From 7861154e26c764f1c4fd5cb0c862d6e0c87d1fca Mon Sep 17 00:00:00 2001
From: Andy Ayers <andya@microsoft.com>
Date: Thu, 1 Sep 2022 18:55:54 -0700
Subject: [PATCH] JIT: assign plausible loop number to new throw helper blocks

These can get added during CSE and may appear in a span of blocks that
comprise an existing loop.

Annotate the new blocks as belonging to an appropriate loop, even though
by rights they don't belong to any loop.

Closes #74739.
---
 src/coreclr/jit/fgbasic.cpp | 52 +++++++++++++++++++++++++++++++++++++
 1 file changed, 52 insertions(+)

diff --git a/src/coreclr/jit/fgbasic.cpp b/src/coreclr/jit/fgbasic.cpp
index f4e078cbaa1bc2..20c880ddc66646 100644
--- a/src/coreclr/jit/fgbasic.cpp
+++ b/src/coreclr/jit/fgbasic.cpp
@@ -6495,6 +6495,58 @@ BasicBlock* Compiler::fgNewBBinRegionWorker(BBjumpKinds jumpKind,
     /* If afterBlk falls through, we insert a jump around newBlk */
     fgConnectFallThrough(afterBlk, newBlk->bbNext);
 
+    // If the loop table is valid, add this block to the appropriate loop.
+    // Note we don't verify (via flow) that this block actually belongs
+    // to the loop, just that it is lexically within the span of blocks
+    // in the loop.
+    //
+    if (optLoopTableValid)
+    {
+        BasicBlock* const bbPrev = newBlk->bbPrev;
+        BasicBlock* const bbNext = newBlk->bbNext;
+
+        if ((bbPrev != nullptr) && (bbNext != nullptr))
+        {
+            BasicBlock::loopNumber const prevLoopNum = bbPrev->bbNatLoopNum;
+            BasicBlock::loopNumber const nextLoopNum = bbNext->bbNatLoopNum;
+
+            if ((prevLoopNum != BasicBlock::NOT_IN_LOOP) && (nextLoopNum != BasicBlock::NOT_IN_LOOP))
+            {
+                if (prevLoopNum == nextLoopNum)
+                {
+                    newBlk->bbNatLoopNum = prevLoopNum;
+                }
+                else
+                {
+                    BasicBlock::loopNumber const prevParentLoopNum = optLoopTable[prevLoopNum].lpParent;
+                    BasicBlock::loopNumber const nextParentLoopNum = optLoopTable[nextLoopNum].lpParent;
+
+                    if (nextParentLoopNum == prevLoopNum)
+                    {
+                        // next is in child loop
+                        newBlk->bbNatLoopNum = prevLoopNum;
+                    }
+                    else if (prevParentLoopNum == nextLoopNum)
+                    {
+                        // prev is in child loop
+                        newBlk->bbNatLoopNum = nextLoopNum;
+                    }
+                    else
+                    {
+                        // next and prev are siblings
+                        assert(prevParentLoopNum == nextParentLoopNum);
+                        newBlk->bbNatLoopNum = prevParentLoopNum;
+                    }
+                }
+            }
+        }
+
+        if (newBlk->bbNatLoopNum != BasicBlock::NOT_IN_LOOP)
+        {
+            JITDUMP("Marked " FMT_BB " as lying within " FMT_LP "\n", newBlk->bbNum, newBlk->bbNatLoopNum);
+        }
+    }
+
 #ifdef DEBUG
     fgVerifyHandlerTab();
 #endif