JIT: Disallow flowgraph modifications during VN opts

src/coreclr/jit/compiler.h

@@ -5417,7 +5417,7 @@ class Compiler
 
     void fgMergeBlockReturn(BasicBlock* block);
 
-    bool fgMorphBlockStmt(BasicBlock* block, Statement* stmt DEBUGARG(const char* msg), bool invalidateDFSTreeOnFGChange = true);
+    bool fgMorphBlockStmt(BasicBlock* block, Statement* stmt DEBUGARG(const char* msg), bool allowFGChange = true, bool invalidateDFSTreeOnFGChange = true);
     void fgMorphStmtBlockOps(BasicBlock* block, Statement* stmt);
 
     bool gtRemoveTreesAfterNoReturnCall(BasicBlock* block, Statement* stmt);

src/coreclr/jit/earlyprop.cpp

@@ -469,7 +469,7 @@ bool Compiler::optFoldNullCheck(GenTree* tree, LocalNumberToNullCheckTreeMap* nu
 
         // Re-morph the statement.
         Statement* curStmt = compCurStmt;
-        fgMorphBlockStmt(compCurBB, nullCheckStmt DEBUGARG("optFoldNullCheck"));
+        fgMorphBlockStmt(compCurBB, nullCheckStmt DEBUGARG("optFoldNullCheck"), /* allowFGChange */ false);
         optRecordSsaUses(nullCheckStmt->GetRootNode(), compCurBB);
         compCurStmt = curStmt;
 

src/coreclr/jit/morph.cpp

@@ -12265,6 +12265,7 @@ Compiler::FoldResult Compiler::fgFoldConditional(BasicBlock* block)
 //    block                       - block containing the statement
 //    stmt                        - statement to morph
 //    msg                         - string to identify caller in a dump
+//    allowFGChange               - whether or not the flow graph can be changed
 //    invalidateDFSTreeOnFGChange - whether or not the DFS tree should be invalidated
 //                                  by this function if it makes a flow graph change
 //
@@ -12277,6 +12278,7 @@ Compiler::FoldResult Compiler::fgFoldConditional(BasicBlock* block)
 //
 bool Compiler::fgMorphBlockStmt(BasicBlock*     block,
                                 Statement* stmt DEBUGARG(const char* msg),
+                                bool            allowFGChange,
                                 bool            invalidateDFSTreeOnFGChange)
 {
     assert(block != nullptr);
@@ -12326,7 +12328,7 @@ bool Compiler::fgMorphBlockStmt(BasicBlock*     block,
     bool removedStmt = fgCheckRemoveStmt(block, stmt);
 
     // Or this is the last statement of a conditional branch that was just folded?
-    if (!removedStmt && (stmt->GetNextStmt() == nullptr) && !fgRemoveRestOfBlock)
+    if (allowFGChange && !removedStmt && (stmt->GetNextStmt() == nullptr) && !fgRemoveRestOfBlock)
     {
         FoldResult const fr = fgFoldConditional(block);
         if (invalidateDFSTreeOnFGChange && (fr != FoldResult::FOLD_DID_NOTHING))
@@ -12369,7 +12371,7 @@ bool Compiler::fgMorphBlockStmt(BasicBlock*     block,
         //
         // For compDbgCode, we prepend an empty BB as the firstBB, it is BBJ_ALWAYS.
         // We should not convert it to a ThrowBB.
-        if ((block != fgFirstBB) || !fgFirstBB->HasFlag(BBF_INTERNAL))
+        if (allowFGChange && ((block != fgFirstBB) || !fgFirstBB->HasFlag(BBF_INTERNAL)))
         {
             // Convert block to a throw bb, or make it rarely run if already a throw.
             //

src/coreclr/jit/redundantbranchopts.cpp

@@ -980,7 +980,8 @@ bool Compiler::optRedundantBranch(BasicBlock* const block)
 
     JITDUMP("\nRedundant branch opt in " FMT_BB ":\n", block->bbNum);
 
-    fgMorphBlockStmt(block, stmt DEBUGARG(__FUNCTION__), /* invalidateDFSTreeOnFGChange */ false);
+    fgMorphBlockStmt(block, stmt DEBUGARG(__FUNCTION__), /* allowFGChange */ true,
+                     /* invalidateDFSTreeOnFGChange */ false);
     Metrics.RedundantBranchesEliminated++;
     return true;
 }

src/coreclr/jit/valuenum.cpp

@@ -10865,11 +10865,7 @@ PhaseStatus Compiler::fgValueNumber()
     }
 
     assert(m_dfsTree != nullptr);
-
-    if (m_loops == nullptr)
-    {
-        m_loops = FlowGraphNaturalLoops::Find(m_dfsTree);
-    }
+    assert(m_loops != nullptr);
 
     m_blockToLoop = BlockToNaturalLoopMap::Build(m_loops);
     // Compute the side effects of loops.