Switches with intrinsic const aren't getting optimized

[stephentoub]

Consider:

[MethodImpl(MethodImplOptions.NoInlining)]
private static int Test<TEnum>()
{
    TypeCode code = Type.GetTypeCode(typeof(TEnum));
    switch (code)
    {
        case TypeCode.Int32:
            return 100;
        case TypeCode.Int64:
            return 200;
        default:
            return 300;
    }
}

called with Test<DayOfWeek>. The optimized code for this ends up being:

; Assembly listing for method ConsoleApp4.Program:Test[int]():int
; Emitting BLENDED_CODE for X64 CPU with AVX - Windows
; Tier-1 compilation
; optimized code
; rsp based frame
; partially interruptible
; No PGO data
; 0 inlinees with PGO data; 0 single block inlinees; 2 inlinees without PGO data

G_M000_IG01:                ;; offset=0000H

G_M000_IG02:                ;; offset=0000H
       B809000000           mov      eax, 9
       83F809               cmp      eax, 9
       7407                 je       SHORT G_M000_IG04

G_M000_IG03:                ;; offset=000AH
       83F80B               cmp      eax, 11
       7408                 je       SHORT G_M000_IG06
       EB0C                 jmp      SHORT G_M000_IG08

G_M000_IG04:                ;; offset=0011H
       B864000000           mov      eax, 100

G_M000_IG05:                ;; offset=0016H
       C3                   ret

G_M000_IG06:                ;; offset=0017H
       B8C8000000           mov      eax, 200

G_M000_IG07:                ;; offset=001CH
       C3                   ret

G_M000_IG08:                ;; offset=001DH
       B82C010000           mov      eax, 300

G_M000_IG09:                ;; offset=0022H
       C3                   ret

; Total bytes of code 35

so even though the Type.GetTypeCode(typeof(TEnum)) is turned into mov eax, 9, the whole switch is still implemented (that mov eax, 9 followed by a cmp eax, 9 is particularly fun). If I instead replace the Type.GetTypeCode(typeof(TEnum)) with TypeCode.Int32, then I do get the expected:

; Assembly listing for method ConsoleApp4.Program:Test[int]():int
; Emitting BLENDED_CODE for X64 CPU with AVX - Windows
; Tier-1 compilation
; optimized code
; rsp based frame
; partially interruptible
; No PGO data

G_M000_IG01:                ;; offset=0000H

G_M000_IG02:                ;; offset=0000H
       B864000000           mov      eax, 100

G_M000_IG03:                ;; offset=0005H
       C3                   ret

; Total bytes of code 6

[ghost]

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Issue Details

---

Consider:

[MethodImpl(MethodImplOptions.NoInlining)]
private static int Test<TEnum>()
{
    TypeCode code = Type.GetTypeCode(typeof(TEnum));
    switch (code)
    {
        case TypeCode.Int32:
            return 100;
        case TypeCode.Int64:
            return 200;
        default:
            return 300;
    }
}

called with Test<DayOfWeek>. The optimized code for this ends up being:

; Assembly listing for method ConsoleApp4.Program:Test[int]():int
; Emitting BLENDED_CODE for X64 CPU with AVX - Windows
; Tier-1 compilation
; optimized code
; rsp based frame
; partially interruptible
; No PGO data
; 0 inlinees with PGO data; 0 single block inlinees; 2 inlinees without PGO data

G_M000_IG01:                ;; offset=0000H

G_M000_IG02:                ;; offset=0000H
       B809000000           mov      eax, 9
       83F809               cmp      eax, 9
       7407                 je       SHORT G_M000_IG04

G_M000_IG03:                ;; offset=000AH
       83F80B               cmp      eax, 11
       7408                 je       SHORT G_M000_IG06
       EB0C                 jmp      SHORT G_M000_IG08

G_M000_IG04:                ;; offset=0011H
       B864000000           mov      eax, 100

G_M000_IG05:                ;; offset=0016H
       C3                   ret

G_M000_IG06:                ;; offset=0017H
       B8C8000000           mov      eax, 200

G_M000_IG07:                ;; offset=001CH
       C3                   ret

G_M000_IG08:                ;; offset=001DH
       B82C010000           mov      eax, 300

G_M000_IG09:                ;; offset=0022H
       C3                   ret

; Total bytes of code 35

so even though the Type.GetTypeCode(typeof(TEnum)) is turned into mov eax, 9, the whole switch is still implemented. If I instead replace the Type.GetTypeCode(typeof(TEnum)) with TypeCode.Int32, then I do get the expected:

; Assembly listing for method ConsoleApp4.Program:Test[int]():int
; Emitting BLENDED_CODE for X64 CPU with AVX - Windows
; Tier-1 compilation
; optimized code
; rsp based frame
; partially interruptible
; No PGO data

G_M000_IG01:                ;; offset=0000H

G_M000_IG02:                ;; offset=0000H
       B864000000           mov      eax, 100

G_M000_IG03:                ;; offset=0005H
       C3                   ret

; Total bytes of code 6

Author:	stephentoub
Assignees:	-
Labels:	area-CodeGen-coreclr
Milestone:	-

[jkotas]

You should see it getting optimized if you switch on GetEnumUnderlyingType:

[MethodImpl(MethodImplOptions.NoInlining)]
private static int Test<TEnum>()
{
    Type type = typeof(TEnum).GetEnumUnderlyingType();
    if (type == typeof(int))
        return 100;
    if (type == typeof(long))
        return 200;
    return 300;
}

[stephentoub]

You should see it getting optimized if you switch on GetEnumUnderlyingType

I do, which is my local workaround.

[jkotas]

I think that switching on GetEnumUnderlyingType is the right forward-looking way to write this. We have been saying that IConvertible and TypeCode are legacy concepts.

[stephentoub]

Ok.

Though I still think outputting the code:

       B809000000           mov      eax, 9
       83F809               cmp      eax, 9

is suspect :)

[jkotas]

Yes, it would be nice to optimize this better. The problem is not specific to intrinsics. Any switch over constant expression that is complex-enough will hit this problem. For example:

[MethodImpl(MethodImplOptions.AggressiveInlining)]
static int Map(int x, int y, bool firstOrSecond)
{
    int t = firstOrSecond ? x : y;
    if (t == 50) return 1;
    if (t == 100) return 2;
    return 3;
}

[MethodImpl(MethodImplOptions.NoInlining)]
static int Test()
{
    switch (Map(50, 100, false))
    {
    case 0:
        return 100;
    case 1:
        return 200;
    default:
        return 0;
    }
}

generates

...
mov      edx, 2
cmp      edx, 1
...

[EgorBo]

Interesting case, thanks, we end-up with this when we hit rationalization:

[EgorBo]

Both Stephen's and Jan's examples are now folded to return const; in Main.
I bet it was improved by @AndyAyersMS's #94689