[cfe-dev] Proposal: Enable BUILD_SHARED_LIBS=ON by default for debug build.

[Yin Ma via cfe-dev]

Hi David,

 

I assume you have a powerful machine. Our drive space is on network mounted 

machined by IT department. The machine is default Ubuntu setup with 8 cores. 

Shared build is showing huge advantage due to size reduction. I will try those two 

debug options to see how it works. 

 

Thanks,

 

Yin

 

From: David Blaikie [mailto:dblaikie at gmail.com] 
Sent: Thursday, January 28, 2016 2:43 PM
To: Yin Ma
Cc: llvm-dev; cfe-dev
Subject: Re: [cfe-dev] Proposal: Enable BUILD_SHARED_LIBS=ON by default for debug build.

 

 

 

On Thu, Jan 28, 2016 at 2:34 PM, Yin Ma via cfe-dev <cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org> > wrote:

Hello, 

 

I understand shared linked is currently a lot slower than static linked.

However, for debug build, where speed is not required. Shared link has 

A lot of advantage. 

 

Static linked clang/llvm on linux is 17G, shared build is only 1.5G.

 

Assuming you're talking about a debug build, you can probably get that down a lot with at least -fdebug-types-section or with -gsplit-dwarf.

 

On a mainstream 

linux host machine with 12G memory, only –j1 can be used to finish final linking step.

 

If you use Ninja, I believe it has a separate pool limit you can tweak for linking (but, again, with -fdebug-types-section or -gsplit-dwarf, you might make the link steps more palatable (-gsplit-dwarf is probably the better/more important choice, though))

But I'd be surprised if the limit is that low - are you using gold? Or binutils ld? Gold is a good/better choice here, and usually I only end up using about a GB of RAM per linking process I Think.

 

With –j8 the linux will link the linking. With 17G footprint, copying from build folder

to install folder is also very slow. 

 

Solid state, on spinning disks? Solid state would help a bit.

 

So it is very reasonable to set 

BUILD_SHARED_LIBS=ON as default for debug build because it would improve usability

and performance.


The thing is, the performance tradeoff also hits test execution time. At least last time I checked (granted, on a 32 core, 64GB of RAM machine) the build time improvement (on a clean build) with shared libs was not saved by a single execution of the regression test suite (which took longer than the savings). Let alone on an incremental build... if I remember my numbers correctly.

Not sure whether it's the right call to change the default - people can change their own settings easily enough.

- Dave

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