JIT: Establish loop invariant base case based on IR (#97182)

Avoid having a cross-phase dependency on loop inversion here. Instead,
validate that the condition is an actual zero-trip test.

Author: jakobbotsch

src/coreclr/jit/compiler.h

@@ -1958,7 +1958,7 @@ struct NaturalLoopIterInfo
     unsigned IterVar = BAD_VAR_NUM;
 
 #ifdef DEBUG
-    // Tree that initializes induction varaible outside the loop.
+    // Tree that initializes induction variable outside the loop.
     // Only valid if HasConstInit is true.
     GenTree* InitTree = nullptr;
 #endif
@@ -2096,6 +2096,8 @@ class FlowGraphNaturalLoop
     void MatchInit(NaturalLoopIterInfo* info, BasicBlock* initBlock, GenTree* init);
     bool MatchLimit(unsigned iterVar, GenTree* test, NaturalLoopIterInfo* info);
     bool CheckLoopConditionBaseCase(BasicBlock* initBlock, NaturalLoopIterInfo* info);
+    bool IsZeroTripTest(BasicBlock* initBlock, NaturalLoopIterInfo* info);
+    bool InitBlockEntersLoopOnTrue(BasicBlock* initBlock);
     template<typename T>
     static bool EvaluateRelop(T op1, T op2, genTreeOps oper);
 public:

src/coreclr/jit/flowgraph.cpp

@@ -5029,7 +5029,7 @@ bool FlowGraphNaturalLoop::AnalyzeIteration(NaturalLoopIterInfo* info)
 
     if (!CheckLoopConditionBaseCase(initBlock, info))
     {
-        JITDUMP("  Loop condition is not always true\n");
+        JITDUMP("  Loop condition may not be true on the first iteration\n");
         return false;
     }
 
@@ -5263,11 +5263,10 @@ bool FlowGraphNaturalLoop::EvaluateRelop(T op1, T op2, genTreeOps oper)
 
 //------------------------------------------------------------------------
 // CheckLoopConditionBaseCase: Verify that the loop condition is true when the
-// loop is entered (or validated immediately on entry).
+// loop is entered.
 //
 // Returns:
-//   True if we could prove that the condition is true at all interesting
-//   points in the loop.
+//   True if we could prove that the condition is true on entry.
 //
 // Remarks:
 //   Currently handles the following cases:
@@ -5306,18 +5305,137 @@ bool FlowGraphNaturalLoop::CheckLoopConditionBaseCase(BasicBlock* initBlock, Nat
     }
 
     // Do we have a zero-trip test?
-    if (initBlock->KindIs(BBJ_COND))
+    if (initBlock->KindIs(BBJ_COND) && IsZeroTripTest(initBlock, info))
     {
-        GenTree* enteringJTrue = initBlock->lastStmt()->GetRootNode();
-        assert(enteringJTrue->OperIs(GT_JTRUE));
-        if (enteringJTrue->gtGetOp1()->OperIsCompare() && ((enteringJTrue->gtGetOp1()->gtFlags & GTF_RELOP_ZTT) != 0))
+        return true;
+    }
+
+    return false;
+}
+
+//------------------------------------------------------------------------
+// IsZeroTripTest: Check whether `initBlock`, a BBJ_COND block that enters the
+// loop in one case and not in the other, implies that the loop invariant is
+// true on entry.
+//
+// Returns:
+//   True if we could prove that the loop invariant is true on entry through
+//   "initBlock".
+//
+bool FlowGraphNaturalLoop::IsZeroTripTest(BasicBlock* initBlock, NaturalLoopIterInfo* info)
+{
+    assert(initBlock->KindIs(BBJ_COND));
+    GenTree* enteringJTrue = initBlock->lastStmt()->GetRootNode();
+    assert(enteringJTrue->OperIs(GT_JTRUE));
+    GenTree* relop = enteringJTrue->gtGetOp1();
+    if (!relop->OperIsCmpCompare())
+    {
+        return false;
+    }
+
+    // Technically optExtractInitTestIncr only handles the "false"
+    // entry case, and preheader creation should ensure that that's the
+    // only time we'll see a BBJ_COND init block. However, it does not
+    // hurt to let this logic be correct by construction.
+    bool enterOnTrue = InitBlockEntersLoopOnTrue(initBlock);
+
+    JITDUMP("  Init block " FMT_BB " enters the loop when condition [%06u] evaluates to %s\n", initBlock->bbNum,
+            Compiler::dspTreeID(relop), enterOnTrue ? "true" : "false");
+
+    GenTree*   limitCandidate;
+    genTreeOps oper;
+
+    if (relop->gtGetOp1()->OperIsScalarLocal() && (relop->gtGetOp1()->AsLclVarCommon()->GetLclNum() == info->IterVar))
+    {
+        JITDUMP("    op1 is the iteration variable\n");
+        oper           = relop->gtOper;
+        limitCandidate = relop->gtGetOp2();
+    }
+    else if (relop->gtGetOp2()->OperIsScalarLocal() &&
+             (relop->gtGetOp2()->AsLclVarCommon()->GetLclNum() == info->IterVar))
+    {
+        JITDUMP("    op2 is the iteration variable\n");
+        oper           = GenTree::SwapRelop(relop->gtOper);
+        limitCandidate = relop->gtGetOp1();
+    }
+    else
+    {
+        JITDUMP("    Relop does not involve iteration variable\n");
+        return false;
+    }
+
+    if (!enterOnTrue)
+    {
+        oper = GenTree::ReverseRelop(oper);
+    }
+
+    // Here we want to prove that [iterVar] [oper] [limitCandidate] implies
+    // [iterVar] [info->IterOper()] [info->Limit()]. Currently we just do the
+    // simple exact checks, but this could be improved. Note that using
+    // `GenTree::Compare` for the limits is ok for a "same value" check for the
+    // limited shapes of limits we recognize.
+    //
+    if ((relop->IsUnsigned() != info->TestTree->IsUnsigned()) || (oper != info->TestOper()) ||
+        !GenTree::Compare(limitCandidate, info->Limit()))
+    {
+        JITDUMP("    Condition guarantees V%02u %s%s [%06u], but invariant requires V%02u %s%s [%06u]\n", info->IterVar,
+                relop->IsUnsigned() ? "(uns) " : "", GenTree::OpName(oper), Compiler::dspTreeID(limitCandidate),
+                info->IterVar, info->TestTree->IsUnsigned() ? "(uns) " : "", GenTree::OpName(info->TestOper()),
+                Compiler::dspTreeID(info->Limit()));
+        return false;
+    }
+
+    JITDUMP("  Condition is established before entry at [%06u]\n", Compiler::dspTreeID(enteringJTrue->gtGetOp1()));
+    return true;
+}
+
+//------------------------------------------------------------------------
+// InitBlockEntersLoopOnTrue: Determine whether a BBJ_COND init block enters the
+// loop in the false or true case.
+//
+// Parameters:
+//   initBlock - A BBJ_COND block that is assumed to dominate the loop, and
+//   only enter the loop in one of the two cases.
+//
+// Returns:
+//   True if the loop is entered if the condition evaluates to true; otherwise false.
+//
+// Remarks:
+//   Handles only limited cases (optExtractInitTestIncr ensures that we see
+//   only limited cases).
+//
+bool FlowGraphNaturalLoop::InitBlockEntersLoopOnTrue(BasicBlock* initBlock)
+{
+    assert(initBlock->KindIs(BBJ_COND));
+
+    if (initBlock->FalseTargetIs(GetHeader()))
+    {
+        return false;
+    }
+
+    if (initBlock->TrueTargetIs(GetHeader()))
+    {
+        return true;
+    }
+
+    // `optExtractInitTestIncr` may look at preds of preds to find an init
+    // block, so try a little bit harder. Today this always happens since we
+    // always have preheaders created in the places we call
+    // FlowGraphNaturalLoop::AnalyzeIteration.
+    for (FlowEdge* enterEdge : EntryEdges())
+    {
+        BasicBlock* entering = enterEdge->getSourceBlock();
+        if (initBlock->FalseTargetIs(entering))
+        {
+            return false;
+        }
+        if (initBlock->TrueTargetIs(entering))
         {
-            JITDUMP("  Condition is established before entry at [%06u]\n",
-                    Compiler::dspTreeID(enteringJTrue->gtGetOp1()));
             return true;
         }
     }
 
+    assert(!"Could not find init block enter side");
     return false;
 }
 

src/coreclr/jit/gentree.h

@@ -503,7 +503,6 @@ enum GenTreeFlags : unsigned int
 
     GTF_RELOP_NAN_UN            = 0x80000000, // GT_<relop> -- Is branch taken if ops are NaN?
     GTF_RELOP_JMP_USED          = 0x40000000, // GT_<relop> -- result of compare used for jump or ?:
-    GTF_RELOP_ZTT               = 0x08000000, // GT_<relop> -- Loop test cloned for converting while-loops into do-while
                                               //               with explicit "loop test" in the header block.
 
     GTF_RET_MERGED              = 0x80000000, // GT_RETURN -- This is a return generated during epilog merging.

src/coreclr/jit/loopcloning.cpp

@@ -1841,17 +1841,6 @@ bool Compiler::optIsLoopClonable(FlowGraphNaturalLoop* loop, LoopCloneContext* c
             return false;
         }
 
-        // TODO-Quirk: Can be removed, loop invariant is validated by
-        // `FlowGraphNaturalLoop::AnalyzeIteration`.
-        if (!iterInfo->TestTree->OperIsCompare() || ((iterInfo->TestTree->gtFlags & GTF_RELOP_ZTT) == 0))
-        {
-            JITDUMP("Loop cloning: rejecting loop " FMT_LP
-                    ". Loop inversion NOT present, loop test [%06u] may not protect "
-                    "entry from head.\n",
-                    loop->GetIndex(), iterInfo->TestTree->gtTreeID);
-            return false;
-        }
-
 #ifdef DEBUG
         const unsigned ivLclNum = iterInfo->IterVar;
         GenTree* const op1      = iterInfo->Iterator();

src/coreclr/jit/optimizer.cpp

@@ -2156,11 +2156,6 @@ bool Compiler::optInvertWhileLoop(BasicBlock* block)
             assert(originalCompareTree->OperIsCompare());
             assert(clonedCompareTree->OperIsCompare());
 
-            // Flag compare and cloned copy so later we know this loop
-            // has a proper zero trip test.
-            originalCompareTree->gtFlags |= GTF_RELOP_ZTT;
-            clonedCompareTree->gtFlags |= GTF_RELOP_ZTT;
-
             // The original test branches to remain in the loop.  The
             // new cloned test will branch to avoid the loop.  So the
             // cloned compare needs to reverse the branch condition.

src/tests/JIT/Regression/JitBlue/Runtime_95315/Runtime_95315.cs

@@ -11,6 +11,7 @@ public class Runtime_95315
     [Fact]
     public static void TestEntryPoint()
     {
+        // Make sure 'Bar' tiers up completely...
         for (int i = 0; i < 4; i++)
         {
             for (int j = 0; j < 200; j++)