Fix unnecessary bounds check with ulong index

[EgorBo]

Closes #76198

string Test(ulong ulValue, string[] names) { if (ulValue < (ulong)names.Length) return names[(uint)ulValue]; return null; }

Codegen diff:

; Method Prog:Test(ulong,System.String[]):System.String:this (FullOpts) sub rsp, 40 mov eax, dword ptr [r8+0x08] - mov ecx, eax - cmp rcx, rdx + cmp rax, rdx jbe SHORT G_M5198_IG05 - cmp edx, eax - jae SHORT G_M5198_IG07 mov eax, edx mov rax, gword ptr [r8+8*rax+0x10] add rsp, 40 ret G_M5198_IG05: xor rax, rax add rsp, 40 ret -G_M5198_IG07: - call CORINFO_HELP_RNGCHKFAIL - int3  -; Total bytes of code: 44 +; Total bytes of code: 32

[EgorBo]

PTAL @AndyAyersMS @dotnet/jit-contrib

Very few diffs, but afair, @stephentoub had to use unsafe to workaround it.

the idea that (ulong)i < (ulong)arr.Length creates an assertion that i is less than arr.Length (unsigned) - basically, the same assertion that we generate for (uint)i < (uint)arr.Length today, I even re-used the same path

[AndyAyersMS]

Can you update the function header comment too?

[EgorBo]

Can you update the function header comment too?

Done

Failure is #101721