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Forward-substitute long calls on 32 bit targets #74502

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Aug 26, 2022
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Forward-substitute long calls on 32 bit targets #74502

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14 changes: 3 additions & 11 deletions src/coreclr/jit/forwardsub.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -673,32 +673,24 @@ bool Compiler::fgForwardSubStatement(Statement* stmt)
// Eg if fwdSubNode is a multi-reg call, parent node must be GT_ASG and the
// local being defined must be specially marked up.
//
if (fwdSubNode->IsMultiRegCall())
if (varTypeIsStruct(fwdSubNode) && fwdSubNode->IsMultiRegCall())
{
GenTree* const parentNode = fsv.GetParentNode();

if (!parentNode->OperIs(GT_ASG))
{
JITDUMP(" multi-reg call, parent not asg\n");
JITDUMP(" multi-reg struct call, parent not asg\n");
return false;
}

GenTree* const parentNodeLHS = parentNode->gtGetOp1();

if (!parentNodeLHS->OperIs(GT_LCL_VAR))
{
JITDUMP(" multi-reg call, parent not asg(lcl, ...)\n");
JITDUMP(" multi-reg struct call, parent not asg(lcl, ...)\n");
return false;
}

#if defined(TARGET_X86) || defined(TARGET_ARM)
if (fwdSubNode->TypeGet() == TYP_LONG)
{
JITDUMP(" TYP_LONG fwd sub node, target is x86/arm\n");
return false;
}
#endif // defined(TARGET_X86) || defined(TARGET_ARM)

Comment on lines -694 to -701
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Just a question. Why do we delete this code that skips TYP_LONG forward substitute node on x86 and arm target?

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It should not be necessary.

The restriction on substituting multi-reg nodes stems from the fact the backend (LSRA and codegen) can only handle a few select forms of uses for them:

  1. STORE_LCL_VAR(multi-reg node)
  2. RETURN(matching multi-reg call | LCL_VAR)

For structs, these restrictions are enforced by the frontend (mainly importer). For LONGs - by decomposition, which means we don't need to worry about them here.

GenTreeLclVar* const parentNodeLHSLocal = parentNodeLHS->AsLclVar();

unsigned const lhsLclNum = parentNodeLHSLocal->GetLclNum();
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