[Vm-dev] Slang inliner effectiveness (was: [Cog] Strange inliner behavior)

[David T. Lewis]

On Sun, Jul 31, 2011 at 12:59:21PM +0200, Stefan Marr wrote:
> 
> Just out of curiosity, in which kind of use cases is the inline-behavior 
> of the used C compiler not sufficient to rely on, instead of manually 
> inline such C code?
> 
> Especially, since people like Mike Pall of the LuaJIT2 claim that GCC 
> with -O3 inlines to aggressively anyway which leads to code bloat that 
> does not fit into typical CPU instruction caches and thus slows things down.
> But since that is just 3rd-hand knowledge, I would like to hear about 
> real experiences.

The slang inliner is amazingly effective. The most obvious use case
is of course the interpreter itself. Try turning off the slang inlining,
apply all the GCC optimization you want, and you will end up with a
painfully slow VM.

As a second use case, which to me was even more convincing, consider
the memory access macros in sqMemoryAccess.h. These are written to be
as efficient as possible for speed. Then look at the slang code in
the MemoryAccess package on SqueakSource/VMMaker. This is a slang
replacement for the memory access macros. When this package is used,
the macros are not used at all, and the memory access code is all
Smalltalk down to the lowest possible level.

I found that using the slang memory access methods, which are fully
inlined by the slang inliner, results in a VM with performance
identical to that of the VM with C macros (to the best of my ability
to measure it with #tinyBenchmarks). I was extremely surprised by this
result, and it tells me that the slang inliner is really very effective
indeed.

Dave