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Question

If I want to achieve better performance from, let's say for example, MySQLdb, I can compile it myself and I will get better performance because it's not compiled on i386, i486 or what ever, just on my CPU. Further I can choose the compile options and so on... Now, I was wondering if this is true also for non-regular Software, such as compiler. Here come the 1st part:

• Will compiling a compiler like GCC result in better performance? and the 2nd part:
• Will the code compiled by my own compiled compiler perform better?

(Yes, I know, I can compile my compiler and benchmark it... but maybe ... someone already knows the answer, and will share it with us =)

Answer 1

In answer to your first question, almost certainly yes. Binary versions of gcc will be the "lowest common denominator" and, if you compile them with special flags more appropriate to your system, it will most likely be faster.

As to your second question, no.

The output of the compiler will be the same regardless of how you've optimised it (unless it's buggy, of course).

In other words, even if you totally stuffed up your compiler flags when compiling gcc , to the point where your particular compiled version of gcc takes a week and a half to compile "Hello World", the actual "Hello World" executable should be identical to the one produced by the "lowest common denominator" gcc (if you use the same flags).

Answer 2

(1) It is possible. If you introduce a new optimization to your compiler, and re-compile it with this optimization included - it is possible that the re-compiled code will perform better.

(2) No!!!! A compiler cannot change the logic of the code! In your case, the logic of the code is the native code produced at the end. So, if compiler A_1 is compiled using compiler A_2 or B, has no affect on the native code produced by A_1 [in here A_1, A_2 are the same compilers, the index is just for clarity].

Answer 3

a.Well, you can compile the compiler to your system, and maybe it will run faster. like any program. (I think that usualy it's not worth it, but do whatever you want).

b. No. Even if you compile the compiler in your computer, it's behavior should not change, and so the code that it generates also doesn't change.

Answer 4

Will compiling a compiler like GCC result in better performance?

A program compiled specifically to the target platform it is used on will usually perform better than a program compiled for a generic platform. Why is this? Knowledge about the harware can help the compiler align data to be cache friendly and choose an instruction ordering that plays well with a CPUs pipelining.

The most benefit is usally achieved by leveraging specific instruction sets such as SSE (in its various versions).

On the other hand, you should ask yourself if a programm like GCC is really CPU bound (much more likely it will be IO bound) and tuning its CPU performance provides any measurable benefit.

Will the code compiled by my own compiled compiler perform better

Hopefully not! Allowing a compiler to optimize a program should never change its behavior. No matter how you compiled your GCC, it should compile code to the same binaries as a generic binary distribution of GCC would.

If code compiled to the specific platform is faster than code compil for a generic platform, why dont we all ship code instead of binaries? Guess what, some linux distros actually follow this phillosophy, such as Gentoo. And while you're at it, make sure to built statically linked binaries, disk space is so cheap nowadays and it gives you at least another 0.001% of performance.

Alright, that was a bit sarcastic. The reason people distribute generic binaries is pretty obvious: It's geneirc, the lowest common denominator and it will work everywhere. Thats a big bonus in terms of flexibility and user friendlyness. I remember once compiling Gnome for my Gentoo box, it took a day or two! (But it must have been so much faster ;-) )

On the other hand, there are occassions where you want to get the best performance possible and it makes sense to build and optimize for specific architctures.

Answer 5

GCC uses a three step bootstraping when building from source. Basically it compiles the source three times to ensure build tools and compiler is build successfully. This bootstraping is used for validation purpose. However it is possible to use the stage 1 as a benchmark for optimizing later stages. You should build GCC with make profiledbootstrap to use this profile based optimization.

This profile based build process increases the performance of "GCC", but not the software compiled with it, as other answers point out.