How can inlining a function be slowing down this program?

[Andrew Haley]

On 06/05/11 00:18, Justin Peel wrote:
> I have been going through the bzip2 source code recently and
> discovered that when I forced a certain function, generateMTFValues
> found in compress.c, to not be inlined using
> __attribute__((noinline)), the code sped up 20% when doing
> compression. That function is only called from inside of one function
> - BZ2_compressBlock in the same source file. The function is the most
> CPU intensive of the entire program during compression of an average
> file. I compiled the code with gcc 4.5.2 using options -Wall -Winline
> -O2. I found the speed-up to happen when compressing both a 15MB and a
> 150MB file.
> 
> I was wondering if anyone could explain to me why inlining is slowing
> down the code in this case?

You have to read the code.

> I asked on Stack Overflow, but no one was fully sure. The guesses on
> there were that the inlining was making the code larger and thus
> causing more misses of the CPU's instruction cache, or that the
> optimizer wasn't able to use the registers as well when the function
> was inlined.
> 
> Also, has anyone else seen this? Does it happen very often?

It's not that unusual.  My first guess is that you get a better
register allocation when it's not inlined, but I'd have to see.  Are
there many local variables?

Andrew.