else
// x1 = 2
// print(c) <-- x is not live here. Let's say 'c' shares the value number with "x0."
//
// If we simply substituted 'c' with "x0", we would be wrong. Ideally, there would be a phi
// node x2 = phi(x0, x1) which can then be used to substitute 'c' with. But because of pruning
// there would be no such phi node. To solve this we'll check if 'x' is live, before replacing
// 'c' with 'x.'
if (!lvaTable[newLclNum].lvVerTypeInfo.IsThisPtr())
{
if (lvaTable[newLclNum].lvAddrExposed)
{
continue;
}
// We compute liveness only on tracked variables. So skip untracked locals.
if (!lvaTable[newLclNum].lvTracked)
{
continue;
}
// Because of this dependence on live variable analysis, CopyProp phase is immediately
// after Liveness, SSA and VN.
if (!VarSetOps::IsMember(this, compCurLife, lvaTable[newLclNum].lvVarIndex))
{
continue;
}
}
unsigned newSsaNum = SsaConfig::RESERVED_SSA_NUM;
if (op->gtFlags & GTF_VAR_DEF)
{
newSsaNum = GetSsaNumForLocalVarDef(op);
}
else // parameters, this pointer etc.
{
newSsaNum = op->AsLclVarCommon()->GetSsaNum();
}
if (newSsaNum == SsaConfig::RESERVED_SSA_NUM)
{
continue;
}
#ifdef DEBUG
if (verbose)
{
JITDUMP("VN based copy assertion for ");
printTreeID(tree);
printf(" V%02d @%08X by ", lclNum, tree->GetVN(VNK_Conservative));
printTreeID(op);
printf(" V%02d @%08X.\n", newLclNum, op->GetVN(VNK_Conservative));
gtDispTree(tree, nullptr, nullptr, true);
}
#endif
lvaTable[lclNum].decRefCnts(block->getBBWeight(this), this);
lvaTable[newLclNum].incRefCnts(block->getBBWeight(this), this);
tree->gtLclVarCommon.SetLclNum(newLclNum);
tree->AsLclVarCommon()->SetSsaNum(newSsaNum);
#ifdef DEBUG
if (verbose)
{
printf("copy propagated to:\n");
gtDispTree(tree, nullptr, nullptr, true);
}
#endif
break;
}
return;
}