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Slightly more aggressive ASG reversal #65920

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Mar 4, 2022
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Slightly more aggressive ASG reversal #65920

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30 changes: 13 additions & 17 deletions src/coreclr/jit/gentree.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -4263,28 +4263,24 @@ unsigned Compiler::gtSetEvalOrder(GenTree* tree)
case GT_IND:
case GT_BLK:
case GT_OBJ:

// In an indirection, the destination address is evaluated prior to the source.
// If we have any side effects on the target indirection,
// we have to evaluate op1 first.
// However, if the LHS is a lclVar address, SSA relies on using evaluation order for its
// renaming, and therefore the RHS must be evaluated first.
// If we have an assignment involving a lclVar address, the LHS may be marked as having
// side-effects.
// However the side-effects won't require that we evaluate the LHS address first:
// - The GTF_GLOB_REF might have been conservatively set on a FIELD of a local.
// - The local might be address-exposed, but that side-effect happens at the actual assignment (not
// when its address is "evaluated") so it doesn't change the side effect to "evaluate" the address
// after the RHS (note that in this case it won't be renamed by SSA anyway, but the reordering is
// safe).
{
// In an ASG(IND(addr), ...), the "IND" is a pure syntactical element,
// the actual indirection will only be realized at the point of the ASG
// itself. As such, we can disard any side effects "induced" by it in
// this logic.
//
// Note that for local "addr"s, liveness depends on seeing the defs and
// uses in correct order, and so we MUST reverse the ASG in that case.
//
if (op1Val->AsIndir()->Addr()->IsLocalAddrExpr())
GenTree* op1Addr = op1->AsIndir()->Addr();

if (op1Addr->IsLocalAddrExpr() || op1Addr->IsInvariant())
{
bReverseInAssignment = true;
tree->gtFlags |= GTF_REVERSE_OPS;
break;
}
if (op1Val->AsIndir()->Addr()->gtFlags & GTF_ALL_EFFECT)
if (op1Addr->gtFlags & GTF_ALL_EFFECT)
{
break;
}
Expand All @@ -4301,7 +4297,7 @@ unsigned Compiler::gtSetEvalOrder(GenTree* tree)
{
break;
}

}
// fall through and set GTF_REVERSE_OPS
FALLTHROUGH;

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