[llvm-dev] Proposal for O1/Og Optimization and Code Generation Pipeline

[Finkel, Hal J. via llvm-dev]

On 3/28/19 9:09 PM, Eric Christopher via llvm-dev wrote:
> Hi All,
>
> I’ve been thinking about both O1 and Og optimization levels and have a
> proposal for an improved O1 that I think overlaps in functionality
> with our desires for Og. The design goal is to rewrite the O1
> optimization and code generation pipeline to include the set of
> optimizations that minimizes build and test time while retaining our
> ability to debug.


+1

The fact that our O1 and O2 pipelines are so similar has generally
suggested to me that our O1 pipeline probably does too much.


>
> This isn’t to minimize efforts around optimized debugging or negate O0
> builds, but rather to provide a compromise mode that encompasses some
> of the benefits of both. In effect to create a “build mode for
> everyday development”.
>
> This proposal is a first approximation guess on direction. I’ll be
> exploring different options and combinations, but I think this is a
> good place to start for discussion. Unless there are serious
> objections to the general direction I’d like to get started so we can
> explore and look at the code as it comes through review.
>
>
> Optimization and Code Generation Pipeline
>
> The optimization passes chosen fall into a few main categories,
> redundancy elimination and basic optimization/abstraction elimination.
> The idea is that these are going to be the optimizations that a
> programmer would expect to happen without affecting debugging. This
> means not eliminating redundant calls or non-redundant loads as those
> could fail in different ways and locations while executing.  These
> optimizations will also reduce the overall amount of code going to the
> code generator helping both linker input size and code generation
> speed.
>
> Dead code elimination
>
>  - Dead code elimination (ADCE, BDCE)


Regarding BDCE: The trivialized values might indeed be irrelevant to
later calculations, but might harm the debugging experience? If BDCE
only was applied at O2 and higher, that's likely not a huge loss.
Regular DCE (meaning without the bit-tracking parts) is probably fine
for O1.


>  - Dead store elimination
>  - Parts of CFG Simplification
>  - Removing branches and dead code paths and not including commoning
> and speculation
>
> Basic Scalar Optimizations
>
>  - Constant propagation including SCCP and IPCP
>  - Constant merging
>  - Instruction Combining
>  - Inlining: always_inline and normal inlining passes
>  - Memory to register promotion
>  - CSE of “unobservable” operations
>  - Reassociation of expressions
>  - Global optimizations - try to fold globals to constants
>
> Loop Optimizations
>
> Loop optimizations have some problems around debuggability and
> observability, but a suggested set of passes would include
> optimizations that remove abstractions and not ones that necessarily
> optimize for performance.
>
>  - Induction Variable Simplification
>  - LICM but not promotion
>  - Trivial Unswitching
>  - Loop rotation
>  - Full loop unrolling
>  - Loop deletion
>
> Pass Structure
>
> Overall pass ordering will look similar to the existing pass layout in
> llvm with passes added or subtracted for O1 rather than a new pass
> ordering. The motivation here is to make the overall proposal easier
> to understand initially upstream while also maintaining existing pass
> pipeline synergies between passes.
>
> Instruction selection
>
> We will use the fast instruction selector (where it exists) for three reasons:
>  - Significantly faster code generation than llvm’s dag based
> instruction selection
>  - Better debugability than selection dag - fewer instructions moved around
>  - Fast instruction selection has been optimized somewhat and
> shouldn’t be an outrageous penalty on most architectures
>
> Register allocation
>
> The fast register allocator should be used for compilation speed.


I'm not sure about this - we should understand the performance impact -
it might be severe.

Thanks again,

Hal


>
> Thoughts?
>
> Thanks!
>
> -eric
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-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory