[Vm-dev] Slang inliner effectiveness

Stefan Marr squeak at stefan-marr.de
Sun Jul 31 14:38:39 UTC 2011

Hi Dave:

On 31/07/11 15:35, David T. Lewis wrote:
> 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.
Interesting. Just for completeness:
When the inlineing isn't done, the generated C functions have compiler 
hints like `inline` and `__attribute__ ((always_inline))`, and are 
generated into the same compilation unit? I guess the last one is true 
since the interp.c is probably the most relevant thing here.

Any guess what the reason could be why the C compiler fails to do proper 

> Dave

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