Microchip Technology XC8 Standard Compiler (Workstation) SW006021-1 SW006021-1 ユーザーズマニュアル

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DS52053B-page 180

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The consequence of integral promotion as illustrated above is that operations are not 
performed with char -type operands, but with int -type operands. However, there are 
circumstances when the result of an operation is identical regardless of whether the 
operands are of type char or int. In these cases, the compiler will not perform the 
integral promotion so as to increase the code efficiency. Consider this example.

unsigned char a, b, c;

a = b + c;

Strictly speaking, this statement requires that the values of b and c should be promoted 
to unsigned int, the addition performed, the result of the addition cast to the type of 
a

, and then the assignment can take place. Even if the result of the unsigned int 

addition of the promoted values of b and c was different to the result of the unsigned 
char

 addition of these values without promotion, after the unsigned int result was 

converted back to unsigned char, the final result would be the same. If an 8-bit addi-
tion is more efficient than a 16-bit addition, the compiler will encode the former.

If, in the above example, the type of a was unsigned int, then integral promotion 
would have to be performed to comply with the ANSI C standard.

The C language does not specify a rotate operator; however, it does allow shifts. The 
compiler will detect expressions that implement rotate operations using shift and logical 
operators and compile them efficiently.

For the following code:

c = (c << 1) | (c >> 7);

if c is unsigned and non-volatile, the compiler will detect that the intended 
operation is a rotate left of 1 bit and will encode the output using the PIC MCU rotate 
instructions. A rotate left of 2 bits would be implemented with code like:

c = (c << 2) | (c >> 6);

This code optimization will also work for integral types larger than a char. If the opti-
mization cannot be applied, or this code is ported to another compiler, the rotate will be 
implemented, but typically with shifts and a bitwise OR operation.

The compiler may encode switch statements using one of several strategies. By 
default, the compiler chooses a strategy based on the case values that are used inside 
the switch statement. Each switch statement is assigned its strategy independently.

The type of strategy can be indicated by using the #pragma switch directive. See 
Section 5.14.4.10 “The #pragma switch Directive”, which also lists the available 
strategy types. There may be more than one strategy associated with each type.

There is information printed in the assembly list file for each switch statement detail-
ing the value being switched and the case values listed. See Section 6.6.4 “Switch 
Statement Information”.