JIT: Avoid inverting already-inverted loops that are IV tested

src/coreclr/jit/compiler.h

@@ -7065,8 +7065,7 @@ class Compiler
 
     bool optIsLoopTestEvalIntoTemp(Statement* testStmt, Statement** newTestStmt);
     unsigned optIsLoopIncrTree(GenTree* incr);
-    bool optExtractTestIncr(
-        BasicBlock* bottom, BasicBlock* top, GenTree** ppTest, GenTree** ppIncr);
+    bool optExtractTestIncr(BasicBlock* cond, GenTree** ppTest, GenTree** ppIncr);
 
     void optSetMappedBlockTargets(BasicBlock*      blk,
                           BasicBlock*      newBlk,

src/coreclr/jit/flowgraph.cpp

@@ -5519,7 +5519,7 @@ bool FlowGraphNaturalLoop::AnalyzeIteration(NaturalLoopIterInfo* info)
         }
 
         GenTree* iterTree = nullptr;
-        if (!comp->optExtractTestIncr(cond, m_header, &test, &iterTree))
+        if (!comp->optExtractTestIncr(cond, &test, &iterTree))
         {
             JITDUMP("    Could not extract an IV\n");
             continue;

src/coreclr/jit/optimizer.cpp

@@ -336,7 +336,6 @@ bool Compiler::optIsLoopTestEvalIntoTemp(Statement* testStmt, Statement** newTes
 //
 // Arguments:
 //      cond       - A BBJ_COND block that exits the loop
-//      header     - Loop header block
 //      ppTest     - [out] The test stmt of the loop if found.
 //      ppIncr     - [out] The incr stmt of the loop if found.
 //
@@ -352,7 +351,7 @@ bool Compiler::optIsLoopTestEvalIntoTemp(Statement* testStmt, Statement** newTes
 //      This method just retrieves what it thinks is the "test" node,
 //      the callers are expected to verify that "iterVar" is used in the test.
 //
-bool Compiler::optExtractTestIncr(BasicBlock* cond, BasicBlock* header, GenTree** ppTest, GenTree** ppIncr)
+bool Compiler::optExtractTestIncr(BasicBlock* cond, GenTree** ppTest, GenTree** ppIncr)
 {
     assert(ppTest != nullptr);
     assert(ppIncr != nullptr);
@@ -1839,6 +1838,18 @@ bool Compiler::optTryInvertWhileLoop(FlowGraphNaturalLoop* loop)
         return false;
     }
 
+    // There may be multiple exits, and one of the other exits may also be a
+    // latch. That latch could be preferable to leave (for example because it
+    // is an IV test).
+    NaturalLoopIterInfo iterInfo;
+    if (loop->AnalyzeIteration(&iterInfo) &&
+        (iterInfo.TestBlock->TrueTargetIs(loop->GetHeader()) != iterInfo.TestBlock->FalseTargetIs(loop->GetHeader())))
+    {
+        // Test block is both a latch and exit, so the loop is already inverted in a preferable way.
+        JITDUMP("No loop-inversion for " FMT_LP " since it is already inverted (with an IV test)\n", loop->GetIndex());
+        return false;
+    }
+
     JITDUMP("Condition in block " FMT_BB " of loop " FMT_LP " is a candidate for duplication to invert the loop\n",
             condBlock->bbNum, loop->GetIndex());