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Arm64: Make unroll code for cpblk non-interruptible #69202

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Merged
kunalspathak merged 4 commits into from
May 17, 2022
Merged

Arm64: Make unroll code for cpblk non-interruptible #69202

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7ffcc79
make unroll code non-interruptible
kunalspathak May 9, 2022
fe439bb
jit format
kunalspathak May 11, 2022
9d08ed9
Disable GC only in specific scenarios:
kunalspathak May 16, 2022
f9e67ab
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kunalspathak May 17, 2022
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27 changes: 25 additions & 2 deletions src/coreclr/jit/codegenarmarch.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -2815,14 +2815,22 @@ void CodeGen::genCodeForCpBlkUnroll(GenTreeBlk* node)
CopyBlockUnrollHelper helper(srcOffset, dstOffset, size);
regNumber srcReg = srcAddrBaseReg;

#ifdef DEBUG
bool isSrcRegAddrAlignmentKnown = false;
bool isDstRegAddrAlignmentKnown = false;
#endif

if (srcLclNum != BAD_VAR_NUM)
{
bool fpBased;
const int baseAddr = compiler->lvaFrameAddress(srcLclNum, &fpBased);

srcReg = fpBased ? REG_FPBASE : REG_SPBASE;

helper.SetSrcOffset(baseAddr + srcOffset);

#ifdef DEBUG
isSrcRegAddrAlignmentKnown = true;
#endif
}

regNumber dstReg = dstAddrBaseReg;
Expand All @@ -2833,8 +2841,11 @@ void CodeGen::genCodeForCpBlkUnroll(GenTreeBlk* node)
const int baseAddr = compiler->lvaFrameAddress(dstLclNum, &fpBased);

dstReg = fpBased ? REG_FPBASE : REG_SPBASE;

helper.SetDstOffset(baseAddr + dstOffset);

#ifdef DEBUG
isDstRegAddrAlignmentKnown = true;
#endif
}

bool canEncodeAllLoads = true;
Expand Down Expand Up @@ -2948,6 +2959,18 @@ void CodeGen::genCodeForCpBlkUnroll(GenTreeBlk* node)
}
#endif

#ifndef JIT32_GCENCODER
if (!node->gtBlkOpGcUnsafe && ((srcOffsetAdjustment != 0) || (dstOffsetAdjustment != 0)))
{
// If node is not already marked as non-interruptible, and if are about to generate code
// that produce GC references in temporary registers not reported, then mark the block
// as non-interruptible.
// Corresponding EnableGC() will be done by the caller of this method.
node->gtBlkOpGcUnsafe = true;
GetEmitter()->emitDisableGC();
}
#endif

if ((srcOffsetAdjustment != 0) && (dstOffsetAdjustment != 0))
{
const regNumber tempReg1 = node->ExtractTempReg(RBM_ALLINT);
Expand Down
10 changes: 1 addition & 9 deletions src/coreclr/jit/lowerarmarch.cpp
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Expand Up @@ -440,18 +440,10 @@ void Lowering::LowerBlockStore(GenTreeBlk* blkNode)

if (blkNode->OperIs(GT_STORE_OBJ))
{
if (!blkNode->AsObj()->GetLayout()->HasGCPtr())
if (!blkNode->AsObj()->GetLayout()->HasGCPtr() || (isDstAddrLocal && (size <= copyBlockUnrollLimit)))
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@BruceForstall BruceForstall May 12, 2022

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This looks like a bug: before, if (isDstAddrLocal && (size <= copyBlockUnrollLimit)), we would set gtBlkOpGcUnsafe=true. Now, we won't unless !isSrcAddrLocal.

So, if both isSrcAddrLocal and isDstAddrLocal are true, we won't disable GC. However, the thing we are copying contains GC pointers in this case, and the GC pointers loaded into temporary registers aren't reported, so we have a GC hole.

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@kunalspathak kunalspathak May 14, 2022

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That's true. I missed that case.

{
blkNode->SetOper(GT_STORE_BLK);
}
else if (isDstAddrLocal && (size <= copyBlockUnrollLimit))
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@BruceForstall BruceForstall May 12, 2022

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It seems like we don't need the isDstAddrLocal check: why can't we use this code path even if copying with src=heap, dst=heap, or src=stack, dst=heap?

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@kunalspathak kunalspathak May 14, 2022

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I was trying to consolidate the blkNode->SetOper(GT_STORE_BLK) case, but in general, we need to disable GC for following unroll cases:

  • isDstAddrLocal = true (as of today)
  • isDstAddrLocal = false (if dst is on heap)
  • isSrcAddrLocal = false (if src is on heap)
  • isSrcAddrLocal = true (since we want to disable GC regardless if dst is on stack/heap, src state won't matter).

So basically, if we are doing GT_STORE_BLK and unrolling, we need to disable GC. With that said, I think we should disable the GC inside genCodeForCpBlkUnroll() only for cases where we will be doing the adjustments i.e. srcOffsetAdjustment != 0 || dstOffsetAdjustment != 0 to limit it to scenarios that are really affected.

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@kunalspathak kunalspathak May 31, 2022

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With that being the case, I missed a case for objects that has GC pointers. Fixed in #70053

{
// If the size is small enough to unroll then we need to mark the block as non-interruptible
// to actually allow unrolling. The generated code does not report GC references loaded in the
// temporary register(s) used for copying.
blkNode->SetOper(GT_STORE_BLK);
blkNode->gtBlkOpGcUnsafe = true;
}
}

if (blkNode->OperIs(GT_STORE_OBJ))
Expand Down