A core requirement for any
programming language intended for one-pass compilation is that all user-defined identifiers, except possibly labels, must be declared before use: • Pascal contains a
forward specification to allow routines to be mutually recursive. • PL/I allows data declarations to be placed anywhere within a program, specifically,
after some references to the not-yet-declared items, so one pass is needed to resolve data declarations, followed by one or more passes to generate code. Some computer hardware (e.g. x86) may have short and long branch instructions: short if the destination is within about 127 bytes, and long otherwise. A one-pass compiler must assume that all jumps are long, whereas a multi-pass compiler can check on jump distance and generate possibly shorter code. Languages which rely on context for statement identification rather than using reserved or stropped keywords may also cause problems. The following examples are from Fortran 77: IF (B) l1,l2 ! two-way branch, where B is a boolean/logical expression IF (N) l1,l2,l3 ! three-way branch, where N is a numeric expression IF (B) THEN ! start conditional block IF (B) THEN = 3.1 ! conditional assignment to variable THEN IF (B) X = 10 ! single conditional statement IF (B) GOTO l4 ! conditional jump IF (N) = 2 ! assignment to a subscripted variable named IF DO 12 I = 1,15 ! start of a count-controlled loop DO 12 I = 1.15 ! assignment of the value 1.15 to a variable called DO12I The entire statement must be scanned to determine what sort of statement it is; only then can translation take place. Hence any potentially ambiguous statements must be processed at least twice. ==See also==