CMM-to-ASM: Register allocation wierdness

[Ben Gamari]

Harendra Kumar <harendra.kumar at gmail.com> writes:

> My earlier experiment was on GHC-7.10.3. I repeated this on GHC-8.0.1 and
> the assembly traced was exactly the same except for a marginal improvement.
> The 8.0.1 code generator removed the r14/r11 swap but the rest of the
> register ring shift remains the same. I have updated the github gist with
> the 8.0.1 trace:
>

Have you tried compiling with -fregs-graph [1] (the graph-coloring
allocator)?

By default GHC uses a very naive linear register allocator which I'd
imagine may produce these sorts of results. At some point there was an
effort to make -fregs-graph the default (see #2790) but it is
unfortunately quite slow despite having a relatively small impact on
produced-code quality in most cases. However, in your case it may be
worth enabling. Note, however, that the graph coloring allocator has a
few quirks of its own (see #8657 and #7697).

It actually came to my attention while researching this that the
-fregs-graph flag is currently silently ignored [2]. Unfortunately this
means you'll need to build a new compiler if you want to try using it.

Simon Marlow: If we really want to disable this option we should at very
least issue an error when the user requests it. However, really it seems
to me like we shouldn't disable it at all; why not just allow the user
to use it and add a note to the documentation stating that the graph
coloring allocator may fail with some programs and if it breaks the user
gets to keep both pieces?

All-in-all, the graph coloring allocator is in great need of some love;
Harendra, perhaps you'd like to have a try at dusting it off and perhaps
look into why it regresses in compiler performance? It would be great if
we could use it by default.

Cheers,

- Ben


[1] http://downloads.haskell.org/~ghc/master/users-guide//using-optimisation.html?highlight=register%20graph#ghc-flag--fregs-graph
[2] https://git.haskell.org/ghc.git/commitdiff/f0a7261a39bd1a8c5217fecba56c593c353f198c
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