Peephole optimisation of JuvixAsm

[lukaszcz]

Simple peephole optimisation on the JuvixAsm level.

The C compiler might not catch some of these as it has no access to JuvixAsm semantics or the type information. This is particularly important for optimising away the single-branch case instructions. C compiler doesn't know that the constructor tag can have only one value, and C semantics mandates that if no branch matches then no branch code is executed (see the C99 standard). So a check is still needed for a single-branch switch and the C compiler cannot optimise it away.

Removing single-branch Case Nodes on the Core level is not good enough. In the Core, a single-branch Case Node cannot be removed if the branch references the constructor arguments.

Examples

• Replace

  case ind {
    ctr: { <code> }
  };
  

  with

  <code>
  

• Replace (where m points to the top of temp stack)

  pusht;
  push tmp[m];
  ret;
  

  with

  ret;
  

  Similarly with tcall or tccall instead of ret.

• Remove

  pusht;
  push tmp[m];
  popt;
  

• Possibly add a load instruction which writes to the top of the stack without pushing it. Replace

  pusht;
  push tmp[m].ctr[k];
  popt;
  

  with

  load $.ctr[k];
  

  and

  pusht;
  push tmp[m].ctr[k];
  ret;
  

  with

  load $.ctr[k];
  ret;
  

  Similarly with tcall or tccall instead of ret.

  Replace

  push V;
  load $.ctr[k];
  

  with

  push V.ctr[k];
  

• Remove

  push V;
  pop;
  

• Replace

  pusht;
  popt;
  

  with

  pop;
  

• Replace

  <instr>;
  popt;
  

  with

  popt;
  <instr>;
  

  if <instr> doesn't reference tmp[m]