Should ghc -msse imply gcc -msse

Johan Tibell johan.tibell at
Thu Jan 17 21:06:10 CET 2013

On Thu, Jan 17, 2013 at 12:01 PM, Johan Tibell <johan.tibell at> wrote:
> I forgot I once raised this on the GHC bug tracker:
> Here's what Simon M had to say back then:
> "The right thing is to put -msse in the cc-options field of your
> .cabal file, if that's what you want.
> I'm distinctly uneasy about having -msse magically pass through to gcc.
> * There are many flags that we do not pass through to gcc, so having
> one that we do pass through could be confusing (and lead to lots more
> requests for more flags to be passed through)
> * What if there is a variant of -msse that gcc supports but we don't?
> Wouldn't we have to keep them in sync?
> I'm going to close this as wontfix, but please feel free to reopen and
> disagree."

One problem with having the user set cc-options in addition to passing
-msse to GHC, is that the user might not realize that he/she needs to
do this. This is bad if you use -fllvm, as your -msse will essentially
just be ignored as the LLVM primitives we use in the LLVM backend
(e.g. for popcnt) won't convert to SSE instructions.

Even worse, LLVM doesn't support a -msse flag, instead you need to use
-mattr=+sse, so the user needs to be aware of this difference and
change his/her flags depending on if we use the native backend or the
LLVM backend.

-- Johan

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