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Re: bug introduced by inlining patch

Hi Dale,

On Wed, Sep 04, 2002 at 10:18:05AM -0700, Dale Johannesen wrote:
> On Wednesday, September 4, 2002, at 12:11 AM, Kurt Garloff wrote:
> The main trunk at  If there's another version
> somewhere that has these things fixed, it needs to be applied here,
> I think.

Well, in my patch collection at
there are patches for documentation included.
But only in the newer patches, and I'm afriad an older one went into 

> I haven't tried it, but it looks like this change
> > read_integral_parameter (option_value, arg - 2,
> >
> > set_param_value ("max-inline-insns", val);
> > set_param_value ("max-inline-insns-single", val/2);
> alters the meaning of -finline-limit from what it was previously (i.e.
> the size limit for an individual function is half what it was).  I think
> that needs some discussion, at a minimum.

In 2.95, it was defaulting to 10000, which means that no real-world 
function was prevented from being inlined. Which could cause excessive
memory and CPU consumption for compiling fairly easy code in C++.
The inliner in 2.95 did not account for recursive inlining.

3.0 had the tree inliner and was accounting for recursive inlining.
It limited your total inlining budget to 1000 statements. (The parameter
was still at 10000, but one statement was assumed to be aequivalent to 
10 RTL insns.) This resulted in extreme compile resource requirements
even for less pathological C++ code, just because the tree inliner has 
less  restriction than the old RTL one. The performance was still worse 
than 2.95.
In 3.0.1 this was changed to 600 (i.e. 60 statements) after 100 (10st) 
showed desastrous performance. With 600, compile times were OK, but 
runtime performance was poor.

Only then, the problem was identified
to be caused by the wrong functions being inlined, as we allow single
functions to eat up the complete inlining budget, which calls for wrong
inlining decisions, of course. A little patch solved it.

Since that moment, the notion of a different single function inlining 
limit as compared to the recursive inlining limit exists and I chose
half the complete by default. -finline-limit in 3.0 meant the complete
inlining budget which was identical to the single function inline size
and it could only keep one meaning after the patch was applied obviously.
The limit of 600 in 3.0.1 worked very well when used as complete budget 
with a single fn limit of 300, so I took that decision.

The patch was improved a lot afterwards by applying some smooth function
that restricted inlining as a smooth decreasing function of the already
inlined code, so more parameters were possible. I did lots of bench-
marking, but the improvement on the first patch was not as huge as the
first patch itself.

Anyway, I was expecting a lot of discussion when posting my patches on 
the gcc list, but I got less reactions than expected. Maybe gcc folks
were not so interested in C++ performance or were just happy with the

> (If anybody's lost, the patch was discussed in Aug 01 and later in
> Apr/May 02; most of the discussion was on gcc, not gcc-patches.
> The discussion does mention documentation changes but they don't
> seem to have been checked in.

That's a pity.

> I'm having a little trouble tracking
> this; Kurt Garloff isn't in MAINTAINERS, so I wonder if somebody
> checked this in for him?)

No, I'm not a gcc MAINTAINER. I lack the time to concentrate on gcc work
and I think there are others who are more qualified than me.

I believe the first version of the patch got checked in by Gerald. 
I don't know what other versions went in later and who did the check ins.
Maybe nobody told me, maybe I don't remember.

But, if this discussion is starting up again, I would like to propose
my patch v4 (from the above webpage) for inclusion.
I would appreciate a lot to get (positive and negative) criticism on it. 

If somebody would like to have the patches rediffed against current
CVS, I can provide those.

PS: Dale, if you're seriously starting to look at the inlining code
 and you happen to have a bit of time, please go ahead and improve 
 the heuristics. One of the most obvious optimizations is keeping track
 of the level (and the estimated iteration count) of loops we're called
 from. Take it into account when taking an inlining decision. It's easy
 yet very efficient.
 Next improvement would be to calculate the complete call graph and then
 take decisions on where to cut it based on inlining limits, loop depth
 and function size. There you would probably start with inlining from
 the leaf functions not the trunk. This is slightly more work, obviously.
 I guess the importance of inlinig heuristics has been underestimated
 previously. Maybe due to the fact that people were focused on C which
 does not make such heavy use of inlining as C++ (and other OOP lan-

Best regards,
Kurt Garloff  <>                          Eindhoven, NL
GPG key: See mail header, key servers         Linux kernel development
SuSE Linux AG, Nuernberg, DE                            SCSI, Security

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