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22007E/0—November 1999
Avoid Unnecessary Integer Division
Copy Frequently De-referenced Pointer Arguments to Local
Variables
Avoid Unnecessary Integer Division
Integer division is the slowest of all integer arithmetic
operations and should be avoided wherever possible. One
possibility for reducing the number of integer divisions is
multiple divisions, in which division can be replaced with
multiplication as shown in the following examples. This
replacement is possible only if no overflow occurs during the
computation of the product. This can be determined by
considering the possible ranges of the divisors.
Example 1 (Avoid):
int i,j,k,m;
m = i / j / k;
Example 2 (Preferred):
int i,j,k,l;
m = i / (j * k);
Avoid frequently de-referencing pointer arguments inside a
function. Since the compiler has no knowledge of whether
aliasing exists between the pointers, such de-referencing can
not be optimized away by the compiler. This prevents data from
being kept in registers and significantly increases memory
traffic.
Note that many compilers have an "assume no aliasing"
optimization switch. This allows the compiler to assume that
two different pointers always have disjoint contents and does
not require copying of pointer arguments to local variables.
Otherwise, copy the data pointed to by the pointer arguments
to local variables at the start of the function and if necessary
copy them back at the end of the function.
AMD Athlon™ Processor x86 Code Optimization
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