Chained pattern matching not eliminating str/arr bounds check

Open

#119,508 opened on Sep 9, 2025 3 comments 0 reactions 1 assignee View on GitHub

@EgorBo is already working on this.

Since Sep 10, 2025.

Assessment

This issue has not been assessed yet.

Description

area-CodeGen-coreclr tenet-performance

The pattern str is { Length: 1 } && str[0] is 'x' or 'y' does not eliminate bounds checks when accessing the first char of the string (or array).

Adding extra indirection with && (sdr[0] is char c) && c is 'x' or 'y' does.

If there is only a single match in the indexer call, e.g. str[0] is 'z' the range check is properly eliminated.

Reproduces on older versions of .NET as well.

class Program
{
    // range check
    public static bool IsFlagsFormat(string? format) => format is { Length: 1 } && format[0] is 'f' or 'F';
    public static bool IsFlagsFormat(char[]? format) => format is { Length: 1 } && format[0] is 'f' or 'F';

    // no range check
    public static bool IsFlagsFormat2(string? format) => format is { Length: 1 } && (format[0] is char c) && (c is 'f' or 'F');
    public static bool IsFlagsFormat2(char[]? format) => format is { Length: 1 } && (format[0] is char c) && (c is 'f' or 'F');
}

https://godbolt.org/z/rKfbffMqE

Program:IsFlagsFormat(System.String):bool (FullOpts):
       push     rbp
       mov      rbp, rsp
       test     rdi, rdi
       je       SHORT G_M36473_IG04
       xor      eax, eax
       cmp      dword ptr [rdi+0x08], 1
       sete     al
       jmp      SHORT G_M36473_IG05
G_M36473_IG04:  ;; offset=0x0014
       xor      eax, eax
G_M36473_IG05:  ;; offset=0x0016
       movzx    rax, al
       test     eax, eax
       je       SHORT G_M36473_IG09
       cmp      dword ptr [rdi+0x08], 0
       jbe      SHORT G_M36473_IG10
       movzx    rax, word  ptr [rdi+0x0C]
       cmp      eax, 70
       je       SHORT G_M36473_IG07
       cmp      eax, 102
       jne      SHORT G_M36473_IG08
G_M36473_IG07:  ;; offset=0x0031
       mov      eax, 1
       jmp      SHORT G_M36473_IG09
G_M36473_IG08:  ;; offset=0x0038
       xor      eax, eax
G_M36473_IG09:  ;; offset=0x003A
       pop      rbp
       ret      
G_M36473_IG10:  ;; offset=0x003C
       call     CORINFO_HELP_RNGCHKFAIL
       int3

Dominant language: C#
Stars: 18.3k
Forks: 5.6k
PR merge metrics: PR metrics pending

Contributor guide

Open the contributing guide

First steps

Read the whole issue, then the project's contributing guide.
Comment on the issue to say you are picking it up — it saves two people doing the same work.
Fork the repository and make your change on a branch.
Open a pull request that references the issue number.

Similar issues

Tsavorite.test fails to compile on main: TestBase.cs references Garnet.test.TestUtils, which is not linked into Tsavorite.test.csproj Open

Difficulty 2/5 1-3 hours Newbie friendliness 86/100

microsoft/garnet#2161 · 2 comments · Opened Sep 20, 2026
v11 update: Validation attributes are no longer experimental Open

:watch: Not Triaged 11.0 fundamentals/subsvc

Difficulty 2/5 1-3 hours Newbie friendliness 92/100

dotnet/AspNetCore.Docs#37699 · Opened Sep 20, 2026
SE 5.3.0 beta 7: In Settings Video player download buttons aren't together Open

Difficulty 2/5 1-3 hours Newbie friendliness 72/100

SubtitleEdit/subtitleedit#15108 · 1 comment · Opened Sep 20, 2026
Kubernetes get-started create-component.md: Go and C# silently default isMinikube instead of requiring it, unlike TS/Python/Java/HCL Open

area/docs-content Bug pulumi/docs

Difficulty 1/5 1-3 hours Newbie friendliness 94/100

pulumi/docs#21762 · Opened Sep 20, 2026
Create parent directories only after the containment check in InstallHelper.TryExtractToDirectory Open

Difficulty 2/5 1-3 hours Newbie friendliness 78/100

PowerShell/PSResourceGet#2056 · Opened Sep 20, 2026

More C# issues