Optimizing used registers when using inline ARM assembly in GCC

Question

I want to write some inline ARM assembly in my C code. For this code, I need to use a register or two more than just the ones declared as inputs and outputs to the function. I know how to use the clobber list to tell GCC that I will be using some extra registers to do my computation.

However, I am sure that GCC enjoys the freedom to shuffle around which registers are used for what when optimizing. That is, I get the feeling it is a bad idea to use a fixed register for my computations.

What is the best way to use some extra register that is neither input nor output of my inline assembly, without using a fixed register?

P.S. I was thinking that using a dummy output variable might do the trick, but I'm not sure what kind of weird other effects that will have...

I'd bet this is possible quite directly, but I'm going to let a C guru answer this. :-) — Noldorin
I know that you can use register aliases and let the compiler choose which registers it wants to use, but can't remember the syntax for that. Or you could bite the bullet and just write a .s file ans assemble it separately for more flexibility. — Michael Dorgan
@MichaelDorgan Thanks for your input. I know the input and output registers work like that, but I haven't seen it used for temporary registers.. Your second solution there kind of does the reverse of what I need, it removes even more optimization options (the designation of input and output registers)... — Madcowswe
If you write your own .s file, you handle your own optimizations. You control the pushing and popping and what gets used where, just have to make sure you follow the ARM rules for register preservation. — Michael Dorgan
You mean if I write the whole program in assembly? Hehe that would definitely be a last resort ;D — Madcowswe

Madcowswe Madcowswe · Accepted Answer · 2012-01-04T11:24:13

Ok, I've found a source that backs up the idea of using dummy outputs instead of hard registers:

4.8 Temporary registers: People also sometimes erroneously use clobbers for temporary registers. The right way is to make up a dummy output, and use “=r” or “=&r” depending on the permitted overlap with the inputs. GCC allocates a register for the dummy value. The difference is that GCC can pick a convenient register, so it has more ﬂexibility.

from page 20 of this pdf.

For anyone who is interested in more info on inline assembly with GCC this website turned out to be very instructive.

Optimizing used registers when using inline ARM assembly in GCC

1 Answers