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JIT: Do not propagate some constants #70378

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merged 11 commits into from
Jun 10, 2022
Merged

JIT: Do not propagate some constants #70378

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593a3ec
Do not propagate some constants
EgorBo Jun 7, 2022
72ab579
Limit to vectors for now
EgorBo Jun 8, 2022
83fc086
Extend impl for other constants
EgorBo Jun 8, 2022
d81b195
Clean up
EgorBo Jun 8, 2022
1d076dd
Remove integers for now
EgorBo Jun 8, 2022
0274989
Merge branch 'main' of github.com:dotnet/runtime into fix-cns-prop
EgorBo Jun 9, 2022
69e4d1d
Address feedback
EgorBo Jun 9, 2022
3920a54
Move to a function
EgorBo Jun 9, 2022
7873570
Remove unused method
EgorBo Jun 9, 2022
bc40d08
fix build
EgorBo Jun 9, 2022
eaede4e
Update src/coreclr/jit/assertionprop.cpp
EgorBo Jun 10, 2022
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35 changes: 35 additions & 0 deletions src/coreclr/jit/assertionprop.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -3177,6 +3177,41 @@ GenTree* Compiler::optVNConstantPropOnTree(BasicBlock* block, GenTree* tree)

if (conValTree != nullptr)
{
if (tree->OperIs(GT_LCL_VAR))
{
bool betterToPropagate = false;
if (conValTree->IsVectorZero() || conValTree->IsFloatPositiveZero())
{
// These are cheap to re-materialize (unlike e.g. "1.0" or "{1,1,1,1}" which end up as
// memory loads from the data section)
betterToPropagate = true;
}
#ifdef TARGET_ARM64
else if (conValTree->IsCnsIntOrI() && unsigned_abs(conValTree->AsIntCon()->IconValue()) < 0x0fff)
{
// This integer constant is likely immable
betterToPropagate = true;
}
#endif

if (!betterToPropagate)
{
// Try to find the block this value was defined in
// and if the current use is inside a loop while the def is not - do not propagate
unsigned lclNum = tree->AsLclVarCommon()->GetLclNum();
unsigned ssaNum = GetSsaNumForLocalVarDef(tree);
if (ssaNum != SsaConfig::RESERVED_SSA_NUM)
{
BasicBlock* defBlock = lvaTable[lclNum].GetPerSsaData(ssaNum)->GetBlock();
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Style nit: lvaTable[lclNum]. -> lvaGetDesc(lclNum)->.

General comment on the algorithm: this walks only one step up in the def chain, so if we have something like this:

var a = const;
for (...) {
    var b = a;
    Use(b);
}

We will still propagate const to Use.

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@SingleAccretion I don't see why it would.
When we're at Use(b) and we query SsaData()->Block from b we get a's Block and it leads to "avoid propagating" path

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@EgorBo EgorBo Jun 9, 2022 •
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Ah, actually for your case b is going to be replaced with a in an earlier phase VN based copy prop

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@SingleAccretion SingleAccretion Jun 9, 2022 •
edited
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When we're at Use(b) and we query SsaData()->Block from b we get a's Block and it leads to "avoid propagating" path

If I annotate this with blocks:

BB01:
var a = const;

for (...) {
BB02:
    var b = a; // b:d:1
    Use(b);    // b:u:1
}

I am pretty sure that when we query the def block for b:u:1, we'd get BB02, since the def b:d:1 occurs inside it.

Ah, actually for your case b is going to be replaced with a in an earlier phase VN based copy prop

I agree that copy propagation helps us here; not sure by how much because of the liveness constraints it has.

In any case, I also agree that there is no reason, for now, to complicate the code with a full UD chain walk, because we are pretty much targeting people hoisting constants out of loops by hands.

if (defBlock != nullptr && !(defBlock->bbFlags & BBF_BACKWARD_JUMP) &&
(block->bbFlags & BBF_BACKWARD_JUMP))
{
return nullptr;
}
}
}
}

// Were able to optimize.
conValTree->gtVNPair = vnPair;
GenTree* sideEffList = optExtractSideEffListFromConst(tree);
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