With a bombshell report late Tuesday, the city’s Independent Budget Office further enflamed the debate over the 421-a tax break.

The break is under attack as a huge giveaway to the real-estate industry, as critics cite the billions in tax revenue the city seems to lose because of it.

Developers note the city’s not truly giving up as much — because, without lower taxes under 421-a, there’d be almost no new rental construction, and so no taxes to pay.

They have a point: The break was born back in 1971 precisely because new building starts had ground to a halt, as high taxes made constructing even market-rate units a sucker’s bet.

Yes, the break’s worth far more in today’s prosperous city. That’s why tweaks to the law now force builders to put up a number of subsidized (“affordable”) units to qualify for 421-a — a tradeoff that Mr. Progressive, Mayor de Blasio, fully endorses.

Naturally, individual builders game this Rube Goldberg system. Hence the One57 building, the “End 421-a” poster child.

Its builder put up 66 subsidized units in The Bronx to “earn” $65.6 million in tax abatements, the IBO notes. If the funds had gone to an affordable-housing charity, the city could’ve netted 367 subsidized aparments.

Problem No. 1: One57 exploited rules that changed years ago; this particular loophole is no more. Problem No. 2: City housing nonprofits often run into trouble, leaving tenants nearly as furious as those who “enjoy” NYCHA public housing.

More important, the IBO notes that 421-a isn’t the main reason One57 residents pay such low property-tax bills. Most of their break comes from the fact that it’s a condo building — which, by the IBO’s count, trims their tax bills more than 80 percent.

In other words, the ultra-wonky IBO is hinting that the class-warriors have missed the real target.