JIT: handle case where we are cloning adjacent sibling loops (#70874)

We can sometimes see adjacent sibling loops (where L1.bottom == L2.head)
and if so, cloning L1 will repurpose L1.bottom and so leave L2 in
an inconsistent state.

Detect this case during optCanonicalizeLoop, and add an intermediary
block to serve as L2's head.

Fixes #70569.

src/coreclr/jit/optimizer.cpp

@@ -2678,6 +2678,9 @@ NO_MORE_LOOPS:
 
     // Make sure that loops are canonical: that every loop has a unique "top", by creating an empty "nop"
     // one, if necessary, for loops containing others that share a "top."
+    //
+    // Also make sure that no loop's "bottom" is another loop's "head".
+    //
     for (unsigned char loopInd = 0; loopInd < optLoopCount; loopInd++)
     {
         // Traverse the outermost loops as entries into the loop nest; so skip non-outermost.
@@ -2923,7 +2926,7 @@ bool Compiler::optCanonicalizeLoopNest(unsigned char loopInd)
 
 //-----------------------------------------------------------------------------
 // optCanonicalizeLoop: ensure that each loop top's back edges come only from
-//   blocks in the same loop.
+//   blocks in the same loop, and that no loop head/bottom blocks coincide.
 //
 // Arguments:
 //   loopInd - index of the loop to consider
@@ -3006,6 +3009,84 @@ bool Compiler::optCanonicalizeLoop(unsigned char loopInd)
         modified |= didCanon;
     }
 
+    // Check if this loopInd head is also the bottom of some sibling.
+    // If so, add a block in between to serve as the new head.
+    //
+    auto repairLoop = [this](unsigned char loopInd, unsigned char sibling) {
+
+        BasicBlock* const h        = optLoopTable[loopInd].lpHead;
+        BasicBlock* const siblingB = optLoopTable[sibling].lpBottom;
+
+        if (h == siblingB)
+        {
+            // We have
+            //
+            //   sibling.B (== loopInd.H) -e-> loopInd.T
+            //
+            // where e is a "critical edge", that is
+            // * sibling.B has other successors (notably sibling.T),
+            // * loopInd.T has other predecessors (notably loopInd.B)
+            //
+            // turn this into
+            //
+            //  sibling.B -> newH (== loopInd.H) -> loopInd.T
+            //
+            // Ideally we'd just call fgSplitEdge, but we are
+            // not keeping pred lists in good shape.
+            //
+            BasicBlock* const t = optLoopTable[loopInd].lpTop;
+            assert(siblingB->bbJumpKind == BBJ_COND);
+            assert(siblingB->bbNext == t);
+
+            JITDUMP(FMT_LP " head " FMT_BB " is also " FMT_LP " bottom\n", loopInd, h->bbNum, sibling);
+
+            BasicBlock* const newH = fgNewBBbefore(BBJ_NONE, t, /*extendRegion*/ true);
+
+            // Anything that flows into sibling will flow here.
+            // So we use sibling.H as our best guess for weight.
+            //
+            newH->inheritWeight(optLoopTable[sibling].lpHead);
+            newH->bbNatLoopNum = optLoopTable[loopInd].lpParent;
+            optUpdateLoopHead(loopInd, h, newH);
+
+            return true;
+        }
+        return false;
+    };
+
+    if (optLoopTable[loopInd].lpParent == BasicBlock::NOT_IN_LOOP)
+    {
+        // check against all other top-level loops
+        //
+        for (unsigned char sibling = 0; sibling < optLoopCount; sibling++)
+        {
+            if (optLoopTable[sibling].lpParent != BasicBlock::NOT_IN_LOOP)
+            {
+                continue;
+            }
+
+            modified |= repairLoop(loopInd, sibling);
+        }
+    }
+    else
+    {
+        // check against all other sibling loops
+        //
+        const unsigned char parentLoop = optLoopTable[loopInd].lpParent;
+
+        for (unsigned char sibling = optLoopTable[parentLoop].lpChild; //
+             sibling != BasicBlock::NOT_IN_LOOP;                       //
+             sibling = optLoopTable[sibling].lpSibling)
+        {
+            if (sibling == loopInd)
+            {
+                continue;
+            }
+
+            modified |= repairLoop(loopInd, sibling);
+        }
+    }
+
     if (modified)
     {
         JITDUMP("Done canonicalizing " FMT_LP "\n\n", loopInd);