For two decades, companies that buy software patents to sue technology giants have been the scourge of Silicon Valley. Reviled as patent trolls, they have attacked everything from Google's online ads to Apple's iPhone features, sometimes winning hundreds of millions of dollars.

But now the trolls are in retreat from the tech titans, interviews and data reviewed by Reuters show.

In the wake of several changes in U.S. law, which make it easier to challenge software patents, patent prices are plummeting, the number of court fights is down, and stock prices of many patent-holding companies have fallen. Some tech firms say they are punching up research budgets as legal costs shrink, while support for major patent reform is under fire as trolls get trounced.

"Their entire business model relies on intimidation, and that has lost its edge," said Efrat Kasznik, president of intellectual property consulting firm Foresight Valuation Group. "If the patents are not enforceable in court anymore... the troll has no legs to stand on."