JIT uses too wide reads for small returned structs #58874
Description
For
using System.Runtime.CompilerServices;
class Program
{
static void Main(string[] args)
{
Test t = default;
Foo(ref t);
}
[MethodImpl(MethodImplOptions.NoInlining)]
static Test Foo(ref Test t)
=> t;
}
struct Test
{
public byte A, B;
}we generate:
; Assembly listing for method Program:Foo(byref):Test
; Emitting BLENDED_CODE for X64 CPU with AVX - Windows
; optimized code
; rsp based frame
; partially interruptible
; No PGO data
; Final local variable assignments
;
; V00 arg0 [V00,T00] ( 3, 3 ) byref -> rcx single-def
;# V01 OutArgs [V01 ] ( 1, 1 ) lclBlk ( 0) [rsp+00H] "OutgoingArgSpace"
;
; Lcl frame size = 0
G_M39029_IG01: ;; offset=0000H
;; bbWeight=1 PerfScore 0.00
G_M39029_IG02: ;; offset=0000H
8B01 mov eax, dword ptr [rcx]
;; bbWeight=1 PerfScore 2.00
G_M39029_IG03: ;; offset=0002H
C3 ret
;; bbWeight=1 PerfScore 1.00
; Total bytes of code 3, prolog size 0, PerfScore 3.30, instruction count 2, allocated bytes for code 3 (MethodHash=7e33678a) for method Program:Foo(byref):Test
; ============================================================It means for instance that the following program (usually) throws AccessViolationException:
using System;
using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;
unsafe class Program
{
static void Main(string[] args)
{
IntPtr page = VirtualAlloc(IntPtr.Zero, 0x1000, 0x1000 | 0x2000, 0x04);
ref Test validRef = ref *(Test*)(page + 0x1000 - sizeof(Test));
validRef = default;
Foo(ref validRef);
}
[MethodImpl(MethodImplOptions.NoInlining)]
static Test Foo(ref Test t)
=> t;
[DllImport("kernel32")]
public static extern IntPtr VirtualAlloc(IntPtr lpAddress, nuint dwSize, uint flAllocationType, uint flProtect);
}
struct Test
{
public byte A, B;
}