JIT: Fix physical promotion creating overlapping local uses (#91088)

Physical promotion could in some cases create uses overlapping illegally
with defs when faced with seemingly last uses of structs. This is a
result of a mismatch between our model for liveness and the actual model
of local uses in the backend. In the actual model, the uses of LCL_VARs
occur at the user, which means that it is possible for there to be no
place at which to insert IR between to local uses.

The example looks like the following. Physical promotion would be faced
with a tree like

```
▌  CALL      void   Program:Foo(Program+S,Program+S)
├──▌  LCL_VAR   struct<Program+S, 4> V01 loc0
└──▌  LCL_VAR   struct<Program+S, 4> V01 loc0
```

When V01 was fully promoted, both of these are logically last uses since
all state of V01 is stored in promoted field locals. Because of that we
would make the following transformation:

```
▌  CALL      void   Program:Foo(Program+S,Program+S)
├──▌  LCL_VAR   struct<Program+S, 4> V01 loc0          (last use)
└──▌  COMMA     struct
   ├──▌  STORE_LCL_FLD int    V01 loc0         [+0]
   │  └──▌  LCL_VAR   int    V02 tmp0
   └──▌  LCL_VAR   struct<Program+S, 4> V01 loc0          (last use)
```

This creates an illegally overlapping use and def; additionally, it is
correct only in a world where the store actually would happen between
the two uses. It is also moderately dangerous to mark both of these as
last uses given the implicit byref transformation.

The fix is to avoid marking a struct use as a last use if we see more
struct uses in the same statement.

Fix #91056

Co-authored-by: Jakob Botsch Nielsen <[email protected]>

Author: github-actions[bot]

src/coreclr/jit/promotion.cpp

@@ -2340,8 +2340,36 @@ void ReplaceVisitor::ReadBackAfterCall(GenTreeCall* call, GenTree* user)
 //
 //   If the remainder of the struct local is dying, then we expect that this
 //   entire struct local is now dying, since all field accesses are going to be
-//   replaced with other locals. The exception is if there is a queued read
-//   back for any of the fields.
+//   replaced with other locals.
+//
+//   There are two exceptions to the above:
+//
+//     1) If there is a queued readback for any of the fields, then there is
+//     live state in the struct local, so it is not dying.
+//
+//     2) If there are further uses of the local in the same statement then we cannot
+//     actually act on the last-use information we would provide here. That's because
+//     uses of locals occur at the user and we do not model that here. In the real model
+//     there are cases where we do not have any place to insert any IR between the two uses.
+//     For example, consider:
+//
+//       ▌  CALL      void   Program:Foo(Program+S,Program+S)
+//       ├──▌  LCL_VAR   struct<Program+S, 4> V01 loc0
+//       └──▌  LCL_VAR   struct<Program+S, 4> V01 loc0
+//
+//     If V01 is promoted fully then both uses of V01 are last uses here; but
+//     replacing the IR with
+//
+//       ▌  CALL      void   Program:Foo(Program+S,Program+S)
+//       ├──▌  LCL_VAR   struct<Program+S, 4> V01 loc0          (last use)
+//       └──▌  COMMA     struct
+//          ├──▌  STORE_LCL_FLD int    V01 loc0         [+0]
+//          │  └──▌  LCL_VAR   int    V02 tmp0
+//          └──▌  LCL_VAR   struct<Program+S, 4> V01 loc0          (last use)
+//
+//     would be illegal since the created store overlaps with the first local,
+//     and does not take into account that both uses occur simultaneously at
+//     the position of the CALL node.
 //
 bool ReplaceVisitor::IsPromotedStructLocalDying(GenTreeLclVarCommon* lcl)
 {
@@ -2362,6 +2390,15 @@ bool ReplaceVisitor::IsPromotedStructLocalDying(GenTreeLclVarCommon* lcl)
         }
     }
 
+    for (GenTree* cur = lcl->gtNext; cur != nullptr; cur = cur->gtNext)
+    {
+        assert(cur->OperIsAnyLocal());
+        if (cur->TypeIs(TYP_STRUCT) && (cur->AsLclVarCommon()->GetLclNum() == lcl->GetLclNum()))
+        {
+            return false;
+        }
+    }
+
     return true;
 }
 
@@ -2577,7 +2614,7 @@ void ReplaceVisitor::WriteBackBeforeCurrentStatement(unsigned lcl, unsigned offs
 
         GenTree*   readBack = Promotion::CreateWriteBack(m_compiler, lcl, rep);
         Statement* stmt     = m_compiler->fgNewStmtFromTree(readBack);
-        JITDUMP("Writing back %s before " FMT_STMT "\n", rep.Description, stmt->GetID());
+        JITDUMP("Writing back %s before " FMT_STMT "\n", rep.Description, m_currentStmt->GetID());
         DISPSTMT(stmt);
         m_compiler->fgInsertStmtBefore(m_currentBlock, m_currentStmt, stmt);
         ClearNeedsWriteBack(rep);

src/tests/JIT/Regression/JitBlue/Runtime_81725/Runtime_81725.csproj

@@ -7,8 +7,6 @@
   </PropertyGroup>
   <ItemGroup>
     <Compile Include="$(MSBuildProjectName).cs" />
-  </ItemGroup>
-  <ItemGroup>
     <CLRTestEnvironmentVariable Include="DOTNET_JitStressModeNames" Value="STRESS_FOLD" />
   </ItemGroup>
 </Project>

src/tests/JIT/Regression/JitBlue/Runtime_85088/Runtime_85088.csproj

@@ -1,5 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
+    <!-- Needed for CLRTestEnvironmentVariable -->
+    <RequiresProcessIsolation>true</RequiresProcessIsolation>
     <Optimize>True</Optimize>
   </PropertyGroup>
   <ItemGroup>

src/tests/JIT/Regression/JitBlue/Runtime_91056/Runtime_91056.cs

@@ -0,0 +1,33 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using Xunit;
+using System.Runtime.CompilerServices;
+
+public class Runtime_91056
+{
+    [Fact]
+    public static void TestEntryPoint()
+    {
+        S s = default;
+        if (False())
+        {
+            s.A = 1234;
+        }
+
+        Foo(0, 0, s, s);
+    }
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    private static bool False() => false;
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    private static void Foo(int a, int b, S s1, S s2)
+    {
+    }
+
+    public struct S
+    {
+        public int A;
+    }
+}

src/tests/JIT/Regression/JitBlue/Runtime_91056/Runtime_91056.csproj

@@ -0,0 +1,12 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <!-- Needed for CLRTestEnvironmentVariable -->
+    <RequiresProcessIsolation>true</RequiresProcessIsolation>
+    <Optimize>True</Optimize>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="$(MSBuildProjectName).cs" />
+    <CLRTestEnvironmentVariable Include="DOTNET_JitStressModeNames" Value="STRESS_PHYSICAL_PROMOTION STRESS_PHYSICAL_PROMOTION_COST STRESS_NO_OLD_PROMOTION" />
+    <CLRTestEnvironmentVariable Include="DOTNET_JitNoCSE" Value="1" />
+  </ItemGroup>
+</Project>