Re: Inlining functions with loops (Richard A. O'Keefe)
9 Dec 1995 19:45:05 -0500

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From: (Richard A. O'Keefe)
Newsgroups: comp.compilers
Date: 9 Dec 1995 19:45:05 -0500
Organization: Comp Sci, RMIT, Melbourne, Australia
References: 95-11-241
Keywords: optimize, C++

"Michael Rice" <> writes:
>I believe the basic problem is the inability to convert such a function
>to a suitable expression tree. That is, loops are syntactically statements
>with no equivalent expression-like construct.

This has never been a problem for Lisp/Scheme inliners. There is no
reason whatsoever why expression trees cannot represent loops.

>What other languages allow inlining functions, and what constructs
>make the function un-inlinable in these languages?

Lisp, and many Scheme implementations, allow inlining. (In Scheme,
use define-integrable.) These languages treat procedure identifiers
as initialised variables, so the inliner needs some form of assurance
that these variables will not be changed. That's about it.

IBM PL/I for OS/2 supports inlining. The manual says
Some procedures and begin-blocks will never be inlined.
These include, but are not limited to:
* Procedures and begin-blocks in packages in which condition
enablement varies [basically because exception handling is
related to block activation]
* Procedures and begin-blocks containing ON or REVERT statements
[again, this is related to exception handling]
* Procedures and begin-blocks containing data-directed input/
output statements (this is like Fortran NAMELIST i/o and has to
do with keeping a symbol table of variables; inlining tends to
involve renaming, which interferes with this. It's a solvable
problem but probably not worth worrying about.)
* Procedures and begin-blocks containing assignments to or
comparisons of ENTRY, FORMAT, or LABEL constants. [These things
are attached to particular activations; there is no analogue in C++]

Loops are *not* included in this list!

Ada 83 and Ada 95 have 'pragma inline'. All the standard says is "for
each such call [to an inlined subprogram] the compiler is free to
follow or to ignore the recommendation expressed by the pragma". Ada
programmers would be surprised if the presence of loops were a reason
for ignoring 'inline'.

>I suppose some pseudo-expression for loops could be created to handle

>I also have to wonder if this is worth the work. Any comments?

If you have a good abstract tree representation, it _isn't_ any extra
work. In Scheme, a loop can appear anywhere a function call can,
whether there is an inline expansion or not, so a compiler has to be
able to deal with it.

Richard A. O'Keefe;; RMIT Comp.Sci.

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