Help JIT elide redundant CharToHexLookup range check

[xtqqczze]

Diffs: MihuBot/runtime-utils#1776

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[EgorBo]

@xtqqczze I'm not convinced those diffs are meaningful. They're just PMI artifacts where we prejit all methods, even with AggressiveInlining. So most of your diffs are just FromUpperChar and FromChar themselves.

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static int FromChar(int c)
        {
            return (c >= CharToHexLookup.Length) ? 0xFF : CharToHexLookup[c];
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static int FromUpperChar(int c)
        {
            return (c > 71) ? 0xFF : CharToHexLookup[c];
        }

here JIT does emit a bound check because noone checks c for being negative so it's expected. But all callers one way or another pass something that is clearly non-negative and after inlining there is no bound check. You may add [AggressiveInlining] over DecodeHexChars if you want.

Also, you can add a Debug.Assert if you want

[xtqqczze]

@EgorBo I was already thinking about AggressiveInlining, diffs: MihuBot/runtime-utils#1777

[EgorBo]

The problem that you effectively change the behavior here. The invalid code that used to throw Invalid range exception now silently uses 0xFF

[xtqqczze]

The invalid code that used to throw Invalid range exception now silently uses 0xFF

HexConverter is internal code, but point taken. There are some projects targeting .NET Framework using these methods.

[xtqqczze]

The problem that you effectively change the behavior here. The invalid code that used to throw Invalid range exception now silently uses 0xFF

When these methods are used as intended, with values that are clearly non-negative, there is no change in behavior, which explains the absence of meaningful diffs.

As such, this change is only expected to improve code generation for projects targeting .NET Framework.

@EgorBo As you suspected, more meaningful diffs can be seen by adding AggressiveInlining to methods like DecodeHexChars: MihuBot/runtime-utils#1778