wiki:Commentary/Compiler/Backends/NCG/RegisterAllocator

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The Register Allocator

Overview

The register allocator is responsible for assigning real/hardware regs (hregs) to each of the virtual regs (vregs) present in the code emitted by the native code generator. It also inserts spill/reload instructions to save vregs to the stack in situations where not enough hregs are available.

GHC currently provides three register allocation algorithms, one which does simple lineear scan and two version of graph coloring. Support for linear scan is likely to be removed in a subequent version.

  • Linear scan
    The linear allocator is turned on by default. This is what you get when you compile with -fasm. The linear allocator does a single pass through the code, allocating registers on a first-come-first-served basis. It is quick, and does a reasonable job for code with little register pressure.

This algorithm has no look-ahead. If say, a particular hreg will be clobbered by a function call, it does not know to avoid allocating to it in the code before the call, and subsequently inserts more spill/reload instructions than strictly needed.

  • Graph coloring (enabled with -fregs-graph)
    The graph coloring algorithm operates on the code for a whole function at a time. From each function it extracts a register conflict graph which has a node for every vreg and an edge between two vregs if they are in use at the same time and thus cannot share the same hreg. The algorithm tries to assign hregs (represented as colors) to the nodes so that no two adjacent nodes share the same color, if it can't then it inserts spill code, rebuilds the graph and tries again.

Graph coloring tends to do better than the linear allocator because the conflict graph helps it avoid the look-ahead problem. The coloring allocator also tries harder to allocate the source and destination of reg-to-reg move instructions to the same hreg. This is done by coalescing (merging) move-related nodes. If this succeeds then the moves can be erased.

  • Graph coloring with iterative coalescing (enabled with -fregs-iterative)
    Iterative coalescing is an improvement over regular graph coloring whereby coalescing passes are interleaved with coloring passes. Iterative coalescing does a better job than regular graph coloring, but is slower because it must alternate between the coloring and coalescing of nodes.

Code map

For an outline of the code see Commentary/Compiler/Backends/NCG/RegisterAllocator/Code

References

If you decide to do some hacking on the register allocator then take a look at (at least) these papers first:

Iterated Register Coalescing
George, Appel, 1996
Decribes the core graph coloring algorithm used.

A Generalised Algorithm for Graph-Coloring Register Allocation
Smith, Ramsey, Holloway, 2004
For a decription of how to deal with overlapping register sets, which aren't fully implemented yet. Explains what the worst, squeese and triv functions are for.

Design and Implementation of a Graph Coloring Register Allocator for GCC
Matz, 2003
For an overview of techniques for inserting spill code.

Hacking/Debugging?

  • Turn on -fasm-lint
    Breaking the allocator can result in compiled programs crashing randomly (if you're lucky) or producing the wrong output. Make sure to always turn on -fasm-lint. Doing this makes the allocator call GraphOps.validateGraph after every spill/color stage. validateGraph checks that all the edges point to valid nodes, that no conflicting nodes have the same color, and if the graph is supposed to be colored then all nodes are really colored.
  • Some useful dump flags

-ddump-asm-regalloc-stages
Shows the code and conflict graph after ever spill/color stage. Also shows spill costs, and what registers were coalesced.

-ddump-asm-stats
Gives statistics about how many spills/reloads/reg-reg-moves are in the output program.

-ddump-asm
Gives the final output code.

-ddump-to-file
Diverts dump output to files. This can be used to get dumps from each module in a nofib benchmark.

cd nofib/real/anna
make EXTRA_HC_OPTS="-O2 -fregs-iterative -ddump-to-file -ddump-asm-regalloc-stages"
  • Graphviz
  • checkSpills

Register pressure in Haskell code

Present GHC compiled code places very little pressure on the register set. Even on x86 with only 3 allocable registers, most modules do not need spill/reloads. This is a mixed blessing - on one hand the conflict graphs are small so we can avoid performance problems related to how the graph is represented, on the other hand it can be hard to find code to test against. Register pressure is expected to increase as the Stg->Cmm transform improves.

In the meantime, here are some good sources for test code:

  • Nofib
    Only a few nofib benchmarks create spills with -O2, two are spectral/hartel/genfft and spectral/sorting.
  • Turn on profiling.
    Register pressure increases significantly when the module is compiled with profiling. checkSpills.report gives tuples of (spills, reloads, reg-reg-moves) present in output code generated by the three algorithms when compiled with -O2 -prof. Left to right are the stats for the linear, graph coloring and iterative coalescing algorithms. Note that most modules compile with no spill/reloads inserted, but a few (notably real/compress2/Encode) need several hundred.

I've found it useful to maintain three darcs repos when working on the allocator. ghc-HEAD-work compiled with -Onot for fast compilation during hacking, ghc-HEAD-prof for testing with profiling turned on, and ghc-HEAD-validate for running the validate script. Patches are created in work, pushed into prof where checkSpills is used to compile the nofib benchmarks with the most register pressure. Once we're happy that the performance is ok, the patch is then pushed into validate for validation before pushing to the main repo on darcs.haskell.org

  • SHA from darcs
    The SHA1.lhs module from the darcs source, compiled with -O2 creates the most register pressure out of any Haskell code that I'm aware of. When compiling SHA1, GHC inlines several worker functions and the native code block that computes the hash ends up being around 1700 instructions long. vregs that live in the middle of the block have in the order of 30 conflict neighbors. (evidently, the conflict graph is too large for most of the graphviz layout algorithms to cope with)

For these reasons, SHA1.lhs can be treated as a good worst-case input to the allocator. In fact, the current linear allocator cannot compile it with -O2 -prof on x86 as it runs out of stack slots, which are allocated from a static pool. Make sure to test any changes to the allocator against this module.

Possible Improvements

These are some ideas for improving the current allocator, most usful first.

  • Work lists for iterative coalescing.
  • Improve spill code cleaner.
  • Revisit choosing of spill candidates.
  • Revisit trivColorable / aliasing of register sets.

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