JIT: Simplify block insertion logic during loop canonicalization

src/coreclr/jit/compiler.h

@@ -5316,6 +5316,8 @@ class Compiler
                                       unsigned    xcptnIndex,
                                       bool        putInTryRegion);
 
+    BasicBlock* fgNewBBatTryRegionEnd(BBKinds jumpKind, unsigned tryIndex);
+
     void fgInsertBBbefore(BasicBlock* insertBeforeBlk, BasicBlock* newBlk);
     void fgInsertBBafter(BasicBlock* insertAfterBlk, BasicBlock* newBlk);
     void fgUnlinkBlock(BasicBlock* block);

src/coreclr/jit/fgbasic.cpp

@@ -6723,6 +6723,39 @@ BasicBlock* Compiler::fgNewBBinRegionWorker(BBKinds     jumpKind,
     return newBlk;
 }
 
+//-----------------------------------------------------------------------------
+// fgNewBBatTryRegionEnd: Creates and inserts a new block at the end of the specified
+// try region, updating the try end pointers in the EH table as necessary.
+//
+// Arguments:
+//    jumpKind - The jump kind of the new block
+//    tryIndex - The index of the try region to insert the new block in
+//
+// Returns:
+//    The new block
+//
+BasicBlock* Compiler::fgNewBBatTryRegionEnd(BBKinds jumpKind, unsigned tryIndex)
+{
+    BasicBlock* const oldTryLast = ehGetDsc(tryIndex)->ebdTryLast;
+    BasicBlock* const newBlock   = fgNewBBafter(jumpKind, oldTryLast, /* extendRegion */ false);
+    newBlock->setTryIndex(tryIndex);
+    newBlock->clearHndIndex();
+
+    // Update this try region's (and all parent try regions') last block pointer
+    //
+    for (unsigned XTnum = tryIndex; XTnum < compHndBBtabCount; XTnum++)
+    {
+        EHblkDsc* const HBtab = ehGetDsc(XTnum);
+        if (HBtab->ebdTryLast == oldTryLast)
+        {
+            assert((XTnum == tryIndex) || (ehGetDsc(tryIndex)->ebdEnclosingTryIndex != EHblkDsc::NO_ENCLOSING_INDEX));
+            fgSetTryEnd(HBtab, newBlock);
+        }
+    }
+
+    return newBlock;
+}
+
 //------------------------------------------------------------------------
 // fgUseThrowHelperBlocks: Determinate does compiler use throw helper blocks.
 //

src/coreclr/jit/optimizer.cpp

@@ -3024,16 +3024,10 @@ bool Compiler::optCreatePreheader(FlowGraphNaturalLoop* loop)
         }
     }
 
-    BasicBlock* insertBefore = loop->GetLexicallyTopMostBlock();
-    if (!BasicBlock::sameEHRegion(insertBefore, header))
-    {
-        insertBefore = header;
-    }
-
-    BasicBlock* preheader = fgNewBBbefore(BBJ_ALWAYS, insertBefore, false);
+    BasicBlock* preheader = fgNewBBbefore(BBJ_ALWAYS, header, false);
     preheader->SetFlags(BBF_INTERNAL);
     fgSetEHRegionForNewPreheaderOrExit(preheader);
-    preheader->bbCodeOffs = insertBefore->bbCodeOffs;
+    preheader->bbCodeOffs = header->bbCodeOffs;
 
     JITDUMP("Created new preheader " FMT_BB " for " FMT_LP "\n", preheader->bbNum, loop->GetIndex());
 
@@ -3136,21 +3130,11 @@ bool Compiler::optCanonicalizeExit(FlowGraphNaturalLoop* loop, BasicBlock* exit)
     {
         // Branches to a BBJ_CALLFINALLY _must_ come from inside its associated
         // try region, and when we have callfinally thunks the BBJ_CALLFINALLY
-        // is outside it. First try to see if the lexically bottom most block
-        // is part of the try; if so, inserting after that is a good choice.
+        // is outside it. Thus, insert newExit at the end of the finally's
+        // try region.
         BasicBlock* finallyBlock = exit->GetTarget();
         assert(finallyBlock->hasHndIndex());
-        BasicBlock* bottom = loop->GetLexicallyBottomMostBlock();
-        if (bottom->hasTryIndex() && (bottom->getTryIndex() == finallyBlock->getHndIndex()) && !bottom->hasHndIndex())
-        {
-            newExit = fgNewBBafter(BBJ_ALWAYS, bottom, true);
-        }
-        else
-        {
-            // Otherwise just do the heavy-handed thing and insert it anywhere in the right region.
-            newExit = fgNewBBinRegion(BBJ_ALWAYS, finallyBlock->bbHndIndex, 0, nullptr, /* putInFilter */ false,
-                                      /* runRarely */ false, /* insertAtEnd */ true);
-        }
+        newExit = fgNewBBatTryRegionEnd(BBJ_ALWAYS, finallyBlock->getHndIndex());
     }
     else
     {