JIT: Optimization opportunity (extra jumps/checks)

[rameel]

godbolt

public static Span<char> Trim(Span<char> s)
{
    while (s.Length != 0 && s[ 0] == '/') s = s.Slice(1);
    while (s.Length != 0 && s[^1] == '/') s = s.Slice(0, s.Length - 1);

    return s;
}

Current codegen:
coreclr trunk-20260521+2a65f102aaad2e0b97bf4554dd510ae57801ae5e

Program:Trim(System.Span`1[char]):System.Span`1[char] (FullOpts):
       push     rbp
       mov      rbp, rsp
       test     esi, esi
       je       SHORT G_M48643_IG04     ; <-- 1: could jump directly to G_M48643_IG06
       align    [0 bytes for IG03]

G_M48643_IG03:
       cmp      word  ptr [rdi], 47
       jne      SHORT G_M48643_IG04     ; <-- 2: could jump directly to G_M48643_IG05
       add      rdi, 2
       dec      esi
       jne      SHORT G_M48643_IG03

G_M48643_IG04:
       test     esi, esi                ; <-- 3: dead test
       je       SHORT G_M48643_IG06     ;        jmp SHORT G_M48643_IG06
       align    [6 bytes for IG05]

G_M48643_IG05:
       lea      eax, [rsi-0x01]
       mov      edx, eax
       cmp      word  ptr [rdi+2*rdx], 47
       jne      SHORT G_M48643_IG06
       mov      esi, eax
       test     esi, esi
       jne      SHORT G_M48643_IG05

G_M48643_IG06:
       mov      rax, rdi
       mov      rdx, rsi
       pop      rbp
       ret

1. since G_M48643_IG04 immediately repeats the same condition
2. we already know esi != 0
3. the condition is already known esi == 0

[dotnet-policy-service]

Tagging subscribers to this area: @JulieLeeMSFT, @jakobbotsch
See info in area-owners.md if you want to be subscribed.

[BoyBaykiller]

Other repro (https://godbolt.org/z/5jWG6oovM):

static bool MissedReuseCmp(byte ch)
{
    if (ch <= 32)
    {
        if (ch != 32)
        {
            Console.WriteLine("Hi!");
            return false;
        }
        return true;
    }
    return false;
}

Program:MissedReuseCmp(byte):bool (FullOpts):
       push     rbp
       mov      rbp, rsp
       movzx    rax, dil
       cmp      eax, 32
       jg       SHORT G_M47556_IG06
       cmp      eax, 32 ; <- can be removed
       jne      SHORT G_M47556_IG05
       mov      eax, 1
       pop      rbp
       ret      
G_M47556_IG05:  ;; offset=0x0019
       mov      rdi, 0x7A0AD4A025E8      ; 'Hi!'
       call     [System.Console:WriteLine(System.String)]
G_M47556_IG06:  ;; offset=0x0029
       xor      eax, eax
       pop      rbp
       ret