Improve Regex support for automatically making loops atomic

[stephentoub]

In .NET 5, we added some support for automatically making some loops atomic. For example, given the expression a+b+c+, we'll convert that into the equivalent of (?>a+)(?>b+)(?>c+): the c+ is at the end of the expression so nothing can backtrack into it, the b+ is always followed by a c and there's nothing b+ can give back that would match c, and similarly the a+ is always followed by a b and there's nothing a+ can give back that would match b. However, tweaking the expression slightly to a*b*c*, we'll convert that into the equivalent of a*b*(?>c*): while we still treat the c* as atomic, when analyzing the a* and b* loops and comparing them against what's guaranteed to come next, we see that the subsequent loop has a 0-lower-bound and give up, even though with a bit more analysis we could see that it's still valid to make all of these atomic. The more we can make atomic, the better, not just for performance but also for readability of code in the source generator.

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Issue Details

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In .NET 5, we added some support for automatically making some loops atomic. For example, given the expression a+b+c+, we'll convert that into the equivalent of (?>a+)(?>b+)(?>c+): the c+is at the end of the expression so nothing can backtrack into it, theb+is always followed by acand there's nothingb+can give back that would matchc, and similarly the a+is always followed by aband there's nothinga+can give back that would matchb. However, tweaking the expression slightly to abc*, we'll convert that into the equivalent of ab(?>c*): while we still treat the c*as atomic, when analyzing thea*andb*` loops and comparing them against what's guaranteed to come next, we see that the subsequent loop has a 0-lower-bound and give up, even though with a bit more analysis we could see that it's still valid to make all of these atomic. The more we can make atomic, the better, not just for performance but also for readability of code in the source generator.

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