-fprof-auto adds cost centers to INLINE functions

[akio]

According to the Section 5.2 of User's Guide, -fprof-auto should add cost centers only to functions that are not marked INLINE. However GHC 7.4 doesn't respect this.

A simple example:

foo :: Integer -> Integer
foo x = x + 100
{-# INLINE foo #-}

main = print $ loop (10000000::Int) 10000000000000000000000
    where
        loop 0 x = x
        loop n x = loop (n-1) (foo x)

Compile and run this like:

% ghc -prof -fprof-auto -O2 test.hs
% ./test +RTS -p
% grep foo test.prof

And you will see 'foo' is listed as a cost center.

This happens with GHC 7.4.1 and 7.4.2-rc1

[simonmar]

This is indeed a bug, however we are inclined to fix the documentation rather than the implementation, i.e. functions with an INLINE pragma will also get cost centres. Would this be a problem for you? What is the reason that you didn't want a cost centre on foo?

[akio]

I used to use an INLINE pragma as a handy way to prevent a cost center from being added to a function. Most of the time I wanted this feature because adding a cost center to a frequently-called small function tends to distort profiling results by adding a significant cost to that function, including costs of lost optimizations.

[simonmar]

Seems reasonable. I'll look into fixing it.

[igloo]

Would a NO_SCC pragma also make sense?

[simonmar]

Yes, NO_SCC would make sense. However, it turns out that adding SCCs to INLINE functions exposes another bug elsewhere that is harder to fix, so I'm going to do as originally suggested and not add SCCs to INLINE functions.

[marlowsd@…]

commit 6181e007f0e1e8eddba7acf0d5fbcbaf46806249

Author: Simon Marlow <marlowsd@gmail.com>
Date:   Fri Jun 15 10:30:35 2012 +0100

    Don't put auto sccs on INLINE functions (#6131)
    
    There was also a bug caused by INLINEs getting SCCs, but unfortunately
    I have lost the test case.  The Note in the code describes the problem
    though.

 compiler/deSugar/Coverage.lhs |   36 +++++++++++++++++++++++++++++++++---
 1 files changed, 33 insertions(+), 3 deletions(-)

[pcapriotti]

Merged as 87ef70b838f43c6425bea97c9a791f2d0bf35e5c.