Extra stloc and ldloc in switch statement compared to if statement

[Tornhoof]

I noticed an extra stloc and ldloc in the following switch statement compared to the equivalent if statement (this is just a shortened version of a longer and nested statement).
The il and disassembly looks pretty much the same except for the stloc.0 and ldloc.0 in the switch version, which adds an extra mov to a register and then compares the register against the value, instead of doing a direct memory location compare against the value (like in the if version).
I looked for other bugs similar to this, but couldn't find anything specific.

The disassembly produced by benchmark.net looks like the following (including source and il annotation):
If

00007ffe`f18327b0 ConsoleApp23.LookupBenchmark.If()
            if (Bytes[2] == (byte)'l')
            ^^^^^^^^^^^^^^^^^^^^^^^^^^
IL_0000: ldsfld System.Byte[] ConsoleApp23.LookupBenchmark::Bytes
IL_0005: ldc.i4.2
IL_0006: ldelem.u1
IL_0007: ldc.i4.s 108
IL_0009: bne.un.s IL_000d
00007ffe`f18327b4 48b900f98df1fe7f0000 mov rcx,7FFEF18DF900h
00007ffe`f18327be ba06000000      mov     edx,6
00007ffe`f18327c3 e828e4ab5f      call    coreclr!coreclr_shutdown_2+0x39680 (00007fff`512f0bf0)
00007ffe`f18327c8 48b8386d0010a9020000 mov rax,2A910006D38h
00007ffe`f18327d2 488b00          mov     rax,qword ptr [rax]
00007ffe`f18327d5 83780802        cmp     dword ptr [rax+8],2
00007ffe`f18327d9 7617            jbe     00007ffe`f18327f2
00007ffe`f18327db 8078126c        cmp     byte ptr [rax+12h],6Ch
00007ffe`f18327df 750a            jne     00007ffe`f18327eb
                return 1;
                ^^^^^^^^^
IL_000b: ldc.i4.1
IL_000c: ret
00007ffe`f18327e1 b801000000      mov     eax,1
00007ffe`f18327e6 4883c428        add     rsp,28h
00007ffe`f18327ea c3              ret
            return 0;
            ^^^^^^^^^
IL_000d: ldc.i4.0
IL_000e: ret
00007ffe`f18327eb 33c0            xor     eax,eax

Switch

00007ffe`f18327b0 ConsoleApp23.LookupBenchmark.Switch()
            switch (Bytes[2])
            ^^^^^^^^^^^^^^^^^
IL_0000: ldsfld System.Byte[] ConsoleApp23.LookupBenchmark::Bytes
IL_0005: ldc.i4.2
IL_0006: ldelem.u1
IL_0007: stloc.0
00007ffe`f18327b4 48b900f98df1fe7f0000 mov rcx,7FFEF18DF900h
00007ffe`f18327be ba06000000      mov     edx,6
00007ffe`f18327c3 e828e4ab5f      call    coreclr!coreclr_shutdown_2+0x39680 (00007fff`512f0bf0)
00007ffe`f18327c8 48b8386d265871020000 mov rax,27158266D38h
00007ffe`f18327d2 488b00          mov     rax,qword ptr [rax]
00007ffe`f18327d5 83780802        cmp     dword ptr [rax+8],2
00007ffe`f18327d9 761a            jbe     00007ffe`f18327f5
00007ffe`f18327db 0fb64012        movzx   eax,byte ptr [rax+12h]
                    return 1;
                    ^^^^^^^^^
IL_0008: ldloc.0
IL_0009: ldc.i4.s 108
IL_000b: bne.un.s IL_000f
00007ffe`f18327df 83f86c          cmp     eax,6Ch
00007ffe`f18327e2 750a            jne     00007ffe`f18327ee
IL_000d: ldc.i4.1
IL_000e: ret
00007ffe`f18327e4 b801000000      mov     eax,1
00007ffe`f18327e9 4883c428        add     rsp,28h
00007ffe`f18327ed c3              ret
            return 0;
            ^^^^^^^^^
IL_000f: ldc.i4.0
IL_0010: ret
00007ffe`f18327ee 33c0            xor     eax,eax

private static readonly byte[] Bytes = Encoding.UTF8.GetBytes("Hello");

[Benchmark]
public int If()
{
    if (Bytes[2] == (byte)'l')
    {
        return 1;
    }

    return 0;
}

[Benchmark]
public int Switch()
{
    switch (Bytes[2])
    {
        case (byte)'l':
        {
            return 1;
        }
    }

    return 0;
}

category:cq theme:basic-cq skill-level:beginner cost:small

[RussKeldorph]

@dotnet/jit-contrib

[AndyAyersMS]

We see this stloc/ldloc pattern elsewhere fairly frequently. If there is just that one load (which we can check for in our up-front IL scan) we should just optimize this pattern to a nop and forward the value to the consumer.

This is a fairly cheap and safe form of forward sub (see #4655).

[AndyAyersMS]

This was fixed at some point... 7.0 codegen below. but sharplab shows the same for 6.0.

; Assembly listing for method X:If():int:this
; Emitting BLENDED_CODE for X64 CPU with AVX - Windows
; optimized code
; rsp based frame
; partially interruptible
; No PGO data
; Final local variable assignments
;
;  V00 this         [V00,T01] (  3,  3   )     ref  ->  rcx         this class-hnd single-def
;  V01 OutArgs      [V01    ] (  1,  1   )  lclBlk (32) [rsp+00H]   "OutgoingArgSpace"
;  V02 tmp1         [V02,T00] (  3,  6   )     ref  ->  rax         single-def "arr expr"
;
; Lcl frame size = 40

G_M50435_IG01:              ;; offset=0000H
       4883EC28             sub      rsp, 40
						;; bbWeight=1    PerfScore 0.25
G_M50435_IG02:              ;; offset=0004H
       488B4108             mov      rax, gword ptr [rcx+8]
       83780802             cmp      dword ptr [rax+8], 2
       7617                 jbe      SHORT G_M50435_IG07
       8078126C             cmp      byte  ptr [rax+18], 108
       750A                 jne      SHORT G_M50435_IG05
						;; bbWeight=1    PerfScore 10.00
G_M50435_IG03:              ;; offset=0014H
       B801000000           mov      eax, 1
						;; bbWeight=0.50 PerfScore 0.12
G_M50435_IG04:              ;; offset=0019H
       4883C428             add      rsp, 40
       C3                   ret      
						;; bbWeight=0.50 PerfScore 0.62
G_M50435_IG05:              ;; offset=001EH
       33C0                 xor      eax, eax
						;; bbWeight=0.50 PerfScore 0.12
G_M50435_IG06:              ;; offset=0020H
       4883C428             add      rsp, 40
       C3                   ret      
						;; bbWeight=0.50 PerfScore 0.62
G_M50435_IG07:              ;; offset=0025H
       E8A6B6015F           call     CORINFO_HELP_RNGCHKFAIL
       CC                   int3     
						;; bbWeight=0    PerfScore 0.00

; Total bytes of code 43, prolog size 4, PerfScore 16.05, instruction count 14, allocated bytes for code 43 (MethodHash=94d33afc) for method X:If():int:this


; Assembly listing for method X:Switch():int:this
; Emitting BLENDED_CODE for X64 CPU with AVX - Windows
; optimized code
; rsp based frame
; partially interruptible
; No PGO data
; Final local variable assignments
;
;  V00 this         [V00,T01] (  3,  3   )     ref  ->  rcx         this class-hnd single-def
;  V01 OutArgs      [V01    ] (  1,  1   )  lclBlk (32) [rsp+00H]   "OutgoingArgSpace"
;  V02 tmp1         [V02,T00] (  3,  6   )     ref  ->  rax         single-def "arr expr"
;
; Lcl frame size = 40

G_M11134_IG01:              ;; offset=0000H
       4883EC28             sub      rsp, 40
						;; bbWeight=1    PerfScore 0.25
G_M11134_IG02:              ;; offset=0004H
       488B4108             mov      rax, gword ptr [rcx+8]
       83780802             cmp      dword ptr [rax+8], 2
       7617                 jbe      SHORT G_M11134_IG07
       8078126C             cmp      byte  ptr [rax+18], 108
       750A                 jne      SHORT G_M11134_IG05
						;; bbWeight=1    PerfScore 10.00
G_M11134_IG03:              ;; offset=0014H
       B801000000           mov      eax, 1
						;; bbWeight=0.50 PerfScore 0.12
G_M11134_IG04:              ;; offset=0019H
       4883C428             add      rsp, 40
       C3                   ret      
						;; bbWeight=0.50 PerfScore 0.62
G_M11134_IG05:              ;; offset=001EH
       33C0                 xor      eax, eax
						;; bbWeight=0.50 PerfScore 0.12
G_M11134_IG06:              ;; offset=0020H
       4883C428             add      rsp, 40
       C3                   ret      
						;; bbWeight=0.50 PerfScore 0.62
G_M11134_IG07:              ;; offset=0025H
       E846B6015F           call     CORINFO_HELP_RNGCHKFAIL
       CC                   int3     
						;; bbWeight=0    PerfScore 0.00

; Total bytes of code 43, prolog size 4, PerfScore 16.05, instruction count 14, allocated bytes for code 43 (MethodHash=0ec8d481) for method X:Switch():int:this