One-pass compiler

<h2 id="difficulties">Difficulties</h2>
<p>A core requirement for any <a href="/facts/Programming_language/cWTYbgWa">programming language</a> intended for one-pass compilation is that all user-defined identifiers, except possibly labels, must be declared before use:
</p>
<ul><li>Pascal contains a forward specification to allow routines to be mutually recursive.</li>
<li>PL/I allows data declarations to be placed anywhere within a program, specifically, <i>after</i> 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.</li></ul>
<p>Any language which allows preprocessing such as PL/1, C, or C++, must have at least two passes, one pass for preprocessing and one or more passes for translation.
</p><p>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.
</p><p>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: 
</p>
 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

<p>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.
</p>
<h2 id="see-also">See also</h2>
<ul><li><a href="/facts/ALGOL_68R/9Ed1AtrD">ALGOL 68R</a></li>
<li><a href="/facts/Burroughs_large_systems/FwY7EqrB">Burroughs Algol</a></li>
<li><a href="/facts/NELIAC/NXJCHHhJ">NELIAC</a></li>
<li><a href="/facts/XPL/kmdXXdhm">XPL</a></li></ul>

<h2 id="references">References</h2>

<ol>
<li id="fn:1"><p>"Single pass, Two pass, and Multi pass Compilers". GeeksforGeeks. 2019-03-13. Retrieved 2023-05-15. <a href="https://www.geeksforgeeks.org/single-pass-two-pass-and-multi-pass-compilers/" target="_blank">https://www.geeksforgeeks.org/single-pass-two-pass-and-multi-pass-compilers/</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li>
</ol>

One-pass compiler open-in-new

One-pass compiler