Embedding Assembly into C

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UniverseIsASimulation

Joined: 23 Sep 2016
Posts: 32

UniverseIsASimulation 25 Aug 2019, 15:03

Hey, guys!
So, I've studied a bit how you can embed Assembly into C, and I've posted an example of how to do that on my website. Do you think what I wrote there is clear and accurate? How can I improve that example?

25 Aug 2019, 15:03

st

Joined: 12 Jul 2019
Posts: 49
Location: Russia

st 26 Aug 2019, 07:10

Thanks, I did not inline asm in GCC in the past, so find out some new things in your example (had to copy-paste its text into an editor with syntax highlighting).

However I am sure, this simplification:

Code:

/*Declare variables used in Assembly as global, so that you don't
  have to deal with both the FPU stack and the system stack
  (of the "esp" or "rsp" registers).
*/

results in:

Code:

#else /*For Windows, you need to write [_result] instead of [result],
and similarly for other variable names (it has to do with the way linker makes EXE files).*/

Therefore all this code is written twice.

I believe GCC features something to access stack variables from asm (and may be to reserve some registers). Hope the Universe will give us a hint. Smile

26 Aug 2019, 07:10

UniverseIsASimulation

Joined: 23 Sep 2016
Posts: 32

UniverseIsASimulation 27 Aug 2019, 06:57

Quote:

I believe GCC features something to access stack variables from asm

Yes, but that, as far as I know, only works if you use AT&T syntax.

27 Aug 2019, 06:57

Furs

Joined: 04 Mar 2016
Posts: 2022

Furs 27 Aug 2019, 15:46

IMO you should either write the entire function in asm, or use inline asm with operands. This is very powerful, because you tell the compiler what are the inputs, outputs, clobbers, jump targets, etc.

When it knows such information, it can produce much better code, and you don't have to deal with global variables. You can specify an operand to reside either in memory or register or whatever else, and GCC will choose based on surrounding code what it thinks it's best, etc.

Raw inline asm by contrast produces sub-optimal code surrounding your asm.

Only thing is that it takes a while to learn it properly. But I love it. Smile

27 Aug 2019, 15:46

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