JIT: Add an "init BB" invariant

src/coreclr/jit/codegenlinear.cpp

@@ -156,10 +156,6 @@ void CodeGen::genCodeForBBlist()
 
     genMarkLabelsForCodegen();
 
-    assert(!compiler->fgFirstBBScratch ||
-           compiler->fgFirstBB == compiler->fgFirstBBScratch); // compiler->fgFirstBBScratch
-                                                               // has to be first.
-
     /* Initialize structures used in the block list iteration */
     genInitialize();
 
@@ -367,9 +363,9 @@ void CodeGen::genCodeForBBlist()
         siBeginBlock(block);
 
         // BBF_INTERNAL blocks don't correspond to any single IL instruction.
-        if (compiler->opts.compDbgInfo && block->HasFlag(BBF_INTERNAL) &&
-            !compiler->fgBBisScratch(block)) // If the block is the distinguished first scratch block, then no need to
-                                             // emit a NO_MAPPING entry, immediately after the prolog.
+        // Add a NoMapping entry unless this is right after the prolog where it
+        // is unnecessary.
+        if (compiler->opts.compDbgInfo && block->HasFlag(BBF_INTERNAL) && !block->IsFirst())
         {
             genIPmappingAdd(IPmappingDscKind::NoMapping, DebugInfo(), true);
         }
@@ -396,9 +392,9 @@ void CodeGen::genCodeForBBlist()
         }
 #endif
 
-        // Emit poisoning into scratch BB that comes right after prolog.
+        // Emit poisoning into the init BB that comes right after prolog.
         // We cannot emit this code in the prolog as it might make the prolog too large.
-        if (compiler->compShouldPoisonFrame() && compiler->fgBBisScratch(block))
+        if (compiler->compShouldPoisonFrame() && block->IsFirst())
         {
             genPoisonFrame(newLiveRegSet);
         }

src/coreclr/jit/compiler.cpp

@@ -3743,20 +3743,6 @@ void Compiler::compInitDebuggingInfo()
         compInitScopeLists();
     }
 
-    if (opts.compDbgCode && (info.compVarScopesCount > 0))
-    {
-        /* Create a new empty basic block. fgExtendDbgLifetimes() may add
-           initialization of variables which are in scope right from the
-           start of the (real) first BB (and therefore artificially marked
-           as alive) into this block.
-         */
-
-        fgEnsureFirstBBisScratch();
-
-        fgNewStmtAtEnd(fgFirstBB, gtNewNothingNode());
-
-        JITDUMP("Debuggable code - Add new %s to perform initialization of variables\n", fgFirstBB->dspToString());
-    }
     /*-------------------------------------------------------------------------
      *
      * Read the stmt-offsets table and the line-number table
@@ -4523,6 +4509,9 @@ void Compiler::compCompile(void** methodCodePtr, uint32_t* methodCodeSize, JitFl
     activePhaseChecks |= PhaseChecks::CHECK_PROFILE;
     DoPhase(this, PHASE_INCPROFILE, &Compiler::fgIncorporateProfileData);
 
+    activePhaseChecks |= PhaseChecks::CHECK_FG_INIT_BLOCK;
+    DoPhase(this, PHASE_CANONICALIZE_ENTRY_BB, &Compiler::fgCanonicalizeFirstBB);
+
     // If we are doing OSR, update flow to initially reach the appropriate IL offset.
     //
     if (opts.IsOSR())
@@ -4691,10 +4680,6 @@ void Compiler::compCompile(void** methodCodePtr, uint32_t* methodCodeSize, JitFl
 
     if (opts.OptimizationEnabled())
     {
-        // Canonicalize entry to have unique entry BB to put IR in for the upcoming phases
-        //
-        DoPhase(this, PHASE_CANONICALIZE_ENTRY, &Compiler::fgCanonicalizeFirstBB);
-
         // Build post-order and remove dead blocks
         //
         DoPhase(this, PHASE_DFS_BLOCKS2, &Compiler::fgDfsBlocksAndRemove);
@@ -4789,10 +4774,6 @@ void Compiler::compCompile(void** methodCodePtr, uint32_t* methodCodeSize, JitFl
             return fgHeadTailMerge(false);
         });
 
-        // Canonicalize entry to give a unique dominator tree root
-        //
-        DoPhase(this, PHASE_CANONICALIZE_ENTRY, &Compiler::fgCanonicalizeFirstBB);
-
         // Compute DFS tree and remove all unreachable blocks.
         //
         DoPhase(this, PHASE_DFS_BLOCKS3, &Compiler::fgDfsBlocksAndRemove);
@@ -5024,12 +5005,6 @@ void Compiler::compCompile(void** methodCodePtr, uint32_t* methodCodeSize, JitFl
 
             assert(opts.optRepeat);
 
-            // We may have optimized away the canonical entry BB that SSA
-            // depends on above, so if we are going for another iteration then
-            // make sure we still have a canonical entry.
-            //
-            DoPhase(this, PHASE_CANONICALIZE_ENTRY, &Compiler::fgCanonicalizeFirstBB);
-
             ResetOptAnnotations();
             RecomputeFlowGraphAnnotations();
 

src/coreclr/jit/compiler.h

@@ -1542,6 +1542,7 @@ enum class PhaseChecks : unsigned int
     CHECK_LIKELIHOODS   = 1 << 5, // profile data likelihood integrity
     CHECK_PROFILE       = 1 << 6, // profile data full integrity
     CHECK_LINKED_LOCALS = 1 << 7, // check linked list of locals
+    CHECK_FG_INIT_BLOCK = 1 << 8, // flow graph has an init block
 };
 
 inline constexpr PhaseChecks operator ~(PhaseChecks a)
@@ -5175,7 +5176,6 @@ class Compiler
     BasicBlock* fgEntryBB = nullptr;        // For OSR, the original method's entry point
     BasicBlock* fgOSREntryBB = nullptr;     // For OSR, the logical entry point (~ patchpoint)
     BasicBlock* fgFirstFuncletBB = nullptr; // First block of outlined funclets (to allow block insertion before the funclets)
-    BasicBlock* fgFirstBBScratch = nullptr;   // Block inserted for initialization stuff. Is nullptr if no such block has been
                                     // created.
     BasicBlockList* fgReturnBlocks = nullptr; // list of BBJ_RETURN blocks
     unsigned        fgEdgeCount = 0;    // # of control flow edges between the BBs
@@ -5224,10 +5224,6 @@ class Compiler
         return getAllocator(cmk).allocate<T>(fgBBNumMax + 1);
     }
 
-    bool fgEnsureFirstBBisScratch();
-    bool fgFirstBBisScratch();
-    bool fgBBisScratch(BasicBlock* block);
-
     void fgExtendEHRegionBefore(BasicBlock* block);
     void fgExtendEHRegionAfter(BasicBlock* block);
 
@@ -5414,6 +5410,7 @@ class Compiler
     };
 
     PhaseStatus fgMorphBlocks();
+    BasicBlock* fgGetFirstILBlock();
     void fgMorphBlock(BasicBlock* block, MorphUnreachableInfo* unreachableInfo = nullptr);
     void fgMorphStmts(BasicBlock* block);
 
@@ -6133,6 +6130,7 @@ class Compiler
     bool fgCheckRemoveStmt(BasicBlock* block, Statement* stmt);
 
     PhaseStatus fgCanonicalizeFirstBB();
+    void fgCreateNewInitBB();
 
     void fgSetEHRegionForNewPreheaderOrExit(BasicBlock* preheader);
 
@@ -6154,6 +6152,8 @@ class Compiler
 
     bool fgCanCompactBlock(BasicBlock* block);
 
+    bool fgCanCompactInitBlock();
+
     void fgCompactBlock(BasicBlock* block);
 
     BasicBlock* fgConnectFallThrough(BasicBlock* bSrc, BasicBlock* bDst);
@@ -6330,6 +6330,7 @@ class Compiler
     void fgDebugCheckBBNumIncreasing();
     void fgDebugCheckBBlist(bool checkBBNum = false, bool checkBBRefs = true);
     void fgDebugCheckBlockLinks();
+    void fgDebugCheckInitBB();
     void fgDebugCheckLinks(bool morphTrees = false);
     void fgDebugCheckStmtsList(BasicBlock* block, bool morphTrees);
     void fgDebugCheckNodeLinks(BasicBlock* block, Statement* stmt);

src/coreclr/jit/compphases.h

@@ -68,7 +68,7 @@ CompPhaseNameMacro(PHASE_OPTIMIZE_FLOW,              "Optimize control flow",
 CompPhaseNameMacro(PHASE_OPTIMIZE_LAYOUT,            "Optimize layout",                false, -1, false)
 CompPhaseNameMacro(PHASE_OPTIMIZE_POST_LAYOUT,       "Optimize post-layout",           false, -1, false)
 CompPhaseNameMacro(PHASE_COMPUTE_DOMINATORS,         "Compute dominators",             false, -1, false)
-CompPhaseNameMacro(PHASE_CANONICALIZE_ENTRY,         "Canonicalize entry",             false, -1, false)
+CompPhaseNameMacro(PHASE_CANONICALIZE_ENTRY_BB,      "Canonicalize entry",             false, -1, false)
 CompPhaseNameMacro(PHASE_SET_BLOCK_WEIGHTS,          "Set block weights",              false, -1, false)
 CompPhaseNameMacro(PHASE_ZERO_INITS,                 "Redundant zero Inits",           false, -1, false)
 CompPhaseNameMacro(PHASE_FIND_LOOPS,                 "Find loops",                     false, -1, false)