Are floating point operations in C associative?

Addition mathematically holds the associative property:

(a + b) + c = a + (b + c)

In the general case, this property does not hold for floating-point numbers because they represent values in a finite precision.

Is a compiler allowed to make the above substitution when generating machine code from a C program as part of an optimization? Where does it exactly say in the C standard?

---

The compiler is not allowed to perform "optimizations", which would result in a different value computed, than the one computed according to abstract machine semantics.

5.1.2.3 Program execution

[#1] The semantic descriptions in this International Standard describe the behavior of an abstract machine in which issues of optimization are irrelevant.

[#3] In the abstract machine, all expressions are evaluated as specified by the semantics.

[#13] EXAMPLE 5 Rearrangement for floating-point expressions is often restricted because of limitations in precision as well as range. The implementation cannot generally apply the mathematical associative rules for addition or multiplication, nor the distributive rule, because of roundoff error, even in the absence of overflow and underflow.

In your example:

(a + b) + c

or even without the parentheses:

a + b + c

we have

   +
  / 
  +  c
 / 
 a  b

and the compiler is required to generate code as if a is summed with b and the result is summed with c .

---

Floating point multiplication in C is not associative.

In C, Floating point multiplication is not associative.

Some evidence is with this C code:

Pick three random float values.
Check if a*(b*c) is ever not equal to (a*b)*c

#include<stdio.h>
#include<time.h>
#include<stdlib.h>
using namespace std;
int main() {
    int counter = 0;
    srand(time(NULL));
    while(counter++ < 10){
        float a = rand() / 100000;
        float b = rand() / 100000;
        float c = rand() / 100000;

        if (a*(b*c) != (a*b)*c){
            printf("Not equaln");
        }
    }
    printf("DONE");
    return 0;
}

The program prints:

Not equal
Not equal
Not equal
Not equal
DONE
RUN FINISHED; exit value 0; real time: 10ms; user: 0ms; system: 0ms

Conclusion:

For my test, three randomly selected floating point multiplication values are associative about 70% of the time.

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