 |
|
|
RENESAS TOOL NEWS on February 1, 2005: RSO-M3T-NC30WA_2-050201D
A Note on Using
C-Compiler Package M3T-NC30WA
--On Right Shifting 4-Byte Variables of Type int--
|
Please take note of the following problem in using the M3T-NC30WA
C-compiler package which is used for the M16C/60, M16C/30, M16C/20, M16C/10,
M16C/Tiny, and R8C/Tiny series of MCUs:
- On right shifting 4-byte variables of type int
- Versions Concerned
M3T-NC30WA V.4.00 Release 1 through V.5.30 Release 1
- Description
Performing a right shift of a 4-byte variable of type int and then
referencing the value previous to the shift may result in incorrect code being generated.
- 2.1 Conditions
- This problem occurs if the following conditions are all satisfied:
| (1) |
Any one or more of the optimizing options -O3, -O4, -O5, -OR,
and -OS are used at compilation. |
| (2) |
A 4-byte variable of type int or the result of an operation
performed on such a variable is right-shifted, and its result is
stored as a 2-byte variable of type int. |
| (3) | The shift count in (2) is greater than 10 and less than 17. |
| (4) |
After storing the shift result in (2) as a 2-byte variable of type int,
a reference is made to the 4-byte variable before the shift. |
- 2.2 Examples
(1) The case where a 4-byte variable of type int right-shifted:
---------------------------------------------------------------
long m, n;
int i;
void func(void)
{
register long a = m;
i = a >> 11; /* Conditions (2) and (3) */
n = a; /* Condition (4) */
}
---------------------------------------------------------------
(2) The case where the result of an operation performed on
a 4-byte variable of type int right-shifted:
---------------------------------------------------------------
long o, p;
int ii;
void func2(void)
{
ii = (o - 1) >> 11; /* Conditions (2) and (3) */
p = (o - 1); /* Condition (4) */
}
---------------------------------------------------------------
- Workaround
| (1) |
If a 4-byte variable of type int to be shifted is qualified as
"register" and compile option "-fenable_register[-fER]" is used,
be sure to keep the following:
| (a) |
Don't qualify a 4-byte variable of type int in Condition (2) as "register". |
| (b) |
Place a dummy asm function between the line containing the
expression for right-shifting a 4-byte variable of type int
(Condition (2)) and the line containing the expression for
referencing the above variable (Condition (4)). |
--------------------------------------------------------------------
long m, n;
int i;
void func(void)
{
long a = m; /* (a); Not qualified as "register" */
i = a >> 11;
asm(); /* (b); Dummy asm function placed */
n = a;
}
-------------------------------------------------------------------- |
| (2) |
Otherwise, place a dummy asm function between the line containing
the expression for right-shifting a 4-byte variable of type int (Condition (2))
and the line containing the expression for referencing the above variable (Condition (4)).
Example (1) The case where a 4-byte variable of type int right-shifted:
-----------------------------------------------------------------
long m, n;
int i;
void func(void)
{
register long a = m;
i = a >> 11;
asm(); /* Dummy asm function placed */
n = a;
}
-----------------------------------------------------------------
Example (2) The case where the result of an operation performed
on a 4-byte variable of type int right-shifted:
-----------------------------------------------------------------
long o, p;
int ii;
void func2(void)
{
ii = (o - 1) >> 11;
asm(); /* Dummy asm function placed */
p = (o - 1);
}
----------------------------------------------------------------- |
- Schedule of Fixing the Problem
This problem has already been fixed in the M3T-NC30WA V.5.30 Release 02 and later.
|
|
