LTO and the inlining of functions only called once.

[Richard Guenther]

On Sat, Oct 10, 2009 at 12:55 PM, Toon Moene <toon@moene.org> wrote:
> Gcc's man page says:
>
>       -finline-functions-called-once
>           Consider all "static" functions called once for inlining into
>           their caller even if they are not marked "inline".  If a call
>           to a given function is integrated, then the function is not
>           output as assembler code in its own right.
>
>           Enabled at levels -O1, -O2, -O3 and -Os.
>
> Now, when using -flto -fwhole-program, *all* functions (that the user
> provided) will be "static inline", no ? - so *all* functions only called
> once in that program will be inlined ?

Well, I think that we should try to not do this across the whole program.
Simply for the reason that a gigantic main function will hit several
non-linear complexity algorithms in GCC.

> I am asking because our most important programs often consist of a chain of
> "routines-that-do-all-the-work" which are all only called once. However, in
> general they are (certainly when viewed from the perspective of a C
> programmer) *huge*.
>
> E.g., our forecast program:
>
>     HLPROG (small "main" program)
>        |
>        | calls
>        V
>     GEMINI (read input files, write output files, and:)
>        |
>        V
>     SL2TIM (time stepping, and:)
>        |
>        V
>     PHCALL (subgrid scale computations)
>        |
>        V
>     PHTASK (split them into tasks over model domain)
>        |
>        V
>     PHYS   (actually hand out the work to:)
>        |
>     ----------------------
>     |    |    |     |    |
>     LSP  CVP  RADIA TURB SOIL
>
>     (large scale precipitation, convective precipitation,
>      radiation, turbulence, soil processes)
>
>     The last five are each around 2,000 lines of Fortran, the P routines are
> each several hundreds of lines, as is SL2TIM.  GEMINI is more than 2,000
> lines.
>
> My question is: How can I be sure that all of them are integrated (note that
> the man page says they are "considered") ?  Does -Winline help here ?
>  Perhaps I should scan the assembler output (HAH!).

-Winline doesn't help here.  Scanning the assember output does (obviously!).

Note that I wouldn't expect such aggressive inlining to have any positive
performance impact - were it not for the fact that we still lack a properly
operating IPA points-to analysis pass (yes, it's on my todo list - but
not for 4.5).

Richard.