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RENESAS TOOL NEWS on September 1, 2006: 060901/tn2
| A Note on Using the C Compiler Package
--M3T-NC30WA--
for the M16C Series of MCUs
--On Evaluating Two or Three Bits
in a Controlling Expression-- |
Please take note of the following problem in using the C compiler package-- M3T-NC30WA--for the M16C series of MCUs:
- On evaluating two or three bits in a controlling expression
- Versions Concerned
The M3T-NC30WA V.1.00 Release 1 through V.5.40 Release 00(A)
(the C compiler package for the M16C/60, M16C/30, M16C/20, M16C/10, M16C/Tiny, and R8C/Tiny series)
- Description
Evaluating two or three bits using the logical AND operator (&&) in a controlling expression may generates incorrect code.
- Conditions
This problem occurs if the following conditions are all satisfied:
| (1) |
|
In the program exists a controlling expression where two or three bits are evaluated using one or two && operators. |
| (2) |
|
Every bit in (1) above is one bit in width. |
| (3) |
|
Every bit in (1) above is a member of a bit field, and the bit field is of type char. |
| (4) |
|
All the bits in (1) are positioned in the lower or upper four bits of the bit field |
| (5) |
|
The positions of the bits in (1) are as follows:
| (a) |
|
If two bits are evaluated, they are separated by 1 or 2 bits from each other. |
| (b) |
|
If three bits are evaluated, any one of them is separated by 1 bit from another. |
Examples of the positions of the evaluated bits in a bit field
(when they are in the lower four bits)
| Bit Position |
7 |
6 |
5 |
4 |
3 |
2 |
1 |
0 |
|---|
| Pattern 1 |
| | | | * | | * | |
|---|
| Pattern 2 |
| | | | * | | | * |
|---|
| Pattern 3 |
| | | | | * | | * |
|---|
| Pattern 4 |
| | | | * | | * | * |
|---|
| Pattern 5 |
| | | | * | * | | * |
|---|
|
| (6) |
|
Optimizing option -O4 is used in compilation.
Example:
------------------------------------------------
struct{
unsigned char b0:1; // Conditions (2), (4), and (5)
unsigned char b1:1;
unsigned char b2:1;
unsigned char b3:1; // Conditions (2), (4), and (5)
unsigned char b4:1;
unsigned char b5:1;
unsigned char b6:1;
unsigned char b7:1;
}bit; // Condition (3)
int main()
{
if(bit.b0 && bit.b3){ // Condition (1)
return 1;
}
}
------------------------------------------------ |
- Workarounds
This problem can be circumvented either of the following ways:
| (1) |
|
Use a nested if statement instead of a && operator.
Example:
------------------------------------------------
. . . . . . . . . .
int main()
{
if(bit.b0){
if(bit.b3){
return 1;
}
}
}
------------------------------------------------ |
| (2) |
|
Use optimizing option -O3 instead of -O4. |
- Schedule of Fixing the Problem
We plan to fix this problem in the next release of the product.
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