JIT: Straighten out flow during early jump threading

After early jump threading had kicked in we can frequently prove that a
branch will go in one direction. This was previously left up to RBO;
instead, try to fold it immediately and straighten out the flow before
we build SSA, so that we can refine the phis.

Fix #101176

src/coreclr/jit/compiler.h

@@ -6128,6 +6128,7 @@ class Compiler
 #endif // FEATURE_EH_WINDOWS_X86
 
     bool fgOptimizeUncondBranchToSimpleCond(BasicBlock* block, BasicBlock* target);
+    bool fgFoldSimpleCondByForwardSub(BasicBlock* block);
 
     bool fgBlockEndFavorsTailDuplication(BasicBlock* block, unsigned lclNum);
 

src/coreclr/jit/fgopt.cpp

@@ -1966,6 +1966,12 @@ bool Compiler::fgBlockIsGoodTailDuplicationCandidate(BasicBlock* target, unsigne
         return false;
     }
 
+    // No point duplicating this block if it would not remove (part of) the joint.
+    if ((target->GetTrueTarget() == target) || (target->GetFalseTarget() == target))
+    {
+        return false;
+    }
+
     Statement* const lastStmt  = target->lastStmt();
     Statement* const firstStmt = target->FirstNonPhiDef();
 
@@ -2265,6 +2271,108 @@ bool Compiler::fgOptimizeUncondBranchToSimpleCond(BasicBlock* block, BasicBlock*
     return true;
 }
 
+//-------------------------------------------------------------
+// fgFoldSimpleCondByForwardSub:
+//   Try to refine the flow of a block that may have just been tail duplicated
+//   or compacted.
+//
+// Arguments:
+//   block - block that was tail duplicated or compacted
+//
+// Returns Value:
+//   true if control flow was changed
+//
+bool Compiler::fgFoldSimpleCondByForwardSub(BasicBlock* block)
+{
+    assert(block->KindIs(BBJ_COND));
+    GenTree* jtrue = block->lastStmt()->GetRootNode();
+    assert(jtrue->OperIs(GT_JTRUE));
+
+    GenTree* relop = jtrue->gtGetOp1();
+    if (!relop->OperIsCompare())
+    {
+        return false;
+    }
+
+    GenTree* op1 = relop->gtGetOp1();
+    GenTree* op2 = relop->gtGetOp2();
+
+    GenTree**            lclUse;
+    GenTreeLclVarCommon* lcl;
+
+    if (op1->OperIs(GT_LCL_VAR) && op2->IsIntegralConst())
+    {
+        lclUse = &relop->AsOp()->gtOp1;
+        lcl    = op1->AsLclVarCommon();
+    }
+    else if (op2->OperIs(GT_LCL_VAR) && op1->IsIntegralConst())
+    {
+        lclUse = &relop->AsOp()->gtOp2;
+        lcl    = op2->AsLclVarCommon();
+    }
+    else
+    {
+        return false;
+    }
+
+    Statement* secondLastStmt = block->lastStmt()->GetPrevStmt();
+    if ((secondLastStmt == nullptr) || (secondLastStmt == block->lastStmt()))
+    {
+        return false;
+    }
+
+    GenTree* prevTree = secondLastStmt->GetRootNode();
+    if (!prevTree->OperIs(GT_STORE_LCL_VAR))
+    {
+        return false;
+    }
+
+    GenTreeLclVarCommon* store = prevTree->AsLclVarCommon();
+    if (store->GetLclNum() != lcl->GetLclNum())
+    {
+        return false;
+    }
+
+    if (!store->Data()->IsIntegralConst())
+    {
+        return false;
+    }
+
+    if (genActualType(store) != genActualType(store->Data()) || (genActualType(store) != genActualType(lcl)))
+    {
+        return false;
+    }
+
+    JITDUMP("Forward substituting local after jump threading. Before:\n");
+    DISPSTMT(block->lastStmt());
+
+    JITDUMP("\nAfter:\n");
+
+    LclVarDsc* varDsc  = lvaGetDesc(lcl);
+    GenTree*   newData = gtCloneExpr(store->Data());
+    if (varTypeIsSmall(varDsc) && fgCastNeeded(store->Data(), varDsc->TypeGet()))
+    {
+        newData = gtNewCastNode(TYP_INT, newData, false, varDsc->TypeGet());
+        newData = gtFoldExpr(newData);
+    }
+
+    *lclUse = newData;
+    DISPSTMT(block->lastStmt());
+
+    JITDUMP("\nNow trying to fold...\n");
+    jtrue->AsUnOp()->gtOp1 = gtFoldExpr(relop);
+    DISPSTMT(block->lastStmt());
+
+    Compiler::FoldResult result = fgFoldConditional(block);
+    if (result != Compiler::FoldResult::FOLD_DID_NOTHING)
+    {
+        assert(block->KindIs(BBJ_ALWAYS));
+        return true;
+    }
+
+    return false;
+}
+
 //-------------------------------------------------------------
 // fgRemoveConditionalJump:
 //    Optimize a BBJ_COND block that unconditionally jumps to the same target
@@ -5176,10 +5284,35 @@ bool Compiler::fgUpdateFlowGraph(bool doTailDuplication /* = false */,
                 {
                     assert(block->KindIs(BBJ_COND));
                     assert(bNext == block->Next());
-                    change     = true;
-                    modified   = true;
-                    bDest      = block->GetTrueTarget();
-                    bFalseDest = block->GetFalseTarget();
+                    change   = true;
+                    modified = true;
+
+                    if (fgFoldSimpleCondByForwardSub(block))
+                    {
+                        // It is likely another pred of the target now can
+                        // similarly have its control flow straightened out.
+                        // Try to compact it and repeat the optimization for
+                        // it.
+                        if (bDest->bbRefs == 1)
+                        {
+                            BasicBlock* otherPred = bDest->bbPreds->getSourceBlock();
+                            JITDUMP("Trying to compact last pred " FMT_BB " of " FMT_BB " that we now bypass\n",
+                                    otherPred->bbNum, bDest->bbNum);
+                            if (fgCanCompactBlock(otherPred))
+                            {
+                                fgCompactBlock(otherPred);
+                                fgFoldSimpleCondByForwardSub(otherPred);
+                            }
+                        }
+
+                        assert(block->KindIs(BBJ_ALWAYS));
+                        bDest = block->GetTarget();
+                    }
+                    else
+                    {
+                        bDest      = block->GetTrueTarget();
+                        bFalseDest = block->GetFalseTarget();
+                    }
                 }
             }