Constant mod over chars can use a cheaper FastMod

[MihaZupan]

#101001 added a faster variant of FastMod that SearchValues now uses when it knows that the value and divisor are both < 2^16 (i.e. chars).
Instead of

runtime/src/libraries/System.Private.CoreLib/src/System/Collections/HashHelpers.cs

Line 107 in e71d737

uint highbits = (uint)(((((multiplier * value) >> 32) + 1) * divisor) >> 32);

we can use

runtime/src/libraries/System.Private.CoreLib/src/System/SearchValues/ProbabilisticMapState.cs

Line 223 in e71d737

ulong result = ((ulong)(multiplier * value) * divisor) >> 32;

---

Is it worth teaching the JIT to do something similar when it knows that values are in range?

int Mod1(char c) => c % 42;
int Mod2(char c) => (int)(((ulong)(102261127u * c) * 42) >> 32);

Test.Mod1(Char)
    L0000: movzx eax, cx
    L0003: mov ecx, eax
    L0005: shr ecx, 1
    L0007: imul rcx, 0x30c30c31
    L000e: shr rcx, 0x22
    L0012: imul ecx, 0x2a
    L0015: sub eax, ecx
    L0017: ret

Test.Mod2(Char)
    L0000: movzx eax, cx
    L0003: imul eax, 0x6186187
    L0009: imul rax, 0x2a
    L000d: shr rax, 0x20
    L0011: ret

sharplab

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