Doc No. : WG21 N1247
Date: 17 April, 2000
Project: Programming Language C++
Reply to: Seiji Hayashida
Programmer-directed Optimizations (Performance Group)
1. Current approaches
The most popular and easy way to improve performance is to use compiler's
optimize options. Compilers support their options for optimization. And some
compilers have optimizing option that gives high priority to code-size or
Other approaches are roughly classified into two categories.
(1) ristrictions (subsetting)
Turn off some functions which affect performance.
Add new directives or compiler options to generate more effective code.
These might be implemented as '#pragma directive' or compiler options.
In general, exception handling and RTTI both have some overhead. For users
who do not use these functions, it is preferable to turn off them using
compiler options. Programmers can get better performance by using these
NOTE: When exceptions are disabled, the specifications of the standard
libraries will be different.
Some compilers support options for subsetting. One of the subsets of the ISO
C++ standard is Embedded C++(EC++). EC++ excludes the following features from
the ISO C++ standard.
- multiple inheritance
EC++ also excludes some library features. The EC++ specification is summarized
by the Embedded C++ Technical Committee (http://www.caravan.net/ec2plus).
Since some compiler venders already support EC++, it is possible to get
better performance by writing programs within this specification.
- memory allocation
Sometimes, memory access time depends on the address of the memory. It is
important to allocate data and text to appropriate addresses (memory sections),
depending on a target CPU.
Some compilers have propriety function for specifying address of data. Many
compilers use #pragma for this function, but each compiler has own syntax.
Although memory alignment is also important for memory access time, it may
waste memory. So, a compiler (linker) must allocate each object with minimum
loss, automatically. Or, programmer must specify the best address manually.
- Initialization of variables
In C++, constructors are always called to initialize variables of class type.
If a global object is initialized to zero by a constructor, the constructor
call may be purely overhead. Because global objects are supposed to be
initialized to zero by default.
- Other directives for optimization
* Convert virtual calls to non-virtual calls
If a virtual function is not overridden by its derived classes, virtual
function calls to it can be converted into non-virtual calls.
A directive like 'final' in Java which specifies such information can
encourage compilers to generate better code.
A new keyword "restrict" has been added to the ISO C(ISO/IEC 9899:1999) for
aiding optimization under the direction of users. If a C++ compiler supports
"restrict", the compiler can produce better code.