JIT: Allow BBJ_COND false target to diverge from bbNext in layout opt…

…imization phase (#96609)

Next step for #93020. Working backwards through the JIT flowgraph phases, this change allows bbFalseTarget to diverge from bbNext in Compiler::optOptimizeLayout and onwards.

src/coreclr/jit/block.cpp

@@ -301,18 +301,20 @@ bool BasicBlock::CanRemoveJumpToNext(Compiler* compiler) const
 }
 
 //------------------------------------------------------------------------
-// CanRemoveJumpToFalseTarget: determine if jump to false target can be omitted
+// CanRemoveJumpToTarget: determine if jump to target can be omitted
 //
 // Arguments:
+//    target - true/false target of BBJ_COND block
 //    compiler - current compiler instance
 //
 // Returns:
-//    true if block is a BBJ_COND that can fall into its false target
+//    true if block is a BBJ_COND that can fall into target
 //
-bool BasicBlock::CanRemoveJumpToFalseTarget(Compiler* compiler) const
+bool BasicBlock::CanRemoveJumpToTarget(BasicBlock* target, Compiler* compiler) const
 {
     assert(KindIs(BBJ_COND));
-    return NextIs(bbFalseTarget) && !hasAlign() && !compiler->fgInDifferentRegions(this, bbFalseTarget);
+    assert(TrueTargetIs(target) || FalseTargetIs(target));
+    return NextIs(target) && !compiler->fgInDifferentRegions(this, target);
 }
 
 //------------------------------------------------------------------------
@@ -1170,7 +1172,7 @@ unsigned BasicBlock::NumSucc() const
             return 1;
 
         case BBJ_COND:
-            if (bbTarget == bbNext)
+            if (bbTrueTarget == bbFalseTarget)
             {
                 return 1;
             }
@@ -1295,7 +1297,7 @@ unsigned BasicBlock::NumSucc(Compiler* comp)
             return 1;
 
         case BBJ_COND:
-            if (bbTarget == bbNext)
+            if (bbTrueTarget == bbFalseTarget)
             {
                 return 1;
             }

src/coreclr/jit/block.h

@@ -616,7 +616,7 @@ struct BasicBlock : private LIR::Range
 
     bool CanRemoveJumpToNext(Compiler* compiler) const;
 
-    bool CanRemoveJumpToFalseTarget(Compiler* compiler) const;
+    bool CanRemoveJumpToTarget(BasicBlock* target, Compiler* compiler) const;
 
     unsigned GetTargetOffs() const
     {
@@ -669,7 +669,6 @@ struct BasicBlock : private LIR::Range
     {
         assert(KindIs(BBJ_COND));
         assert(bbTrueTarget != nullptr);
-        assert(target != nullptr);
         return (bbTrueTarget == target);
     }
 
@@ -696,15 +695,16 @@ struct BasicBlock : private LIR::Range
     {
         assert(KindIs(BBJ_COND));
         assert(bbFalseTarget != nullptr);
-        assert(target != nullptr);
         return (bbFalseTarget == target);
     }
 
     void SetCond(BasicBlock* trueTarget, BasicBlock* falseTarget)
     {
+        // Switch lowering may temporarily set a block to a BBJ_COND
+        // with a null false target if it is the last block in the list.
+        // This invalid state is eventually fixed, so allow it in the below assert.
+        assert((falseTarget != nullptr) || (falseTarget == bbNext));
         assert(trueTarget != nullptr);
-        // TODO-NoFallThrough: Allow falseTarget to diverge from bbNext
-        assert(falseTarget == bbNext);
         bbKind        = BBJ_COND;
         bbTrueTarget  = trueTarget;
         bbFalseTarget = falseTarget;

src/coreclr/jit/codegenarm.cpp

@@ -1317,6 +1317,13 @@ void CodeGen::genCodeForJTrue(GenTreeOp* jtrue)
     regNumber reg = genConsumeReg(op);
     inst_RV_RV(INS_tst, reg, reg, genActualType(op));
     inst_JMP(EJ_ne, compiler->compCurBB->GetTrueTarget());
+
+    // If we cannot fall into the false target, emit a jump to it
+    BasicBlock* falseTarget = compiler->compCurBB->GetFalseTarget();
+    if (!compiler->compCurBB->CanRemoveJumpToTarget(falseTarget, compiler))
+    {
+        inst_JMP(EJ_jmp, falseTarget);
+    }
 }
 
 //------------------------------------------------------------------------

src/coreclr/jit/codegenarm64.cpp

@@ -4650,6 +4650,13 @@ void CodeGen::genCodeForJTrue(GenTreeOp* jtrue)
     GenTree*  op  = jtrue->gtGetOp1();
     regNumber reg = genConsumeReg(op);
     GetEmitter()->emitIns_J_R(INS_cbnz, emitActualTypeSize(op), compiler->compCurBB->GetTrueTarget(), reg);
+
+    // If we cannot fall into the false target, emit a jump to it
+    BasicBlock* falseTarget = compiler->compCurBB->GetFalseTarget();
+    if (!compiler->compCurBB->CanRemoveJumpToTarget(falseTarget, compiler))
+    {
+        inst_JMP(EJ_jmp, falseTarget);
+    }
 }
 
 //------------------------------------------------------------------------
@@ -4877,6 +4884,13 @@ void CodeGen::genCodeForJumpCompare(GenTreeOpCC* tree)
 
         GetEmitter()->emitIns_J_R(ins, attr, compiler->compCurBB->GetTrueTarget(), reg);
     }
+
+    // If we cannot fall into the false target, emit a jump to it
+    BasicBlock* falseTarget = compiler->compCurBB->GetFalseTarget();
+    if (!compiler->compCurBB->CanRemoveJumpToTarget(falseTarget, compiler))
+    {
+        inst_JMP(EJ_jmp, falseTarget);
+    }
 }
 
 //---------------------------------------------------------------------

src/coreclr/jit/codegencommon.cpp

@@ -397,7 +397,7 @@ void CodeGen::genMarkLabelsForCodegen()
                 block->GetTrueTarget()->SetFlags(BBF_HAS_LABEL);
 
                 // If we need a jump to the false target, give it a label
-                if (!block->CanRemoveJumpToFalseTarget(compiler))
+                if (!block->CanRemoveJumpToTarget(block->GetFalseTarget(), compiler))
                 {
                     JITDUMP("  " FMT_BB " : branch target\n", block->GetFalseTarget()->bbNum);
                     block->GetFalseTarget()->SetFlags(BBF_HAS_LABEL);

src/coreclr/jit/codegenlinear.cpp

@@ -2621,9 +2621,10 @@ void CodeGen::genCodeForJcc(GenTreeCC* jcc)
     inst_JCC(jcc->gtCondition, compiler->compCurBB->GetTrueTarget());
 
     // If we cannot fall into the false target, emit a jump to it
-    if (!compiler->compCurBB->CanRemoveJumpToFalseTarget(compiler))
+    BasicBlock* falseTarget = compiler->compCurBB->GetFalseTarget();
+    if (!compiler->compCurBB->CanRemoveJumpToTarget(falseTarget, compiler))
     {
-        inst_JMP(EJ_jmp, compiler->compCurBB->GetFalseTarget());
+        inst_JMP(EJ_jmp, falseTarget);
     }
 }
 

src/coreclr/jit/codegenloongarch64.cpp

@@ -4330,6 +4330,13 @@ void CodeGen::genCodeForJumpCompare(GenTreeOpCC* tree)
     assert(regs != 0);
 
     emit->emitIns_J(ins, compiler->compCurBB->GetTrueTarget(), regs); // 5-bits;
+
+    // If we cannot fall into the false target, emit a jump to it
+    BasicBlock* falseTarget = compiler->compCurBB->GetFalseTarget();
+    if (!compiler->compCurBB->CanRemoveJumpToTarget(falseTarget, compiler))
+    {
+        inst_JMP(EJ_jmp, falseTarget);
+    }
 }
 
 //---------------------------------------------------------------------
@@ -4874,16 +4881,31 @@ void CodeGen::genCodeForTreeNode(GenTree* treeNode)
 
         case GT_JCC:
         {
+#if !FEATURE_FIXED_OUT_ARGS
             BasicBlock* tgtBlock = compiler->compCurBB->KindIs(BBJ_COND) ? compiler->compCurBB->GetTrueTarget()
                                                                          : compiler->compCurBB->GetTarget();
-#if !FEATURE_FIXED_OUT_ARGS
             assert((tgtBlock->bbTgtStkDepth * sizeof(int) == genStackLevel) || isFramePointerUsed());
 #endif // !FEATURE_FIXED_OUT_ARGS
 
             GenTreeCC* jcc = treeNode->AsCC();
             assert(jcc->gtCondition.Is(GenCondition::EQ, GenCondition::NE));
             instruction ins = jcc->gtCondition.Is(GenCondition::EQ) ? INS_bceqz : INS_bcnez;
-            emit->emitIns_J(ins, tgtBlock, (int)1 /* cc */);
+
+            if (compiler->compCurBB->KindIs(BBJ_COND))
+            {
+                emit->emitIns_J(ins, compiler->compCurBB->GetTrueTarget(), (int)1 /* cc */);
+
+                // If we cannot fall into the false target, emit a jump to it
+                BasicBlock* falseTarget = compiler->compCurBB->GetFalseTarget();
+                if (!compiler->compCurBB->CanRemoveJumpToTarget(falseTarget, compiler))
+                {
+                    inst_JMP(EJ_jmp, falseTarget);
+                }
+            }
+            else
+            {
+                emit->emitIns_J(ins, compiler->compCurBB->GetTarget(), (int)1 /* cc */);
+            }
         }
         break;
 

src/coreclr/jit/codegenriscv64.cpp

@@ -3920,6 +3920,13 @@ void CodeGen::genCodeForJumpCompare(GenTreeOpCC* tree)
     assert(regs != 0);
 
     emit->emitIns_J(ins, compiler->compCurBB->GetTrueTarget(), regs); // 5-bits;
+
+    // If we cannot fall into the false target, emit a jump to it
+    BasicBlock* falseTarget = compiler->compCurBB->GetFalseTarget();
+    if (!compiler->compCurBB->CanRemoveJumpToTarget(falseTarget, compiler))
+    {
+        inst_JMP(EJ_jmp, falseTarget);
+    }
 }
 
 //---------------------------------------------------------------------

src/coreclr/jit/codegenxarch.cpp

@@ -1453,6 +1453,13 @@ void CodeGen::genCodeForJTrue(GenTreeOp* jtrue)
     regNumber reg = genConsumeReg(op);
     inst_RV_RV(INS_test, reg, reg, genActualType(op));
     inst_JMP(EJ_jne, compiler->compCurBB->GetTrueTarget());
+
+    // If we cannot fall into the false target, emit a jump to it
+    BasicBlock* falseTarget = compiler->compCurBB->GetFalseTarget();
+    if (!compiler->compCurBB->CanRemoveJumpToTarget(falseTarget, compiler))
+    {
+        inst_JMP(EJ_jmp, falseTarget);
+    }
 }
 
 //------------------------------------------------------------------------

src/coreclr/jit/compiler.h

@@ -5956,7 +5956,7 @@ class Compiler
 
     void fgUpdateLoopsAfterCompacting(BasicBlock* block, BasicBlock* bNext);
 
-    BasicBlock* fgConnectFallThrough(BasicBlock* bSrc, BasicBlock* bDst);
+    BasicBlock* fgConnectFallThrough(BasicBlock* bSrc, BasicBlock* bDst, bool noFallThroughQuirk = false);
 
     bool fgRenumberBlocks();
 
@@ -6003,8 +6003,6 @@ class Compiler
 
     bool fgOptimizeSwitchBranches(BasicBlock* block);
 
-    bool fgOptimizeBranchToNext(BasicBlock* block, BasicBlock* bNext, BasicBlock* bPrev);
-
     bool fgOptimizeSwitchJumps();
 #ifdef DEBUG
     void fgPrintEdgeWeights();