Capture stackalloc size expression only if needed

src/Compilers/CSharp/Portable/Lowering/LocalRewriter/LocalRewriter_BinaryOperator.cs

@@ -1290,10 +1290,10 @@ private BoundExpression MakeNullableHasValue(SyntaxNode syntax, BoundExpression
             BoundExpression rightNeverNull = NullableAlwaysHasValue(loweredRight);
 
             BoundExpression boundTempX = leftNeverNull ?? loweredLeft;
-            boundTempX = CaptureNullableOperandInTempIfNeeded(boundTempX, sideeffects, locals);
+            boundTempX = CaptureExpressionInTempIfNeeded(boundTempX, sideeffects, locals);
 
             BoundExpression boundTempY = rightNeverNull ?? loweredRight;
-            boundTempY = CaptureNullableOperandInTempIfNeeded(boundTempY, sideeffects, locals);
+            boundTempY = CaptureExpressionInTempIfNeeded(boundTempY, sideeffects, locals);
 
             BoundExpression callX_GetValueOrDefault = MakeOptimizedGetValueOrDefault(syntax, boundTempX);
             BoundExpression callY_GetValueOrDefault = MakeOptimizedGetValueOrDefault(syntax, boundTempY);
@@ -1330,7 +1330,7 @@ private BoundExpression MakeNullableHasValue(SyntaxNode syntax, BoundExpression
                 type: type);
         }
 
-        private BoundExpression CaptureNullableOperandInTempIfNeeded(BoundExpression operand, ArrayBuilder<BoundExpression> sideeffects, ArrayBuilder<LocalSymbol> locals)
+        private BoundExpression CaptureExpressionInTempIfNeeded(BoundExpression operand, ArrayBuilder<BoundExpression> sideeffects, ArrayBuilder<LocalSymbol> locals)
         {
             if (CanChangeValueBetweenReads(operand))
             {

src/Compilers/CSharp/Portable/Lowering/LocalRewriter/LocalRewriter_StackAlloc.cs

 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
+using System.Diagnostics;
 using Microsoft.CodeAnalysis.CSharp.Symbols;
-using System.Collections.Immutable;
+using Microsoft.CodeAnalysis.PooledObjects;
 using Roslyn.Utilities;
-using System.Diagnostics;
 
 namespace Microsoft.CodeAnalysis.CSharp
 {
@@ -35,7 +35,9 @@ public override BoundNode VisitStackAllocArrayCreation(BoundStackAllocArrayCreat
             else if (type.OriginalDefinition == _compilation.GetWellKnownType(WellKnownType.System_Span_T))
             {
                 var spanType = (NamedTypeSymbol)stackAllocNode.Type;
-                var countTemp = _factory.StoreToTemp(rewrittenCount, out BoundAssignmentOperator countTempAssignment);
+                var sideEffects = ArrayBuilder<BoundExpression>.GetInstance();
+                var locals = ArrayBuilder<LocalSymbol>.GetInstance();
+                var countTemp = CaptureExpressionInTempIfNeeded(rewrittenCount, sideEffects, locals);
                 var stackSize = RewriteStackAllocCountToSize(countTemp, elementType);
                 stackAllocNode = new BoundConvertedStackAllocExpression(stackAllocNode.Syntax, elementType, stackSize, spanType);
 
@@ -44,8 +46,8 @@ public override BoundNode VisitStackAllocArrayCreation(BoundStackAllocArrayCreat
 
                 return new BoundSequence(
                     syntax: stackAllocNode.Syntax,
-                    locals: ImmutableArray.Create(countTemp.LocalSymbol),
-                    sideEffects: ImmutableArray.Create<BoundExpression>(countTempAssignment),
+                    locals: locals.ToImmutableAndFree(),
+                    sideEffects: sideEffects.ToImmutableAndFree(),
                     value: ctorCall,
                     type: spanType);
             }

src/Compilers/CSharp/Test/Emit/CodeGen/CodeGenReadonlyStructTests.cs

@@ -1077,24 +1077,19 @@ public static void Main()
 
             CompileAndVerify(comp, expectedOutput: "10", verify: Verification.Fails).VerifyIL("Test.Main", @"
 {
-  // Code size       29 (0x1d)
+  // Code size       26 (0x1a)
   .maxstack  2
-  .locals init (System.Span<int> V_0, //x
-                int V_1)
-  IL_0000:  ldc.i4.s   10
-  IL_0002:  stloc.1
-  IL_0003:  ldloc.1
-  IL_0004:  conv.u
-  IL_0005:  ldc.i4.4
-  IL_0006:  mul.ovf.un
-  IL_0007:  localloc
-  IL_0009:  ldloc.1
-  IL_000a:  newobj     ""System.Span<int>..ctor(void*, int)""
-  IL_000f:  stloc.0
-  IL_0010:  ldloca.s   V_0
-  IL_0012:  call       ""int System.Span<int>.Length.get""
-  IL_0017:  call       ""void System.Console.WriteLine(int)""
-  IL_001c:  ret
+  .locals init (System.Span<int> V_0) //x
+  IL_0000:  ldc.i4.s   40
+  IL_0002:  conv.u
+  IL_0003:  localloc
+  IL_0005:  ldc.i4.s   10
+  IL_0007:  newobj     ""System.Span<int>..ctor(void*, int)""
+  IL_000c:  stloc.0
+  IL_000d:  ldloca.s   V_0
+  IL_000f:  call       ""int System.Span<int>.Length.get""
+  IL_0014:  call       ""void System.Console.WriteLine(int)""
+  IL_0019:  ret
 }");
         }