Re: New wikibook about efficient C++

Carlo Milanesi wrote:


  The text contains mostly irrelevant and sometimes erroneous
suggestions. Most of the suggested micro-optimizations will usually not
make your program any faster at all and thus are completely irrelevant.
  Some examples:

"Don't check that a pointer is non-null before calling delete on it."

  While it's true that you don't have to check for null before deleting,
speedwise that's mostly irrelevant. In most systems with most compilers
the 'delete' itself is a rather heavy operation, and the extra clock
cycles added by the conditional will not make the program relevantly
slower. It might become more relevant if you have a super-fast
specialized memory allocator where a 'delete' takes next to nothing of
time, and humongous amounts of objects are deleted in a short time.
However, in normal situations it's irrelevant.

"Declare const every member function that does not change the state of
the object on which it is applied."

  Mostly irrelevant speedwise.

"Instead of writing a for loop over an STL container, use an STL
algorithm with a function-object or a lambda expression"

  Why is that any faster than the for loop? (In fact, it might even be
slower if, for whatever reason, the compiler is unable to inline the
lambda function.)

"Though, every time a function containing substantial code is inlined,
the machine code is duplicated, and therefore the total size of the
program is increased, causing a general slowing down."

  Mostly not true. There is no necessary correlation between code size
and speed. In fact, sometimes a longer piece of code may perform faster
than a shorter one (for example loop unrolling performed by the compiler
sometimes produces faster code, even in modern processors).

"Among non-tiny functions, only the performance critical ones are to be
declared inline at the optimization stage."

  The main reason to decide whether to declare a larger function
'inline' or not is not about speed. The compiler has heuristics for this
and will not inline the function if it estimates that it would be
counter-productive. The main reason to use or avoid 'inline' has more to
do with the quality of the source code.

"In addition, every virtual member function occupies some more space"

  Irrelevant, unless you are developing for an embedded system with a
*very* small amount of memory.

"Do not null a pointer after having called the delete operator on it, if
you are sure that this pointer will no more be used."

  Irrelevant. The 'delete' itself will usually be so slow that the
additional assignment won't change the anything.

"Garbage collection, that is automatic reclamation of unreferenced
memory, provides the ease to forget about memory deallocation, and
prevents memory leaks. Such feature is not provided by the standard
library, but is provided by non-standard libraries. Though, such memory
management technique causes a performance worse than explicit
deallocation (that is when the delete operator is explicitly called)."

  This is simply not true. In fact, GC can be made faster than explicit
deallocation, at least compared to the default memory allocator used by
most C++ compilers.

"To perform input/output operations, instead of calling the standard
library functions, call directly the operating system primitives."

  Dubious advice. The general (and portable) advice for fast I/O is to
use fread() and fwrite() for large blocks of data (the C++ equivalents
may actually be equally fast, if called rarely). If very small amounts
of data (such as characters) need to be read or written individually,
use the correspondent C I/O functions.

  In places where I/O speed is irrelevant, this advice is
counter-productive.

"Look-up table"

  This was relevant in the early 90's. Nowadays it's less evident. With
modern CPUs sometimes using a lookup table instead of a seemingly "slow"
function might actually be slower, depending on a ton of factors.

"Instead of doing case-insensitive comparisons between a strings,
transform all the letters to uppercase (or to lowercase), and then do
case-sensitive comparisons."

  Yeah, because converting the string does not take time?