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JIT doesn't fold typeof(_Canon) == typeof(vt) #79081
Description
@am11 noticed in #78874 (comment)
public static bool Test<T>()
{
Type k = typeof(T);
return k == typeof(sbyte) || k == typeof(byte);
}codegen for shared T:
G_M37465_IG01: ;; offset=0000H
4883EC28 sub rsp, 40
48894C2420 mov qword ptr [rsp+20H], rcx
G_M37465_IG02: ;; offset=0009H
488B4938 mov rcx, qword ptr [rcx+38H]
488B09 mov rcx, qword ptr [rcx]
E8AB49505F call CORINFO_HELP_TYPEHANDLE_TO_RUNTIMETYPE
48BA205359CFE3010000 mov rdx, 0x1E3CF595320 ; 'System.SByte'
483BC2 cmp rax, rdx
7418 je SHORT G_M37465_IG05
G_M37465_IG03: ;; offset=0024H
48BAE00059CFE3010000 mov rdx, 0x1E3CF5900E0 ; 'System.Byte'
483BC2 cmp rax, rdx
0F94C0 sete al
0FB6C0 movzx rax, al
G_M37465_IG04: ;; offset=0037H
4883C428 add rsp, 40
C3 ret
G_M37465_IG05: ;; offset=003CH
B801000000 mov eax, 1
G_M37465_IG06: ;; offset=0041H
4883C428 add rsp, 40
C3 ret
; Total bytes of code 70Expected codegen:
return false;
Activity
am11 commentedon Dec 1, 2022
Thanks. The non-generic version is also slightly inefficient: https://godbolt.org/z/cW47njasd.
stephentoub commentedon Dec 1, 2022
I assume fixing this would enable this to be removed?
runtime/src/libraries/System.Private.CoreLib/src/System/Threading/Tasks/Task.cs
Line 5232 in fe85cd4
EgorBo commentedon Dec 1, 2022
Yep, I have a fix already but it requires another JIT-EE update so I can't use SPMI to get diffs and have to run it locally 😞
EgorBo commentedon Dec 1, 2022
@stephentoub hm.. actually your case should be foldable as is 🤔 e.g.:
This issue is mostly about a case where a local is introduced.
stephentoub commentedon Dec 1, 2022
That line was added a decade ago (or, at least, it's evolved from one that was):
https://referencesource.microsoft.com/#mscorlib/system/runtime/compilerservices/AsyncMethodBuilder.cs,737
If it's not helping with code gen and it's not helping to improve the JIT's throughput in any meaningful way or otherwise helping with anything, let's delete it :)
EgorBo commentedon Dec 1, 2022
Ah makes sense, it probably slightly improves JIT's throughput for this specific method but unlikely it's worth additional code in C# 🙂
EgorBo commentedon Dec 1, 2022
That one must be a different issue - jit pushes generic context for VM, we have an issue for that somewhere (quite old one)