JIT: Fix downwards loop transformation with multiply executed exiting blocks (#103723)

The transformation to turn loops into downwards counted loops was
missing a check for whether the block containing the exit test was
potentially executed multiple times.

Author: jakobbotsch

src/coreclr/jit/compiler.h

@@ -2230,6 +2230,8 @@ class FlowGraphNaturalLoop
     bool CanDuplicate(INDEBUG(const char** reason));
     void Duplicate(BasicBlock** insertAfter, BlockToBlockMap* map, weight_t weightScale);
 
+    bool MayExecuteBlockMultipleTimesPerIteration(BasicBlock* block);
+
 #ifdef DEBUG
     static void Dump(FlowGraphNaturalLoop* loop);
 #endif // DEBUG

src/coreclr/jit/flowgraph.cpp

@@ -5768,6 +5768,39 @@ void FlowGraphNaturalLoop::Duplicate(BasicBlock** insertAfter, BlockToBlockMap*
     });
 }
 
+//------------------------------------------------------------------------
+// MayExecuteBlockMultipleTimesPerIteration:
+//   Check if the loop may execute a particular loop block multiple times for
+//   each iteration.
+//
+// Parameters:
+//   block - The basic block
+//
+// Returns:
+//   True if so. May return true even if the true answer is false.
+//
+bool FlowGraphNaturalLoop::MayExecuteBlockMultipleTimesPerIteration(BasicBlock* block)
+{
+    assert(ContainsBlock(block));
+
+    if (ContainsImproperHeader())
+    {
+        // To be more precise we could check if 'block' can reach itself
+        // without going through the header, but this case is rare.
+        return true;
+    }
+
+    for (FlowGraphNaturalLoop* child = GetChild(); child != nullptr; child = child->GetSibling())
+    {
+        if (child->ContainsBlock(block))
+        {
+            return true;
+        }
+    }
+
+    return false;
+}
+
 //------------------------------------------------------------------------
 // IterConst: Get the constant with which the iterator is modified
 //

src/coreclr/jit/inductionvariableopts.cpp

@@ -1056,6 +1056,12 @@ bool Compiler::optMakeLoopDownwardsCounted(ScalarEvolutionContext& scevContext,
     // At this point we know that the single exit dominates all backedges.
     JITDUMP("  All backedges are dominated by exiting block " FMT_BB "\n", exiting->bbNum);
 
+    if (loop->MayExecuteBlockMultipleTimesPerIteration(exiting))
+    {
+        JITDUMP("  Exiting block may be executed multiple times per iteration; cannot place decrement in it\n");
+        return false;
+    }
+
     Scev* backedgeCount = scevContext.ComputeExitNotTakenCount(exiting);
     if (backedgeCount == nullptr)
     {

src/tests/JIT/opt/Loops/DownwardsLoopMultiplyExecutedExitingBlock.cs

@@ -0,0 +1,70 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System;
+using System.Runtime.CompilerServices;
+using Xunit;
+
+public class DownwardsLoopMultiplyExecutedExiting
+{
+    [Fact]
+    public static int ExitFromNestedLoop()
+    {
+        int n = Get10();
+        int i = 0;
+        int result = 0;
+        if (n < 0)
+            return -1;
+
+        do
+        {
+            int j = 0;
+            do
+            {
+                if (i >= n)
+                    return result;
+
+                j++;
+                result++;
+            } while (j < 10);
+
+            i++;
+        } while (true);
+    }
+
+    [Fact]
+    public static int ExitFromNestedIrreducibleLoop()
+    {
+        int n = Get10();
+        int i = 0;
+        int result = -1;
+        if (n < 0)
+            return -1;
+
+        do
+        {
+            int j = 0;
+            if (AlwaysFalse())
+                goto InsideLoop;
+
+            LoopHeader:
+            j++;
+            result++;
+
+            InsideLoop:;
+            if (i >= n)
+                return result;
+
+            if (j < 10)
+                goto LoopHeader;
+
+            i++;
+        } while (true);
+    }
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    private static bool AlwaysFalse() => false;
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    private static int Get10() => 10;
+}

src/tests/JIT/opt/Loops/DownwardsLoopMultiplyExecutedExitingBlock.csproj

@@ -0,0 +1,9 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <DebugType>PdbOnly</DebugType>
+    <Optimize>True</Optimize>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="$(MSBuildProjectName).cs" />
+  </ItemGroup>
+</Project>