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[APX] fix bugs in NDD code gen #115809

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[APX] fix bugs in NDD code gen #115809

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fix bugs in NDD code gen.
Ruihan-Yin May 20, 2025
3a050c2
formatting
Ruihan-Yin May 21, 2025
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5 changes: 3 additions & 2 deletions src/coreclr/jit/emitxarch.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -6420,7 +6420,7 @@ regNumber emitter::emitInsBinary(instruction ins, emitAttr attr, GenTree* dst, G
}
else
{
fmt = useNDD ? emitInsModeFormat(ins, IF_RWR_RRD_ARD) : emitInsModeFormat(ins, IF_RRD_ARD);
fmt = useNDD ? IF_RWR_RRD_ARD : emitInsModeFormat(ins, IF_RRD_ARD);
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Copilot AI May 21, 2025

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By directly assigning IF_RWR_RRD_ARD when useNDD is true, ensure that this constant provides equivalent behavior as the previous call to emitInsModeFormat(ins, IF_RWR_RRD_ARD) and doesn't bypass any essential formatting logic.

Suggested change
fmt = useNDD ? IF_RWR_RRD_ARD : emitInsModeFormat(ins, IF_RRD_ARD);
fmt = useNDD ? emitInsModeFormat(ins, IF_RWR_RRD_ARD) : emitInsModeFormat(ins, IF_RRD_ARD);

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Is this because the instruction is annotated as IUM_RW so we end up picking IF_RRW_RRD_ARD instead of IUM_RWR_RRD_ARD?

Is this something we should more generally handle or have a centralized helper for? I would imagine it applies to many of the other code paths in here as well, such as for IF_RWR_RRD_SRD, IF_RWR_RRD_MRD and IF_RWR_RRD_RRD

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Yes, the function cannot give the right IF for NDD instructions, so some further code gen optimization failed. Same thing applies to RR, SV, AM. I had to manually set them right.

}
}
else
Expand Down Expand Up @@ -10572,7 +10572,8 @@ regNumber emitter::emitIns_BASE_R_R_RM(
regNumber r = REG_NA;
assert(regOp->isUsedFromReg());

if (DoJitUseApxNDD(ins) && regOp->GetRegNum() != targetReg)
if (DoJitUseApxNDD(ins) && regOp->GetRegNum() != targetReg &&
!IsRedundantMov(INS_mov, IF_RWR_RRD, attr, targetReg, regOp->GetRegNum(), false))
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This probably deserves a comment explaining what IsRedundantMov is trying to handle as its a bit non-obvious at first.

What we're basically trying to do here is all the normal handling that emitins_Mov would do, but trying to avoid the last few lines of code that actually create the instrDesc since the caller is nddAware. So I wonder if we could refactor this a bit instead...

More ideally we could do something like the following, that way we can avoid redundant checks in the worst case and keep it understandable:

bool useApxNdd = DoJitUseApxNDD(ins);

if (TryEmitIns_Mov(..., useApxNdd) && useApxNdd)
{
    // The move is necessary, but we're using APX NDD
    return emitInsBinary(ins, attr, regOp, rmOp, targetReg);
}

// The move was either unnecessary or already emitted because we can't use APX NDD
return emitInsBinary(ins, attr, treeNode, rmOp);

emitIns_Mov would just have the last few lines changed to:

     if (IsRedundantMov(ins, fmt, attr, dstReg, srcReg, canSkip))
     {
-        return;
+        // Move is redundant, no need to emit anything
+        return false;
     }

     if (EmitMovsxAsCwde(ins, size, dstReg, srcReg))
     {
-        return;
+        // Move is redundant, no need to emit anything
+        return false;
     }

+    if (useApxNdd)
+    {
+        // Move is required, but the caller is APX NDD aware
+        return true;
+    }

     instrDesc* id = emitNewInstrSmall(attr);
     id->idIns(ins);
     id->idInsFmt(fmt);
     id->idReg1(dstReg);
     id->idReg2(srcReg);

     UNATIVE_OFFSET sz = emitInsSizeRR(id);
     id->idCodeSize(sz);

     dispIns(id);
     emitCurIGsize += sz;
     
+    // Move is required and the caller is not APX NDD aware, we emitted the move
+    return true;

{
r = emitInsBinary(ins, attr, regOp, rmOp, targetReg);
}
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