[commit: ghc] master: WIP on combined Step 1 and 3 for Trees That Grow, HsExpr (e3ec2e7)

[Ben Gamari]

On November 11, 2017 10:03:37 PM EST, Joachim Breitner <mail at joachim-breitner.de> wrote:
>Hi Alan,
>
>the combined effect of 
>
>commit e3ec2e7ae94524ebd111963faf34b84d942265b4
>WIP on combined Step 1 and 3 for Trees That Grow, HsExpr
>
>
>and 
>
>commit 438dd1cbba13d35f3452b4dcef3f94ce9a216905
>WIP on Doing a combined Step 1 and 3 for Trees That Grow
>
>
>causes a 15% regression in the time it takes to build GHC, according to
>https://perf.haskell.org/ghc/#compare/fe6848f544c2a14086bcef388c46f4070c22d287/e3ec2e7ae94524ebd111963faf34b84d942265b4
>
>
>Is that a known, expected and accepted regression?
>
>Joachim

I noted this on D4177 and discussed the effect with Alan. Indeed there is quite a sizeable regression in compilation time but thankfully this is not because GHC itself is slower. Rather, it simply requires more work to compile. I did a set of nofib runs with and without the first TTG patch and found that compiler allocations remained essentially unchanged.

A 15% regression in the compilation time of GHC is indeed hard to stomach but Alan had said that much of this will likely disappear in the future. If this is the case then a temporary regression is in my opinion acceptable.

Cheers,

- Ben