AMD 250 manual Sign of Integer Operands, Examples

Software Optimization Guide for AMD64 Processors 25112 Rev. 3.06 September 2005

2.24Sign of Integer Operands

Optimization

Where there is a choice of using either a signed or an unsigned type, take into consideration that some operations are faster with unsigned types while others are faster for signed types.

Application

This optimization applies to:

•32-bit software

Rationale

In many cases, the type of data to be stored in an integer variable determines whether a signed or an unsigned integer type is appropriate. For example, to record the weight of a person in pounds, no negative numbers are required, so an unsigned type is appropriate. However, recording temperatures in degrees Celsius may require both positive and negative numbers, so a signed type is needed.

Integer-to-floating-point conversion using integers larger than 16 bits is faster with signed types, as the AMD64 architecture provides instructions for converting signed integers to floating-point but has no instructions for converting unsigned integers. In a typical case, a 32-bit integer is converted by a compiler to assembly as follows:

Examples

Listing 18. (Avoid)

double x;	====> mov [temp+4], 0
unsigned int i;	mov	eax, i
	mov	[temp], eax
x = i;	fild		QWORD PTR [temp]
	fstp		QWORD PTR [x]

The preceding code is slow not only because of the number of instructions, but also because a size mismatch prevents store-to-load forwarding to the FILD instruction. Instead, use the following code:

Listing 19. (Preferred)

double x;	====>	fild	DWORD	PTR	[i]
int i;		fstp	QWORD	PTR	[x]

x = i;

Computing quotients and remainders in integer division by constants is faster when performed on unsigned types. The following typical case is the compiler output for a 32-bit integer divided by 4:

48 C and C++ Source-Level Optimizations Chapter 2