reassociation

Regrouping a sum or product, such as (a + b) + c into a + (b + c); harmless for integers, but it changes float results.

Reassociation rewrites an expression using associativity; because each floating-point operation rounds, it can change the result, so compilers may do it to FP code only when told (for example -ffast-math, or -fassociative-math together with -fno-signed-zeros; Clang ignores -fassociative-math on its own).