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RENESAS TOOL NEWS on March 1, 2004: RSO-M3T-NC308WA_1-040301D
A Note on Using C Compilers
M3T-NC308WA and M3T-NC30WA
--On Testing Equality/Inequality Between a Variable and a Constant--
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Please take note of the following problem in using C compilers (with an assembler and integrated development environment) M3T-NC308WA and M3T-NC30WA
- On testing equality/inequality between a variable of type signed or unsigned char and a constant
- Products and Versions Concerned
M3T-NC308WA V.5.00 Release 1 and V.5.10 Release 1
for the M32C/80 and M16C/80 series MCUs
M3T-NC30WA V.5.00 Release 1 through V.5.10 Release 1
for the M16C/60, M16C/30, M16C/Tiny, M16C/20, M16C/10, and R8C/Tiny series MCUs
- Description
Testing equality/inequality between a variable of type signed or unsigned char and a constant may generate incorrect code.
- 2.1 Conditions
- This problem occurs if the following five conditions are satisfied:
| (1) |
In an if statement exist two or more controlling expressions testing equality/inequality between a variable of type signed or unsigned char and a constant. |
| (2) |
The variables in (1) are such as arrays or aggregates (whose elements are arranged continuously in memory space).
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| (3) |
Operators used in the controlling expressions in (1) are all "==" or "!=".
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| (4) |
The controlling expressions in (1) are associated with each other by a logical OR operator.
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| (5) |
The following compile options are used:
a. Any of those, -O1, -O2, -O3, -OR, and -OS, and -Ocompare_byte_to_word (-OCBTW), or
b. O4 only, or
c. O5 only
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- 2.2 Example
-
-------------------------------------------------------------------
char c[2]; /* Condition (2) */
int func( void )
{
if( c[0] == 1 || c[1] == 1 ) /* Conditions (1), (2), (3),
and (4) */
return 0;
else
return 1;
}
-------------------------------------------------------------------
- Workaround
This problem can be circumvented in any of the following ways:
| (1) |
Split the if statement.
-----------------------------------------
char c[2];
int func( void )
{
if( c[0] == 1 ) /* One of split if statement */
return 0;
else if( c[1] == 1 ) /* The other of split if statement */
return 0;
else
return 1;
}
----------------------------------------- |
| (2) |
Don't use -Ocompare_byte_to_word (-OCBTW) together with any of the options -O1, -O2, -O3, -OR, and -OS. |
| (3) |
Use -O3 instead of -O4 or -O5. |
- Schedule of Fixing the Problem
In the M3T-NC308WA, we plan to fix this problem in its next release.
In the M3T-NC30WA, this problem has been resolved in its V.5.20 Release 1, so please upgrade your product to it.
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