Confusing warnings from GHC HEAD

Simon Peyton-Jones simonpj at
Wed Sep 1 06:59:37 EDT 2010

It's a bit of a moveable feast.  With -fno-spec-constr-count you allow GHC to specialise as much as it pleases, which is good for perf (maybe) but can lead to major code size expansion.


From: Johan Tibell [mailto:johan.tibell at]
Sent: 31 August 2010 10:16
To: Simon Peyton-Jones
Cc: Bryan O'Sullivan; glasgow-haskell-users at
Subject: Re: Confusing warnings from GHC HEAD

On Tue, Aug 31, 2010 at 11:06 AM, Simon Peyton-Jones <simonpj at<mailto:simonpj at>> wrote:
I can't reproduce this.  With the enclosed module and HEAD, I get the warning; but when I add -fspec-constr-count=5, the warning goes away and I get the specialised rules.

Is this the right fix in general? I try to keep my code warning free but I don't want to bump the count unless it's always a win. If it's not always a win perhaps it should be a warning that's on by default?

-- Johan

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