JIT: Add an "init BB" invariant (dotnet#110404)

This adds an invariant that there always exists an "init BB" throughout
the JIT's phases. The init BB has the following properties:
- It is only executed once, so it does not have any predecessors
- It is not inside a try region, hence it dominates all other blocks in
  the function

There are no further requirements on the BB. The init BB does not have
to be `BBJ_ALWAYS` (unlike the old "scratch BB" concept). This is mainly
because it philosophically does not make sense to insert IR at the end
of the init BB, since earlier phases can have inserted arbitrary IR in
them.

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);
         }
@@ -388,17 +384,17 @@ void CodeGen::genCodeForBBlist()
 
 #ifdef SWIFT_SUPPORT
         // Reassemble Swift struct parameters on the local stack frame in the
-        // scratch BB right after the prolog. There can be arbitrary amounts of
+        // init BB right after the prolog. There can be arbitrary amounts of
         // codegen related to doing this, so it cannot be done in the prolog.
-        if (compiler->fgBBisScratch(block) && compiler->lvaHasAnySwiftStackParamToReassemble())
+        if (block->IsFirst() && compiler->lvaHasAnySwiftStackParamToReassemble())
         {
             genHomeSwiftStructParameters(/* handleStack */ true);
         }
 #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
@@ -4526,6 +4512,9 @@ 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, &Compiler::fgCanonicalizeFirstBB);
+
     // If we are doing OSR, update flow to initially reach the appropriate IL offset.
     //
     if (opts.IsOSR())
@@ -4694,10 +4683,6 @@ 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);
@@ -4792,10 +4777,6 @@ 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);
@@ -5027,12 +5008,6 @@ 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)
@@ -5197,8 +5198,6 @@ 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
     unsigned        fgBBcount = 0;      // # of BBs in the method (in the linked list that starts with fgFirstBB)
@@ -5246,10 +5245,6 @@ 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);
 
@@ -5439,6 +5434,7 @@ class Compiler
     };
 
     PhaseStatus fgMorphBlocks();
+    BasicBlock* fgGetFirstILBlock();
     void fgMorphBlock(BasicBlock* block, MorphUnreachableInfo* unreachableInfo = nullptr);
     void fgMorphStmts(BasicBlock* block);
 
@@ -6158,6 +6154,7 @@ class Compiler
     bool fgCheckRemoveStmt(BasicBlock* block, Statement* stmt);
 
     PhaseStatus fgCanonicalizeFirstBB();
+    void fgCreateNewInitBB();
 
     void fgSetEHRegionForNewPreheaderOrExit(BasicBlock* preheader);
 
@@ -6179,6 +6176,8 @@ class Compiler
 
     bool fgCanCompactBlock(BasicBlock* block);
 
+    bool fgCanCompactInitBlock();
+
     void fgCompactBlock(BasicBlock* block);
 
     BasicBlock* fgConnectFallThrough(BasicBlock* bSrc, BasicBlock* bDst);
@@ -6355,6 +6354,7 @@ 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);