[llvm] [llvm][Support] fix convertToSnakeFromCamelCase (PR #68375)

[via llvm-commits]

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@@ -98,16 +98,18 @@ std::string llvm::convertToSnakeFromCamelCase(StringRef input) {
 
   std::string snakeCase;
   snakeCase.reserve(input.size());
-  for (char c : input) {
-    if (!std::isupper(c)) {
-      snakeCase.push_back(c);
-      continue;
-    }
-
-    if (!snakeCase.empty() && snakeCase.back() != '_')
+  auto check = [&input](size_t j, std::function<bool(int)> check) {
+    return j < input.size() ? check(input[j]) : false;
+  };
+  for (size_t i = 0; i < input.size(); ++i) {
+    snakeCase.push_back(input[i]);
+    if (check(i, isupper) && check(i + 1, isupper) && check(i + 2, islower))
+      snakeCase.push_back('_');
+    if ((check(i, islower) || check(i, isdigit)) && check(i + 1, isupper))
       snakeCase.push_back('_');
-    snakeCase.push_back(llvm::toLower(c));
   }
+  std::transform(snakeCase.begin(), snakeCase.end(), snakeCase.begin(),
+                 [](unsigned char c) { return std::tolower(c); });
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rkayaith wrote:

can move the `tolower` into the loop now

https://github.com/llvm/llvm-project/pull/68375