Coding Practices which enable the compiler/optimizer to make a faster program

Question

Many years ago, C compilers were not particularly smart. As a workaround K&R invented the register keyword, to hint to the compiler, that maybe it woul

Answer 1

For performance, focus first on writing maintenable code - componentized, loosely coupled, etc, so when you have to isolate a part either to rewrite, optimize or simply profile, you can do it without much effort.

Optimizer will help your program's performance marginally.

Answer 2

Put small and/or frequently called functions at the top of the source file. That makes it easier for the compiler to find opportunities for inlining.

Answer 3

Here's a coding practice to help the compiler create fast code—any language, any platform, any compiler, any problem:

Do not use any clever tricks which force, or even encourage, the compiler to lay variables out in memory (including cache and registers) as you think best. First write a program which is correct and maintainable.

Next, profile your code.

Then, and only then, you might want to start investigating the effects of telling the compiler how to use memory. Make 1 change at a time and measure its impact.

Expect to be disappointed and to have to work very hard indeed for small performance improvements. Modern compilers for mature languages such as Fortran and C are very, very good. If you read an account of a 'trick' to get better performance out of code, bear in mind that the compiler writers have also read about it and, if it is worth doing, probably implemented it. They probably wrote what you read in the first place.

Answer 4

If you've got small functions you call repeatedly, i have in the past got large gains by putting them in headers as "static inline". Function calls on the ix86 are surprisingly expensive.

Reimplementing recursive functions in a non-recursive way using an explicit stack can also gain a lot, but then you really are in the realm of development time vs gain.

Answer 5

A neat technique I learned from @MSalters comment on this answer allows compilers to do copy elision even when returning different objects according to some condition:

// before
BigObject a, b;
if(condition)
  return a;
else
  return b;

// after
BigObject a, b;
if(condition)
  swap(a,b);
return a;

Answer 6

I've actually seen this done in SQLite and they claim it results in performance boosts ~5%: Put all your code in one file or use the preprocessor to do the equivalent to this. This way the optimizer will have access to the entire program and can do more interprocedural optimizations.