Integer vs floating division -> Who is responsible for providing the result?

Question

I've been programming for a while in C++, but suddenly had a doubt and wanted to clarify with the Stackoverflow community.

When an integer is divided by another integer, we all know the result is an integer and like wise, a float divided by float is also a float.

But who is responsible for providing this result? Is it the compiler or DIV instruction?

Answer 1

The compiler will decide at compile time what form of division is required based on the types of the variables being used - at the end of the day a DIV (or FDIV) instruction of one form or another will get involved.

Answer 2

That depends on whether or not your architecture has a DIV instruction. If your architecture has both integer and floating-point divide instructions, the compiler will emit the right instruction for the case specified by the code. The language standard specifies the rules for type promotion and whether integer or floating-point division should be used in each possible situation.

If you have only an integer divide instruction, or only a floating-point divide instruction, the compiler will inline some code or generate a call to a math support library to handle the division. Divide instructions are notoriously slow, so most compilers will try to optimize them out if at all possible (eg, replace with shift instructions, or precalculate the result for a division of compile-time constants).

Answer 3

Hardware divide instructions almost never include conversion between integer and floating point. If you get divide instructions at all (they are sometimes left out, because a divide circuit is large and complicated), they're practically certain to be "divide int by int, produce int" and "divide float by float, produce float". And it'll usually be that both inputs and the output are all the same size, too.

The compiler is responsible for building whatever operation was written in the source code, on top of these primitives. For instance, in C, if you divide a float by an int, the compiler will emit an int-to-float conversion and then a float divide.

(Wacky exceptions do exist. I don't know, but I wouldn't put it past the VAX to have had "divide float by int" type instructions. The Itanium didn't really have a divide instruction, but its "divide helper" was only for floating point, you had to fake integer divide on top of float divide!)

Answer 4

Your question doesn't really make sense. The DIV instruction doesn't do anything by itself. No matter how loud you shout at it, even if you try to bribe it, it doesn't take responsibility for anything

When you program in a programming language [X], it is the sole responsibility of the [X] compiler to make a program that does what you described in the source code.

If a division is requested, the compiler decides how to make a division happen. That might happen by generating the opcode for the DIV instruction, if the CPU you're targeting has one. It might be by precomputing the division at compile-time, and just inserting the result directly into the program (assuming both operands are known at compile-time), or it might be done by generating a sequence of instructions which together emulate a divison.

But it is always up to the compiler. Your C++ program doesn't have any effect unless it is interpreted according to the C++ standard. If you interpret it as a plain text file, it doesn't do anything. If your compiler interprets it as a Java program, it is going to choke and reject it.

And the DIV instruction doesn't know anything about the C++ standard. A C++ compiler, on the other hand, is written with the sole purpose of understanding the C++ standard, and transforming code according to it.

The compiler is always responsible.

Answer 5

One of the most important rules in the C++ standard is the "as if" rule:

The semantic descriptions in this International Standard define a parameterized nondeterministic abstract machine. This International Standard places no requirement on the structure of conforming implementations. In particular, they need not copy or emulate the structure of the abstract machine. Rather, conforming implementations are required to emulate (only) the observable behavior of the abstract machine as explained below.

Which in relation to your question means it doesn't matter what component does the division, as long as it gets done. It may be performed by a DIV machine code, it may be performed by more complicated code if there isn't an appropriate instruction for the processor in question.

It can also:

1. Replace the operation with a bit-shift operation if appropriate and likely to be faster.
2. Replace the operation with a literal if computable at compile-time or an assignment if e.g. when processing x / y it can be shown at compile time that y will always be 1.
3. Replace the operation with an exception throw if it can be shown at compile time that it will always be an integer division by zero.

Answer 6

Practically

The C99 standard defines "When integers are divided, the result of the / operator is the algebraic quotient with any fractional part discarded." And adds in a footnote that "this is often called 'truncation toward zero.'"

History

Historically, the language specification is responsible.

Pascal defines its operators so that using / for division always returns a real (even if you use it to divide 2 integers), and if you want to divide integers and get an integer result, you use the div operator instead. (Visual Basic has a similar distinction and uses the \ operator for integer division that returns an integer result.)

In C, it was decided that the same distinction should be made by casting one of the integer operands to a float if you wanted a floating point result. It's become convention to treat integer versus floating point types the way you describe in many C-derived languages. I suspect this convention may have originated in Fortran.