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RENESAS TOOL NEWS on September 1, 2004: RSO-M3T-NC30WA-040901D
A Note on Using
C Compiler Package M3T-NC30WA
--On Inverting the Sign of a Signed Variable--
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Please take note of the following problem in using the M3T-NC30WA
C-compiler package (used for the M16C/60, M16C/30, M16C/Tiny, M16C/20,
M16C/10, and R8C/Tiny series MCUs):
- On inverting the sign of a signed variable
- Versions Concerned
M3T-NC30WA V.5.00 Release 1 through V.5.30 Release 1
- Description
Inverting the sign of a signed variable may result in incorrect code being generated if the variable is less than zero.
- 2.1 Conditions
- This problem occurs if all the following conditions are satisfied:
| (1) | Any of the optimizing options -O1, -O2, -O3, -O4, -O5, -OR, and -OS is selected at compilation. |
| (2) | A signed variable is compared with zero in the controlling expression of an "if" statement. |
| (3) | The type of the variable in (2) is any of these, signed char, signed int, and signed short. |
| (4) | If the variable is less than zero in the comparison in (2), in the true statement exists only an expression where the above-mentioned variable is assigned to another with its sign being inverted. |
- 2.2 Example
------------------------------------------------------------------------
signed char s1,s2; /* Condition (3) */
void func(void)
{
if(s1 < 0) /* Condition (2) */
s2 = -s1; /* Condition (4) and (5) */
}
------------------------------------------------------------------------
In the above example, -s1 is assigned to s2 even if the controlling
expression is unsatisfied.
- Workaround
Place a dummy asm function immediately after the controlling expression.
-------------------------------------------------------------------------
signed char s1,s2;
void func(void)
{
if(s1 < 0){
asm(); /* Dummy asm function placed */
s2 = -s1;
}
}
-------------------------------------------------------------------------
- Schedule of Fixing the Problem
We plan to fix this problem in our next release of the product.
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