Implementation notes for IfWithConstantSimplifier

The IfWithConstantSimplifier simplifies JIfStatements (on synthetic variables) by inserting goto and label statements.

The implementation visits these ‘if’ statements in post-order traversal so that we do not insert invalid goto statements.

Example

Let's consider the following IR where:

• <COND> is a variable of boolean type
• <THEN> represents a list of statements of the ‘then’ block
• <ELSE> represents a list of statements of the ‘else’ block

 [...]
 if (<COND>) {
   -tac0 = true;
 } else {
   -tac0 = false;
 }
 if (-tac0) {
   -tac1 = true;
 } else {
   -tac1 = false;
 }
 if (-tac1) {
   <THEN>
 } else {
   <ELSE>
 }
 [...]

The IfWithConstantSimplifier is able to optimize away the synthetic variables -tac0 and -tac1.

In post-order, we visit the if (-tac1) statement first. Here is how the IR is transformed:

[...]
if (<COND>) {
  -tac0 = true;
} else {
  -tac0 = false;
}
if (-tac0) {
  goto ifSimplierThen_0;
} else {
  goto ifSimplierElse_0;
}
{
  ifSimplierThen_0: {
  }
  <THEN>
  goto ifSimplifierStepOverElse_0;
}
{
  ifSimplierElse_0: {
  }
  <ELSE>
}
ifSimplifierStepOverElse_0: {
}
[...]

This first optimization removed the synthetic variable -tac1 and introduced labels ifSimplierThen_0 and ifSimplierElse_0, respectively corresponding to the start of the ‘then’ and ‘else’ block. The label ifSimplifierStepOverElse_0 allows the ‘then’ block to step over the ‘else’ block.

Then we visit the statement if (-tac0). Here is how the IR is transformed:

[...]
if (<COND>) {
  goto ifSimplierThen_1;
} else {
  goto ifSimplierElse_1;
}
{
  ifSimplierThen_1: {
  }
  goto ifSimplierThen_0;
}
{
  ifSimplierElse_1: {
  }
  goto ifSimplierElse_0;
}
{
  ifSimplierThen_0: {}
  }
  <THEN>
  goto ifSimplifierStepOverElse_0;
}
{
  ifSimplierElse_0: {
  }
  <ELSE>
}
ifSimplifierStepOverElse_0: {
}
[...]

This second optimization removed the synthetic variable -tac0 and introduced labels ifSimplierThen_1 and ifSimplierElse_1, respectively corresponding to the start of the ‘then’ and ‘else’ block (that were already transformed in the previous step).

The post-order traversal allows to know that it is not necessary to insert the “step over” statements (goto + label) because the ‘then’ block already contains a goto statement, thus already step over the ‘else’ block.