Reset the bbNatLoopNum of block for removed loop

[kunalspathak]

When a loop is removed, we should also reset the bbNatLoopNum present at the blocks to reflect the updated information.

Fixes: #68769

[kunalspathak]

@dotnet/jit-contrib

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Issue Details

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When a loop is removed, we should also reset the bbNatLoopNum present at the blocks to reflect the updated information.

Fixes: #68769

Author:	kunalspathak
Assignees:	-
Labels:	area-CodeGen-coreclr
Milestone:	-

[kunalspathak]

I intentionally added an assert about assert(auxBlock->bbNatLoopNum == loopNum); and realized that this is not always true and we hit the assert even while compiling SPC because auxBlock could be inside a nested loop and bbNatLoopNum could be pointing to that.

But I also realized that we don't update the lpParent field of child loops if a parent is removed and that is one of the reason we have this:

runtime/src/coreclr/jit/optimizer.cpp

Lines 9587 to 9590 in 6111bc7

	if (optAnyChildNotRemoved(loopNum))
	{
	JITDUMP("Removed loop " FMT_LP " has one or more live children\n", loopNum);
	}

runtime/src/coreclr/jit/optimizer.cpp

Lines 9537 to 9539 in 6111bc7

	// optAnyChildNotRemoved: Recursively check the child loops of a loop to see if any of them
	// are still live (that is, not marked as LPFLG_REMOVED). This check is done when we are
	// removing a parent, just to notify that there is something odd about leaving a live child.

If we have live children, at the minimal, we should at least update the lpParent to record that lpParent is no longer its parent.

[BruceForstall]

Suggested change

	for (BasicBlock::loopNumber l = optLoopTable[loopNum].lpChild; //
	l != BasicBlock::NOT_IN_LOOP; //
	l = optLoopTable[l].lpSibling)
	{
	if ((optLoopTable[l].lpFlags & LPFLG_REMOVED) == 0)
	{
	JITDUMP("Resetting parent of loop number " FMT_LP " from " FMT_LP " to " FMT_LP ".\n", l,
	optLoopTable[l].lpParent, optLoopTable[loopNum].lpParent);
	optLoopTable[l].lpParent = optLoopTable[loopNum].lpParent;
	}
	}
	for (BasicBlock::loopNumber l = loop.lpChild; //
	l != BasicBlock::NOT_IN_LOOP; //
	l = optLoopTable[l].lpSibling)
	{
	if ((optLoopTable[l].lpFlags & LPFLG_REMOVED) == 0)
	{
	JITDUMP("Resetting parent of loop number " FMT_LP " from " FMT_LP " to " FMT_LP ".\n", l,
	optLoopTable[l].lpParent, loop.lpParent);
	optLoopTable[l].lpParent = loop.lpParent;
	}
	}