RequirementMachine: Speed up term simplification with a prefix trie #38775
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…ontain type variables
…icate rewrite rules RewriteSystem::addRule() did two things before adding the rewrite rule: - Simplify both sides. If both sides are now equal, discard the rule. - If the last symbol on the left hand side was a superclass or concrete type symbol, simplify the substitution terms in that symbol. The problem is that the second step can produce a term which can be further simplified, and in particular, one that is exactly equal to the left hand side of some other rule. To fix this, swap the order of the two steps. The only wrinkle is now we have to check for a concrete type symbol at the end of _both_ the left hand side and right hand side, since we don't orient the rule until we simplify both sides. I don't have a reduced test case for this one, but it was revealed by compiler_crashers_2_fixed/0109-sr4737.swift after I introduced the trie.
Previously RewriteSystem::simplify() would attempt to apply every rewrite rule at every position in the original term, which was obviously a source of overhead. The trie itself is somewhat unoptimized; for example, with a bit of effort it could merge a node with its only child, if nodes stored a range of elements to compare rather than a single element.
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This reduces the time spent in the requirement machine while building the standard library from 20s to 5s. More improvements are coming soon.