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Check if loop body occured before loopTop and if so unmark alignment #91854

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Sep 12, 2023
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Check if loop body occured before loopTop and if so unmark alignment #91854

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34 changes: 31 additions & 3 deletions src/coreclr/jit/compiler.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -5276,6 +5276,13 @@ PhaseStatus Compiler::placeLoopAlignInstructions()
weight_t minBlockSoFar = BB_MAX_WEIGHT;
BasicBlock* bbHavingAlign = nullptr;
BasicBlock::loopNumber currentAlignedLoopNum = BasicBlock::NOT_IN_LOOP;
bool visitedLoopNum[BasicBlock::MAX_LOOP_NUM];
memset(visitedLoopNum, false, sizeof(visitedLoopNum));
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It's weird to use memset to zero a bool array, but ok.


#ifdef DEBUG
int visitedBlockForLoopNum[BasicBlock::MAX_LOOP_NUM];
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block nums can't be negative; use unsigned?

memset(visitedBlockForLoopNum, false, sizeof(visitedBlockForLoopNum));
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It's weird to use false as a value to zero an int array.

#endif

if ((fgFirstBB != nullptr) && fgFirstBB->isLoopAlign())
{
Expand All @@ -5298,7 +5305,7 @@ PhaseStatus Compiler::placeLoopAlignInstructions()
}
}

// If there is a unconditional jump (which is not part of callf/always pair)
// If there is an unconditional jump (which is not part of callf/always pair)
if (opts.compJitHideAlignBehindJmp && (block->bbJumpKind == BBJ_ALWAYS) && !block->isBBCallAlwaysPairTail())
{
// Track the lower weight blocks
Expand Down Expand Up @@ -5352,12 +5359,19 @@ PhaseStatus Compiler::placeLoopAlignInstructions()
madeChanges = true;
unmarkedLoopAlign = true;
}
else if ((block->bbNatLoopNum != BasicBlock::NOT_IN_LOOP) && (block->bbNatLoopNum == loopTop->bbNatLoopNum))
else if (visitedLoopNum[loopTop->bbNatLoopNum])
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Don't you need to check that block->bbNatLoopNum != BasicBlock::NOT_IN_LOOP?

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Not really. block is the previous block before loopTop and we want to check here if we have seen a loop body related to loopTop->bbNatLoopNum.

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Oh, sorry, I meant, don't you need to check loopTop->bbNatLoopNum != BasicBlock::NOT_IN_LOOP?

It appears that sometimes it does equal NOT_IN_LOOP, leading to reading beyond the array bounds.

{
#ifdef DEBUG
char buffer[50];
sprintf_s(buffer, 50, "loop block " FMT_BB " appears before top of loop",
visitedBlockForLoopNum[loopTop->bbNatLoopNum]);
#endif

// In some odd cases we may see blocks within the loop before we see the
// top block of the loop. Just bail on aligning such loops.
//
loopTop->unmarkLoopAlign(this DEBUG_ARG("loop block appears before top of loop"));

loopTop->unmarkLoopAlign(this DEBUG_ARG(buffer));
madeChanges = true;
unmarkedLoopAlign = true;
}
Expand Down Expand Up @@ -5392,6 +5406,20 @@ PhaseStatus Compiler::placeLoopAlignInstructions()
break;
}
}

if (block->bbNatLoopNum != BasicBlock::NOT_IN_LOOP)
{
#ifdef DEBUG
if (!visitedLoopNum[block->bbNatLoopNum])
{
// Record the first block for which bbNatLoopNum was seen for
// debugging purpose.
visitedBlockForLoopNum[block->bbNatLoopNum] = block->bbNum;
}
#endif
// If this block is part of loop, mark the loopNum as visited.
visitedLoopNum[block->bbNatLoopNum] = true;
}
}

assert(loopsToProcess == 0);
Expand Down