Improve single-range IgnoreCase codegen for RegexCompiler / source generator

[stephentoub]

Given a set like [A-F] with RegexOptions.IgnoreCase, we'll produce a set that's actually [A-Fa-f] (we do a limited form of this today and will do a more complete job of it after #61048). When emitting matching code for this, we'll emit it today as:

((ch = span[i]) < 128 && ("\0\0\0\0~\0~\0"[ch >> 4] & (1 << (ch & 0xF))) != 0)

but we could instead emit it as:

(((uint)span[i] | 0x20) - 'a' <= (uint)('f' - 'a'))

This will entail updating

runtime/src/libraries/System.Text.RegularExpressions/gen/RegexGenerator.Emitter.cs

Line 3073 in 6f40e48

private static string MatchCharacterClass(bool hasTextInfo, RegexOptions options, string chExpr, string charClass, bool caseInsensitive, HashSet<string>? additionalDeclarations)

to handle the additional special-case. It already has a special-case for a range, and it already has a special-case for casing, we just need to add one that combines them. There may be additional cases to consider as well.

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Issue Details

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Given a set like [A-F] with RegexOptions.IgnoreCase, we'll produce a set that's actually [A-Fa-f] (we do a limited form of this today and will do a more complete job of it after #61048). When emitting matching code for this, we'll emit it today as:

((ch = span[i]) < 128 && ("\0\0\0\0~\0~\0"[ch >> 4] & (1 << (ch & 0xF))) != 0)

but we could instead emit it as:

(((uint)span[i] | 0x20) - 'a' <= (uint)('f' - 'a'))

This will entail updating

runtime/src/libraries/System.Text.RegularExpressions/gen/RegexGenerator.Emitter.cs

Line 3073 in 6f40e48

private static string MatchCharacterClass(bool hasTextInfo, RegexOptions options, string chExpr, string charClass, bool caseInsensitive, HashSet<string>? additionalDeclarations)

to handle the additional special-case. It already has a special-case for a range, and it already has a special-case for casing, we just need to add one that combines them. There may be additional cases to consider as well.

Author:	stephentoub
Assignees:	-
Labels:	area-System.Text.RegularExpressions
Milestone:	7.0.0