Perform reachability analysis before codegen

src/coreclr/jit/compiler.cpp

@@ -4944,6 +4944,9 @@ void Compiler::compCompile(void** methodCodePtr, uint32_t* methodCodeSize, JitFl
         }
     }
 
+    // Remove dead blocks
+    DoPhase(this, PHASE_REMOVE_DEAD_BLOCKS, &Compiler::fgRemoveDeadBlocks);
+
     // Insert GC Polls
     DoPhase(this, PHASE_INSERT_GC_POLLS, &Compiler::fgInsertGCPolls);
 

src/coreclr/jit/compiler.h

@@ -5196,7 +5196,6 @@ class Compiler
     bool     fgHaveValidEdgeWeights;   // true if we were successful in computing all of the edge weights
     bool     fgSlopUsedInEdgeWeights;  // true if their was some slop used when computing the edge weights
     bool     fgRangeUsedInEdgeWeights; // true if some of the edgeWeight are expressed in Min..Max form
-    bool     fgNeedsUpdateFlowGraph;   // true if we need to run fgUpdateFlowGraph
     weight_t fgCalledCount;            // count of the number of times this method was called
                                        // This is derived from the profile data
                                        // or is BB_UNITY_WEIGHT when we don't have profile data
@@ -5774,10 +5773,14 @@ class Compiler
 
     void fgComputeEnterBlocksSet(); // Compute the set of entry blocks, 'fgEnterBlks'.
 
-    bool fgRemoveUnreachableBlocks(); // Remove blocks determined to be unreachable by the bbReach sets.
+    // Remove blocks determined to be unreachable by the 'canRemoveBlock'.
+    template <typename CanRemoveBlockBody>
+    bool fgRemoveUnreachableBlocks(CanRemoveBlockBody canRemoveBlock);
 
     void fgComputeReachability(); // Perform flow graph node reachability analysis.
 
+    void fgRemoveDeadBlocks(); // Identify and remove dead blocks.
+
     BasicBlock* fgIntersectDom(BasicBlock* a, BasicBlock* b); // Intersect two immediate dominator sets.
 
     void fgDfsInvPostOrder(); // In order to compute dominance using fgIntersectDom, the flow graph nodes must be

src/coreclr/jit/compphases.h

@@ -84,6 +84,7 @@ CompPhaseNameMacro(PHASE_OPTIMIZE_BRANCHES,      "Redundant branch opts",
 CompPhaseNameMacro(PHASE_ASSERTION_PROP_MAIN,    "Assertion prop",                 "AST-PROP", false, -1, false)
 CompPhaseNameMacro(PHASE_OPT_UPDATE_FLOW_GRAPH,  "Update flow graph opt pass",     "UPD-FG-O", false, -1, false)
 CompPhaseNameMacro(PHASE_COMPUTE_EDGE_WEIGHTS2,  "Compute edge weights (2, false)","EDG-WGT2", false, -1, false)
+CompPhaseNameMacro(PHASE_REMOVE_DEAD_BLOCKS,     "Remove dead blocks",             "DEAD-BLK", false, -1, false)
 CompPhaseNameMacro(PHASE_INSERT_GC_POLLS,        "Insert GC Polls",                "GC-POLLS", false, -1, true)
 CompPhaseNameMacro(PHASE_DETERMINE_FIRST_COLD_BLOCK, "Determine first cold block", "COLD-BLK", false, -1, true)
 CompPhaseNameMacro(PHASE_RATIONALIZE,            "Rationalize IR",                 "RAT",      false, -1, false)

src/coreclr/jit/emit.cpp

@@ -6556,7 +6556,8 @@ unsigned emitter::emitEndCodeGen(Compiler* comp,
 #ifdef DEBUG
         if (emitComp->opts.disAsm || emitComp->verbose)
         {
-            printf("\t\t\t\t\t\t;; bbWeight=%s PerfScore %.2f", refCntWtd2str(ig->igWeight), ig->igPerfScore);
+            printf("\t\t\t\t\t\t;; size=%d bbWeight=%s PerfScore %.2f", ig->igSize, refCntWtd2str(ig->igWeight),
+                   ig->igPerfScore);
         }
         *instrCount += ig->igInsCnt;
 #endif // DEBUG

src/coreclr/jit/fgbasic.cpp

@@ -31,7 +31,6 @@ void Compiler::fgInit()
     fgHaveValidEdgeWeights   = false;
     fgSlopUsedInEdgeWeights  = false;
     fgRangeUsedInEdgeWeights = true;
-    fgNeedsUpdateFlowGraph   = false;
     fgCalledCount            = BB_ZERO_WEIGHT;
 
     /* We haven't yet computed the dominator sets */

src/coreclr/jit/fgopt.cpp

@@ -367,17 +367,14 @@ void Compiler::fgComputeEnterBlocksSet()
     BlockSetOps::AddElemD(this, fgEnterBlks, fgFirstBB->bbNum);
     assert(fgFirstBB->bbNum == 1);
 
-    if (compHndBBtabCount > 0)
+    /* Also 'or' in the handler basic blocks */
+    for (EHblkDsc* const HBtab : EHClauses(this))
     {
-        /* Also 'or' in the handler basic blocks */
-        for (EHblkDsc* const HBtab : EHClauses(this))
+        if (HBtab->HasFilter())
         {
-            if (HBtab->HasFilter())
-            {
-                BlockSetOps::AddElemD(this, fgEnterBlks, HBtab->ebdFilter->bbNum);
-            }
-            BlockSetOps::AddElemD(this, fgEnterBlks, HBtab->ebdHndBeg->bbNum);
+            BlockSetOps::AddElemD(this, fgEnterBlks, HBtab->ebdFilter->bbNum);
         }
+        BlockSetOps::AddElemD(this, fgEnterBlks, HBtab->ebdHndBeg->bbNum);
     }
 
 #if defined(FEATURE_EH_FUNCLETS) && defined(TARGET_ARM)
@@ -420,16 +417,28 @@ void Compiler::fgComputeEnterBlocksSet()
 // are converted to `throw` blocks. Internal throw helper blocks and the single return block (if any)
 // are never considered unreachable.
 //
+// Arguments:
+//   canRemoveBlock - Method that determines if a block can be removed or not. In earlier phases, it
+//       relies on the reachability set. During final phase, it depends on the DFS walk of the flowgraph
+//       and considering blocks that are not visited as unreachable.
+//
 // Return Value:
 //    Return true if changes were made that may cause additional blocks to be removable.
 //
-// Assumptions:
-//    The reachability sets must be computed and valid.
+// Notes:
+//    Unreachable blocks removal phase happens twice.
+//
+//    During early phases RecomputeLoopInfo, the logic to determine if a block is reachable
+//    or not is based on the reachability sets, and hence it must be computed and valid.
+//
+//    During late phase, all the reachable blocks from fgFirstBB are traversed and everything
+//    else are marked as unreachable (with exceptions of handler/filter blocks and BBJ_ALWAYS
+//    blocks in Arm). As such, it is not dependent on the validity of reachability sets.
 //
-bool Compiler::fgRemoveUnreachableBlocks()
+template <typename CanRemoveBlockBody>
+bool Compiler::fgRemoveUnreachableBlocks(CanRemoveBlockBody canRemoveBlock)
 {
     assert(!fgCheapPredsValid);
-    assert(fgReachabilitySetsValid);
 
     bool hasUnreachableBlocks = false;
     bool changed              = false;
@@ -451,18 +460,10 @@ bool Compiler::fgRemoveUnreachableBlocks()
         }
         else
         {
-            // If any of the entry blocks can reach this block, then we skip it.
-            if (!BlockSetOps::IsEmptyIntersection(this, fgEnterBlks, block->bbReach))
-            {
-                continue;
-            }
-
-#if defined(FEATURE_EH_FUNCLETS) && defined(TARGET_ARM)
-            if (!BlockSetOps::IsEmptyIntersection(this, fgAlwaysBlks, block->bbReach))
+            if (!canRemoveBlock(block))
             {
                 continue;
             }
-#endif // defined(FEATURE_EH_FUNCLETS) && defined(TARGET_ARM)
         }
 
         // Remove all the code for the block
@@ -572,6 +573,24 @@ void Compiler::fgComputeReachability()
     // The dominator algorithm expects that all blocks can be reached from the fgEnterBlks set.
     unsigned passNum = 1;
     bool     changed;
+
+    auto canRemoveBlock = [&](BasicBlock* block) -> bool {
+        // If any of the entry blocks can reach this block, then we skip it.
+        if (!BlockSetOps::IsEmptyIntersection(this, fgEnterBlks, block->bbReach))
+        {
+            return false;
+        }
+
+#if defined(FEATURE_EH_FUNCLETS) && defined(TARGET_ARM)
+        if (!BlockSetOps::IsEmptyIntersection(this, fgAlwaysBlks, block->bbReach))
+        {
+            return false;
+        }
+#endif // defined(FEATURE_EH_FUNCLETS) && defined(TARGET_ARM)
+
+        return true;
+    };
+
     do
     {
         // Just to be paranoid, avoid infinite loops; fall back to minopts.
@@ -601,7 +620,7 @@ void Compiler::fgComputeReachability()
         // Use reachability information to delete unreachable blocks.
         //
 
-        changed = fgRemoveUnreachableBlocks();
+        changed = fgRemoveUnreachableBlocks(canRemoveBlock);
 
     } while (changed);
 
@@ -624,6 +643,95 @@ void Compiler::fgComputeReachability()
     fgComputeDoms();
 }
 
+//------------------------------------------------------------------------
+// fgRemoveDeadBlocks: Identify all the unreachable blocks and remove them.
+//         Handler and filter blocks are considered as reachable and hence won't
+//         be removed. For Arm32, do not remove BBJ_ALWAYS block of
+//         BBJ_CALLFINALLY/BBJ_ALWAYS pair.
+//
+void Compiler::fgRemoveDeadBlocks()
+{
+    jitstd::list<BasicBlock*> worklist(jitstd::allocator<void>(getAllocator(CMK_Reachability)));
+    worklist.push_back(fgFirstBB);
+
+    // Do not remove handler blocks
+    for (EHblkDsc* const HBtab : EHClauses(this))
+    {
+        if (HBtab->HasFilter())
+        {
+            worklist.push_back(HBtab->ebdFilter);
+        }
+        worklist.push_back(HBtab->ebdHndBeg);
+    }
+
+#if defined(FEATURE_EH_FUNCLETS) && defined(TARGET_ARM)
+    // For ARM code, prevent creating retless calls by adding the BBJ_ALWAYS to the "fgAlwaysBlks" list.
+    for (BasicBlock* const block : Blocks())
+    {
+        if (block->bbJumpKind == BBJ_CALLFINALLY)
+        {
+            assert(block->isBBCallAlwaysPair());
+
+            // Don't remove the BBJ_ALWAYS block that is only here for the unwinder.
+            worklist.push_back(block->bbNext);
+        }
+    }
+#endif // defined(FEATURE_EH_FUNCLETS) && defined(TARGET_ARM)
+
+    unsigned prevFgCurBBEpoch = fgCurBBEpoch;
+    EnsureBasicBlockEpoch();
+
+    if (prevFgCurBBEpoch != fgCurBBEpoch)
+    {
+        // If Epoch has changed, reset the doms computed as well because
+        // in future, during insert gc polls or lowering, when we compact
+        // blocks during flowgraph update, it might propagate the invalid
+        // bbReach as well (although Epoch adjustment resets fgReachabilitySetsValid).
+        fgDomsComputed = false;
+    }
+
+    BlockSet visitedBlocks(BlockSetOps::MakeEmpty(this));
+
+    // Visit all the reachable blocks, everything else can be removed
+    while (!worklist.empty())
+    {
+        BasicBlock* block = *(worklist.begin());
+        worklist.pop_front();
+
+        if (BlockSetOps::IsMember(this, visitedBlocks, block->bbNum))
+        {
+            continue;
+        }
+
+        BlockSetOps::AddElemD(this, visitedBlocks, block->bbNum);
+
+        for (BasicBlock* succ : block->Succs(this))
+        {
+            worklist.push_back(succ);
+        }
+    }
+
+    // A block is unreachable if no path was found from
+    // any of the fgFirstBB, handler, filter or BBJ_ALWAYS (Arm) blocks.
+    auto isBlockRemovable = [&](BasicBlock* block) -> bool {
+        return !BlockSetOps::IsMember(this, visitedBlocks, block->bbNum);
+    };
+
+    bool changed = fgRemoveUnreachableBlocks(isBlockRemovable);
+
+#ifdef DEBUG
+    if (verbose && changed)
+    {
+        printf("\nAfter dead block removal:\n");
+        fgDispBasicBlocks(verboseTrees);
+        printf("\n");
+    }
+
+    fgVerifyHandlerTab();
+    fgDebugCheckBBlist(false);
+#endif // DEBUG
+}
+
 //-------------------------------------------------------------
 // fgDfsInvPostOrder: Helper function for computing dominance information.
 //
@@ -2163,6 +2271,7 @@ void Compiler::fgCompactBlocks(BasicBlock* block, BasicBlock* bNext)
     //
     if (fgDomsComputed && (block->bbNum > fgDomBBcount))
     {
+        assert(fgReachabilitySetsValid);
         BlockSetOps::Assign(this, block->bbReach, bNext->bbReach);
         BlockSetOps::ClearD(this, bNext->bbReach);
 
@@ -2512,8 +2621,7 @@ bool Compiler::fgOptimizeBranchToEmptyUnconditional(BasicBlock* block, BasicBloc
     //
     if (fgIsUsingProfileWeights() && !fgEdgeWeightsComputed)
     {
-        fgNeedsUpdateFlowGraph = true;
-        optimizeJump           = false;
+        optimizeJump = false;
     }
 
     if (optimizeJump)
@@ -2875,8 +2983,7 @@ bool Compiler::fgOptimizeSwitchBranches(BasicBlock* block)
             //
             if (fgIsUsingProfileWeights() && !fgEdgeWeightsComputed)
             {
-                fgNeedsUpdateFlowGraph = true;
-                optimizeJump           = false;
+                optimizeJump = false;
             }
 
             if (optimizeJump)
@@ -5683,7 +5790,6 @@ bool Compiler::fgReorderBlocks()
 
     if (changed)
     {
-        fgNeedsUpdateFlowGraph = true;
 #if DEBUG
         // Make sure that the predecessor lists are accurate
         if (expensiveDebugCheckLevel >= 2)
@@ -5908,8 +6014,7 @@ bool Compiler::fgUpdateFlowGraph(bool doTailDuplication)
                         // because we can't allow fall-through into the cold region.
                         if (!fgEdgeWeightsComputed || fgInDifferentRegions(block, bDest))
                         {
-                            fgNeedsUpdateFlowGraph = true;
-                            optimizeJump           = false;
+                            optimizeJump = false;
                         }
                     }
 
@@ -6194,8 +6299,6 @@ bool Compiler::fgUpdateFlowGraph(bool doTailDuplication)
         }
     } while (change);
 
-    fgNeedsUpdateFlowGraph = false;
-
 #ifdef DEBUG
     if (verbose && modified)
     {