JIT: Avoid internal register allocation when possible for PUTARG_SPLIT #76443
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We can almost always avoid allocating an internal register, which may be expensive if lr is picked since we cannot use thumb encoding for it. The only case where we need an internal register is when we have a source that is in a register, and we have a single taget register to place that conflicts with that source register. The to-stack copies then need an intermediate scratch register to avoid clobbering the source register.
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Should fix regression seen in #76017 (comment). |
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Tagging subscribers to this area: @JulieLeeMSFT, @jakobbotsch Issue DetailsWe can almost always avoid allocating an internal register, which may be expensive if lr is picked since we cannot use thumb encoding for it. The only case where we need an internal register is when we have a source that is in a register, and we have a single taget register to place that conflicts with that source register. The to-stack copies then need an intermediate scratch register to avoid clobbering the source register.
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cc @dotnet/jit-contrib PTAL @BruceForstall |
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| // Find first register to place. If we are placing addrReg, then |
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Just that I understand, we basically pick the first register to place after addrReg and round robin back so addrReg is placed last?
For r0, r1, addrReg, r3, r4, we will place: r3, r4, r0, r1, addrReg?
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If yes, can you call that out in comment with an example. Initially it was not clear to me, if that's what we meant.
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Exactly. I'll add a comment like you suggested.
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| if (treeNode->GetRegNumByIdx(i) == addrReg) | ||
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| firstRegToPlace = i + 1; |
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| firstRegToPlace = i + 1; | |
| firstRegToPlace = (i + 1) % treeNode->gtNumRegs; |
That way, you can remove the code from below loop:
if (curRegIndex == treeNode->gtNumRegs)
{
curRegIndex = 0;
structOffset = 0;
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It still needs to be handled in below loop (since the loop there also increments curRegIndex). So I think I'll leave it as is.
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We can almost always avoid allocating an internal register, which may be expensive if lr is picked since we cannot use thumb encoding for it.
The only case where we need an internal register is when we have a source that is in a register, and we have a single taget register to place that conflicts with that source register. The to-stack copies then need an intermediate scratch register to avoid clobbering the source register.