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A peephole optimizer sweeps through the code array, looking at a small window of instructions, looking for ways to improve the code.
For example, sequence
goto 15 label 15
can be replaced by
label 15
Some preprocessing allows the peephole optimizer to expand its view beyond just a few contiguous instructions.
For example, a pass over the code array can locate all labels. Then, when a branch instruction is encountered, the optimizer can look at the code that follows the given label. Upon seeing
if x = y goto 5 … label 5 goto 6
the optimizer can replace the conditional branch instruction by
if x = y goto 6
and can look at the same instruction again, possibly leading to more changes.
A peephole optimizer can also eliminate tests with constant conditions. For example,
if 0 = 1 goto 10
can be removed. You might expect such tests rarely to occur, but in fact they can be common.
Tracing instructions are typically protected by tests, which can check defined constants. For example, if 'tracing' is a constant that is defined to be 0, then source code
if(tracing > 0) {
print(…);
}
leads to a test with a constant condition. A bit more sophisticated optimizer can also eliminate the unreachable code for the print line.
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