Re: Whole program optimization and functions-only-called-once.

[Richard Guenther <richard dot guenther at gmail dot com>]

On Wed, Nov 4, 2009 at 8:19 PM, Toon Moene <toon@moene.org> wrote:
> Jan,
>
> I had some time to study the example I sent you a couple of weeks ago.
>
> According to visible inspection of the source code, there are 5 functions
> (subroutines in Fortran parlance) that are called once:
>
> MAIN ? calls
> HLPROG calls
> GEMINI calls
> SL2TIM calls
> PHCALL calls
> PHTASK
>
> I.e., the last five should be candidates for inlining of "functions only
> called once".
>
> However, ccrPOljB.o.047i.inline says:
>
> Deciding on functions called once:
>
> Considering gemini_.clone.1 size 11443.
> ?Called once from hlprog 462 insns.
> ?Inlined into hlprog which now has 10728 size for a net change of -12620
> size.
>
> Considering hlprog size 10728.
> ?Called once from main 7 insns.
> ?Not inlined because --param large-function-growth limit reached.
>
> Inlined 1 calls, eliminated 1 functions, size 45477 turned to 32857 size.
>
> The dump option -fdump-ipa-all also gives me the call graph, of which I copy
> here the relevant part:
>
> phcall_.clone.3/11(-1) @0x7fd198c16400 (clone of phcall/33)
> availability:local 8281 time, 972 benefit 1351 size, 291 benefit 984 bytes
> stack usage reachable local finalized inlinable
> ?called by: sl2tim/49 (0.44 per call) sl2tim_.clone.0/16 (0.44 per call)
> phtask_.clone.2/12(-1) @0x7fd198c16500 (clone of phtask/41)
> availability:local 26416 time, 4268 benefit 4541 size, 880 benefit 480 bytes
> stack usage reachable local finalized inlinable
> ?called by: phcall_.clone.3/11 (3.52 per call) phcall/33 (3.52 per call)
> sl2tim_.clone.0/16(-1) @0x7fd198c16900 (clone of sl2tim/49)
> availability:local 207312 time, 26617 benefit 5169 size, 941 benefit 3856
> bytes stack usage reachable local finalized inlinable
> ?called by: gemini_.clone.1/40 (1.00 per call) gemini/0 (1.00 per call)
> gemini_.clone.1phtask/40(-1) @0x7fd198c35000 (inline copy in hlprog/17)
> (clone of gemini/0) availability:local 147324 time, 2770 benefit 11443 size,
> 1177 benefit 11635 bytes stack usage reachable local finalized inlinable
> ?called by: hlprog/17 (3.57 per call) (inlined)
> phtask/41(-1) @0x7fd198c35100 availability:local 26416 time, 4268 benefit
> 4541 size, 880 benefit 480 bytes stack usage reachable body local finalized
> inlinable
> ?called by:
> phcall/33(-1) @0x7fd198c33a00 availability:local 8281 time, 972 benefit 1351
> size, 291 benefit 984 bytes stack usage reachable body local finalized
> inlinable
> ?called by:
> hlprog/17(-1) @0x7fd198c16a00 availability:local 560 time, 10 benefit
> (516762 after inlining) 462 size, 1 benefit (10728 after inlining) 4216
> bytes stack usage 15851 bytes after inlining reachable body local finalized
> inlinable
> ?called by: main/29 (1.00 per call)
> sl2tim/49(-1) @0x7fd198c35900 availability:local 207312 time, 26617 benefit
> 5169 size, 941 benefit 3856 bytes stack usage reachable body local finalized
> inlinable
> ?called by:
> gemini/0(-1) @0x7fd198bef800 availability:local 147324 time, 2770 benefit
> 11443 size, 1177 benefit 11635 bytes stack usage reachable body local
> finalized inlinable
> ?called by:
>
> So if we have to believe this summary,
>
> HLPROG is called by MAIN, but is not suitable for inlining (I can live with
> that).
> GEMINI is not called, but GEMINI.clone is (by HLPROG) and is inlined.
> SL2TIM is not called, but SL2TIM.clone is called by GEMINI and GEMINI.clone;
> because it is called twice, it is not considered a
> function-only-called-once.
> PHCALL is not called, but PHCALL.clone is called by SL2TIM and SL2TIM.clone;
> because it is called twice, it is not considered a
> function-only-called-once.
> PHTASK is not called, but PHTASK.clone is called by PHCALL and PHCALL.clone;
> because it is called twice, it is not considered a
> function-only-called-once.
>
> I don't think this is really what we want with functions-only-called-once:
> If only the .clone version of a function is used, than a function that's
> only called once *inside this clone* is a function-only-called-once.
>
> I hope this analysis helps,

I think the underlying issue is

phtask/41(-1) @0x7fd198c35100 availability:local 26416 time, 4268
benefit 4541 size, 880 benefit 480 bytes stack usage reachable body
local finalized inlinable
 called by:
phcall/33(-1) @0x7fd198c33a00 availability:local 8281 time, 972
benefit 1351 size, 291 benefit 984 bytes stack usage reachable body
local finalized inlinable
 called by:

that these are not called but still reachable (they should not be reachable
anymore, instead the clones are now reachable).  I think there already is
a bug about cloning not updating cgraph reachability and not reclaiming
nodes after IPA transform application.

Richard.

> --
> Toon Moene - e-mail: toon@moene.org - phone: +31 346 214290
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>