[cfe-dev] Controlling the LTO optimization level

[Bob Wilson]

> On Mar 18, 2015, at 4:27 PM, Peter Collingbourne <peter at pcc.me.uk> wrote:
> 
> Hi all,
> 
> I wanted to start a thread to discuss ways to control the optimization
> level when using LTO. We have found that there are use cases for the LTO
> mechanism beyond whole-program optimization, in which full optimization
> is not always needed or desired. We started that discussion over in
> http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20150316/266560.html
> and I thought I'd summarize the problem and possible solutions here:
> 
> Problem
> -------
> 
> As currently implemented, the control flow integrity checks in Clang rely on
> a so-called bit set lowering pass to implement its checks efficiently. The
> current implementation of the bit set lowering pass requires whole-program
> visibility. The full details of why are described in the design document at:
> http://clang.llvm.org/docs/ControlFlowIntegrityDesign.html
> 
> We currently achieve whole-program visibility using LTO. The trouble with LTO
> is that it comes with a significant compile time cost -- on large programs
> such as Chrome, compiling with link-time optimization can be over 7x slower
> (over 3 hours has been measured) than compiling without.

We’ve had some recent improvements that speed things up considerably, and hopefully things will continue to get faster, but I’m sure there will always be cases where LTO is slower.

> 
> So I would like there to be a way for users to choose whether to apply
> optimizations, and how much optimization to apply.
> 
> Achieving this requires a design for how users should specify the level of
> optimization to apply, as well as a design for changes to the clang driver
> and the various LTO plugins so that the plugin knows whether optimizations
> are required.
> 
> Solutions
> ---------
> 
> 1) Controlled at compile time
> 
> Strawman proposal for command line syntax:
> 
> -flto-level=X means optimize at level X. At link time, the LTO plugin will
> take the maximum of all -flto-level flags and optimize at that level.
> 
> -flto-level is inferred from other flags if not specified:
> 
> -flto implies -flto-level=2.
> If -flto not specified, -O >= 1 implies -flto-level=1.
> Otherwise, default to -flto-level=0.
> 
> This is probably easier to implement in a supported way. We can pass the
> LTO level to the linker via module flags as shown in the patches attached to
> http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20150316/266778.html
> 
> 2) Controlled at link time
> 
> -flto-level has the same semantics as in the previous sub-section, except it is
> instead passed at link time.
> 
> This is to a certain extent possible to implement with libLTO by passing
> -mllvm flags to the linker, or with gold by passing -plugin-opt flags.
> 
> According to Duncan, passing flags to libLTO this way is unsupported --
> if we did want to accept flags at link time, and we absolutely don't want
> to pass flags to the linker that way, I suppose we could do something like
> have the clang driver synthesize a module containing the module flags we want.

Option (2) makes more sense to me, but I don’t like the idea of introducing a new command line option. At least for now, this seems like a fairly special-purpose request for CFI. I haven’t heard anyone else asking for LTO with minimal optimization. How about if you just pass the “-mllvm” options yourself when using CFI?

If it turns out that there are lots of people who want this feature, I could imagine that we might someday repurpose the existing -O optimization options to pass something to the linker to control LTO optimization. The downside of that is the clang driver doesn’t know whether the link will involve LTO or not, so it would have to pass those flags to the linker all the time. That’s not a real problem, but it’s just extra complexity that doesn’t seem justified unless it benefits more people.

> Optimization Levels
> -------------------
> 
> We need to decide what the various optimization levels mean. The thing that
> works best for the CFI use case is for -flto-level=2 to mean what -flto
> currently means, for -flto-level=1 to mean "run only the globaldce and
> simplifycfg passes", and for -flto-level=0 to mean "run no passes", but this
> may not be the correct thing to do in every situation where we only want a
> few passes to run at link time. We may want to make -flto-level a cc1-level
> flag until we've had more experience and found more use cases.
> 
> Thanks,
> -- 
> Peter
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