Loop fusion

In computer science, loop fusion (or loop jamming) is a compiler optimization and loop transformation which replaces multiple loops with a single one. It is possible when two loops iterate over the same range and do not reference each other's data.

Loop fusion does not always improve run-time speed. On some architectures, two loops may actually perform better than one loop because, for example, there is increased data locality within each loop. In these cases, a single loop may be transformed into two, which is called loop fission.

Example in C

  int i, a[100], b[100];
  for (i = 0; i < 100; i++)
    a[i] = 1;                     
  for (i = 0; i < 100; i++)
    b[i] = 2;

is equivalent to:

  int i, a[100], b[100];
  for (i = 0; i < 100; i++)
  {
    a[i] = 1; 
    b[i] = 2;
  }

Compiler optimizations

Basic block	Peephole optimization

Loop optimization	Induction variable Strength reduction Loop fusion Loop inversion Loop interchange Loop-invariant code motion Loop nest optimization Loop unrolling Loop splitting Loop unswitching Software pipelining Automatic parallelization

Data-flow analysis	Common subexpression elimination Constant folding Induction variable recognition and elimination Dead store elimination Use-define chain Live variable analysis Available expression

SSA-based	Global value numbering Sparse conditional constant propagation

Code generation	Register allocation Instruction selection Instruction scheduling Rematerialization

Functional	Tail call elimination Deforestation

Global	Interprocedural optimization

Other	Bounds-checking elimination Dead code elimination Inline expansion Jump threading

Static analysis	Alias analysis Pointer analysis Shape analysis Escape analysis Array access analysis Dependence analysis Control flow analysis Data flow analysis

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Loop fusion

Example in C

See also