unblock cloning of loops where the header is a try begin (#108604)

Author: AndyAyersMS

src/coreclr/jit/flowgraph.cpp

@@ -5964,9 +5964,9 @@ bool FlowGraphNaturalLoop::CanDuplicate(INDEBUG(const char** reason))
 
     Compiler*       comp   = m_dfsTree->GetCompiler();
     BasicBlockVisit result = VisitLoopBlocks([=](BasicBlock* block) {
-        if (comp->bbIsTryBeg(block))
+        if (!BasicBlock::sameEHRegion(block, GetHeader()))
         {
-            INDEBUG(*reason = "Loop has a `try` begin");
+            INDEBUG(*reason = "Loop not entirely within one EH region");
             return BasicBlockVisit::Abort;
         }
 

src/coreclr/jit/loopcloning.cpp

@@ -1929,7 +1929,6 @@ bool Compiler::optIsLoopClonable(FlowGraphNaturalLoop* loop, LoopCloneContext* c
                 loop->GetIndex());
         return false;
     }
-
     assert(!requireIterable || !lvaVarAddrExposed(iterInfo->IterVar));
 
     if (requireIterable)

src/coreclr/jit/optimizer.cpp

@@ -2866,15 +2866,28 @@ bool Compiler::optCreatePreheader(FlowGraphNaturalLoop* loop)
 {
     BasicBlock* header = loop->GetHeader();
 
-    // If the header is already a try entry then we need to keep it as such
-    // since blocks from within the loop will be jumping back to it after we're
-    // done. Thus, in that case we insert the preheader in the enclosing try
-    // region.
-    unsigned headerEHRegion    = header->hasTryIndex() ? header->getTryIndex() : EHblkDsc::NO_ENCLOSING_INDEX;
-    unsigned preheaderEHRegion = headerEHRegion;
-    if ((headerEHRegion != EHblkDsc::NO_ENCLOSING_INDEX) && bbIsTryBeg(header))
+    // If all loop backedges sources are within the same try region as the loop header,
+    // then the preheader can be in the same try region as the header.
+    //
+    // Otherwise the preheader must be in the enclosing try region.
+    //
+    unsigned preheaderEHRegion    = EHblkDsc::NO_ENCLOSING_INDEX;
+    bool     inSameRegionAsHeader = true;
+    if (header->hasTryIndex())
     {
-        preheaderEHRegion = ehTrueEnclosingTryIndexIL(headerEHRegion);
+        preheaderEHRegion = header->getTryIndex();
+        for (FlowEdge* backEdge : loop->BackEdges())
+        {
+            BasicBlock* const backedgeSource = backEdge->getSourceBlock();
+            if (!bbInTryRegions(preheaderEHRegion, backedgeSource))
+            {
+                // Preheader should be in the true enclosing region of the header.
+                //
+                preheaderEHRegion    = ehTrueEnclosingTryIndexIL(preheaderEHRegion);
+                inSameRegionAsHeader = false;
+                break;
+            }
+        }
     }
 
     if (!bbIsHandlerBeg(header) && (loop->EntryEdges().size() == 1))
@@ -2893,7 +2906,16 @@ bool Compiler::optCreatePreheader(FlowGraphNaturalLoop* loop)
 
     BasicBlock* preheader = fgNewBBbefore(BBJ_ALWAYS, header, false);
     preheader->SetFlags(BBF_INTERNAL);
-    fgSetEHRegionForNewPreheaderOrExit(preheader);
+
+    if (inSameRegionAsHeader)
+    {
+        fgExtendEHRegionBefore(header);
+    }
+    else
+    {
+        fgSetEHRegionForNewPreheaderOrExit(preheader);
+    }
+
     preheader->bbCodeOffs = header->bbCodeOffs;
 
     JITDUMP("Created new preheader " FMT_BB " for " FMT_LP "\n", preheader->bbNum, loop->GetIndex());