Concept of register variables(datatype:register) in C language?

Question

I just to want to get an idea about how the register variables are handled in C program executables. ie in which location(or register) it exactly get stored in case of an embedded system and in a X86 machine(C program executable in a desktop PC)?

What about this view? (correct me if am wrong)

Suppose we have declared/initialized one variable inside a function as 'int' datatype. Normally it will go to the stack segment and it will be there in that section only at run time ,when the caller calls the callee containing the local variable. But if we declare above local variable as 'register int' then also it'll go to the stack segment. But on run time , the processor put that local variable from stack to its general purpose register locations(because of extra compiler inserted code due to 'register' keyword) and a fast access of the same from there.

That is the only difference between them is at run time access and there is no memory loading differences between them.

__Kanu

if the compiler does choose to stick it into a register it'll probably pick the "best" one (presumably the one that will be used longest from now for something else) — TofuBeer, Sep 28 '10 at 19:18
meaning the compiler should know that register address exactly and also it should go to that register set while loading the executable right? — Renjith G, Sep 28 '10 at 19:22
from all the asm language I know (been a long time), yes the compiler generates that at compile time. From a programmer point of view you should not care about where it is though. — TofuBeer, Sep 28 '10 at 19:30
As per my understanding (Please correct me if am wrong) Compiler is dependent on the target architecture only right? — Renjith G, Sep 28 '10 at 19:47
Does this answer your question? ["register" keyword in C?](https://stackoverflow.com/questions/578202/register-keyword-in-c) — Sankalp, Aug 09 '21 at 04:27

score 10 · Accepted Answer · answered Sep 28 '10 at 19:13

10

The register keyword in C (rarely ever seen anymore) is only a hint to the compiler that it may be useful to keep a variable in a register for faster access.

The compiler is free to ignore the hint, and optimize as it sees best.

Since modern compilers are much better than humans at understanding usage and speed, the register keyword is usually ignored by modern compilers, and in some cases, may actually slow down execution speed.

answered Sep 28 '10 at 19:13

abelenky

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" that it may be useful to keep a variable in a register for faster access" to which register memory(general purpose registers/cache/scratch pad/RAM/ ?) it occupies? – Renjith G Sep 28 '10 at 19:17
3

Also note that if you declare a variable with the `register` keyword, you can't then take the address. – Brian Hooper Sep 28 '10 at 19:18
@Renjith: "register" refers ONLY to CPU registers (typically named AX, BX, ES, etc), and not cache, RAM, L1, L2, or anything else you referred to. We are talking only about the final, lowest level CPU registers. – abelenky Sep 28 '10 at 23:36
OK. fine. In the case of an X86 machine, what is the maximum number of register variable declaration in a c program allowed? – Renjith G Sep 29 '10 at 04:56
@Renjith: there is no limit. Register is only a *hint* to the compiler, not a directive. You can "suggest" that dozens or hundreds of variables be stored in registers. The compiler will be unable to accommodate all your requests, and just pick a few as it is able. – abelenky Sep 29 '10 at 06:35
Actually, you were right the first time: a modern compiler will ignore _all_ register requests and pick its own anyway. – MSalters Jan 11 '11 at 10:24

score 7 · Answer 2 · answered Sep 28 '10 at 19:21

From K&R C:

A register variable advises the compiler that the variable in question will be heavily used. The idea is that register variables are to be placed in machine registers, which may result in smaller & faster programs. But compilers are free to ignore this advice.

It is not possible to take the address of a register variable, regardless of whether the variable is actually placed in a register.

Hence,

register int x;
int *y = &x; // is illegal

So, you must weigh in the cons of not being able to get the address of the register variable.

score 4 · Answer 3 · answered Sep 28 '10 at 19:35

In addition to crypto's answer (that has my vote) just see the name register for the keyword as a historical misnomer. It has not much to do with registers as you learn it in class e.g for the von Neumann processor model, but is just a hint to the compiler that this variable doesn't need an address.

On modern machines an addressless variable can be realized by different means (e.g an immediate assembler operator) or optimized away completely. Tagging a variable as register can be a useful optimization hint for the compiler and also a useful discipline for the programmer.

+1: See also here for more info: http://gustedt.wordpress.com/2010/08/17/a-common-misconsception-the-register-keyword/ — Matt Joiner, Sep 29 '10 at 07:57

score 1 · Answer 4 · answered Sep 28 '10 at 20:37

When a compiler takes its internal code and the backend turns it into machine/assembler for the target processor, it keeps track of the registers it is generating instructions for as it creates the code. When it needs to allocate a register to load or keep track of a variable if there is an unused working variable then it marks it as used and generates the instructions using that register. But if all the working registers have something in them then it will usually evict the contents of one of those registers somewhere, often ram for example global memory or the stack if that variable had a home. The compiler may or may not be smart about that decision and may evict a variable that is highly used. By using the register keyword, depending on the compiler, you may be able to influence that decision, it may choose to keep the register keyword variables in registers and evict non-register keyword variables to memory as needed.

score 0 · Answer 5 · answered Sep 28 '10 at 20:01

which location(or register) it exactly get stored in case of an embedded system and in a > X86 machine(C program executable in a desktop PC)?

You don't know without opening up the assembly output, which will be liable to shift based on compiler choices. It's a good idea to check the assembly just for educational purposes.

If you need to read and write particular registers that precisely, you should write inline assembly or link in an assembly module.

Typically when using a standard C compiler for x86/amd64 (gcc, icc, cl), you can reasonably assume that the compiler will optimize sufficiently well for most purposes.

If, however, you are using a non-standard compiler, e.g., one cooked up for a new embedded system, it is a good idea to consider hand optimization. If the architecture is new, it might also be a good idea to consider hand optimization.

Concept of register variables(datatype:register) in C language?

5 Answers5