“Commitment” is a cornerstone of the marriage contract. And it’s damned important in a romantic comedy about marriages as well.

Say this for the cast of writer-director Nicholas Stoller’s “You’re Cordially Invited.” These kids — and Will Ferrell and Reese Witherspoon, Celia Weston on down to Nick Jonas — commit the f out of this, to use a word that earned and earned and earned again this comedy’s R-rating.

It’s a tidy but rude “dueling weddings” farce that doesn’t break new ground in the genre, and Stoller (“Forgetting Sarah Marshall”) peppers the script with all those f-bombs in a vain attempt to give it “edge.” There’s no edge to this. At all.

But here’s what it gets just soooo right. Everybody on set performs at a fever pitch, even the slower talking, colorful colloquia quipping Southerners.

The energy level is never quite manic but it never flags.

Stoller follows wedding rom-com master P.J. Hogan’s (“My Best Friend’s Wedding/Muriel’s Wedding”) long-established golden rules for the genre. The efforts to sabotage True Love have to be believable, petty and yet redeemably selfish.

And it’s a wedding. What’s a wedding or a wedding movie without singing?

As it co-stars Witherspoon, it’s a lot closer to her “Sweet Home Alabama” sweetspot than a great wedding rom-com. But Witherspoon, playing an ex-Atlantan now a Left Coast reality TV mogul, is given a couple of grand sparring partners that help this come off.

Ferrell, playing an over-the-top doting dad whose wedding is double-booked into the same S.C. island hotel, knows “over-the-top.” And towering over Reese, he offers no quarter.

And Witherspoon’s fellow Southerner Celia Weston (“Dead Man Walking”) plays the Steel Magnolia mom whom TV producer Margot fled to the other coast to get away from, the matriarch of a drawling Atlanta clan so Southern fried contemptuous of everything about Margot they might as well be wearing red baseball caps.

“The sins of the country have been blamed on The South,” Mama drawls, as we notice the lily whiteness of their wedding party.

Ferrell is blessed with having the gonzo Geraldine Viswanathan (“Drive-Away Dolls”) play his very young, mercurial daughter. Yeah, she inherited her Dad’s hair-trigger temper. Yes, she indulges his baking her cake and doing her wedding day hair (that’s not his profession).

And noooo, there’s nothing Frank-and-Nancy icky about father and daughter dueting Kenny & Dolly’s “Islands in the Stream.”

The meanness of two people and two families competing for a double-booked “exclusive” weekend is somewhat undercut by characters sweetly rising to the occasion and doing the right thing. But that doesn’t mean rehearsals, weddings and receptions won’t get down and dirty and downright out of hand.

The laughs are a combination of slapstick low-hanging fruit and little random bits of yokel whimsy. Witherspoon’s sister Neve (Meredith Hagner) is A) marrying a cowboy male exotic dancer (drawling scene-stealer Jimmy Tatro) and B) secretly pregnant.

Southern enough for you?

Pop star Jonas plays a hip young (Southern) pastor who figures crooning a little Creed should go with every marriage ceremony.

Jim’s daughter’s “wedding planner” is “a drunken child” (Keyla Monterroso Mejia) and utterly incompetent and unready for the real world. And the bridesmaids are also Gen Z cheap shot in the making.

The picture, which is earning dismissive reviews in some quarters, wouldn’t work without the oddball, mismatched chemistry between Witherspoon and Ferrell, who are a walking sight gag when they’re in the same shot.

But again, they “commit” to her drunken toast scene, his gator wrestling one, her withering, pissy put-downs of her family — and his — and Ferrell’s not-quite-Boomer “anything for a laugh” ethos turned loose on the delicate sensitivities of 20thing bridesmaids.

And say what you want, but that’s funny, till death do us part.

Rating: R, for profanity (lots and lots of F-bombs)

Cast: Reese Witherspoon, Will Ferrell, Geraldine Viswanathan, Meredith Hagner, Celia Weston, Jimmy Tatro, Leanne Morgan, Stony Blyden, Keyla Monterroso Mejia, Nick Jonas, and Jack McBrayer

Credits: Scripted and directed by Nicholas Stoller. An Amazon Prime Video release.

Running time: 1:52