Compiling with -O2 is 7x slower than -O

[simonpj]

Serge writes that compiling DoCon with -O2 -fvia-C "does not gain more performance, but leads to 7 times longer compilation". That seems a big slow down, worth looking into.

The offending DoCon bundle is attached

[simonpj]

DoCon? 2.12 pre-release (for testing purposes)

[simonpj]

Ah, this seems to be to do with -fviaC not to do with -O2. Serge writes:

With -O2, the compile time is ok, but when I add -fvia-C, it takes 7 times longer -- this is for ghc-6.9-earlySeptember to compile DoCon-2.12-pre.

You do not control over the speed of gcc itself. But a small change in the GHC compiler may, in principle, lead to many times faster compilation by gcc of the code produced by GHC. This may occur some problem in GHC as well as some problem in gcc.

This is only my observation, I do not pretend for a bug report in this case. I tried -fvia-C for curiosity: "by occasion, would it increase the code performance?". If this ratio of 7 looks curious to you, you could investigate this.

And personally, I am satisfied with -O (without via-C).

[igloo]

-fvia-C is now deprecated.