Hardcore (IMDB) is a 1979 crime drama film written and directed by Paul Schrader.
The plot is that Jake (George C. Scott), a mid-Western family man and devout Calvinist, searches through the sex work underground for his daughter after she disappears in Los Angeles, then turns up in a porn film.
Hardcore bears a strong resemblance to 8MM. Both are Orpheus narratives: a man descends into the underworld to find a lost loved one. It also taps into the American captivity narrative. It provides an interesting glimpse into the sex work underworld of Los Angeles in the late 1970s. Again, like 8MM, there’s no particular investigation of the people who work in the sex industry. (Somebody did do enough research to drop names like the Mitchell brothers.)