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RENESAS TOOL NEWS on May 1, 2004: RSO-M3T-CC32R_2-040501D
A Note on Using C-Compiler Package
M3T-CC32R
--On Making a Change to a Variable by Using a Bit Field--
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Please take note of the following problem in using the M3T-CC32R C-compiler package for the M32R family MCUs:
- On making a change to a variable by using a bit field
- Versions Concerned
M3T-CC32R V.4.00 Release 1 through V.4.20 Release 1A
- Description
When an integer-type or floating-type variable is referenced, correct reference may not be made by improper optimization.
- 2.1 Conditions
- This problem occurs if the following five conditions are satisfied:
| (1) |
Any of the optimizing options -O4, -O5, -O6, and -O7 is used;
or -Ospace only or -Otime only is selected. |
| (2) |
An assignment is made to an integer-type or floating-type variable. |
| (3) |
An assignment is made to a bit field by using a pointer to a structure containing a bit field and a return value of an address (&) variable. |
| (4) |
The pointer in (3) points to the same address where the variable in (2) is stored. |
| (5) |
The variable in (2) is referenced after the assignment in (3). |
- 2.2 Examples
-
Source file 1: sample1.c
--------------------------------------------------------------------
unsigned long src1, dst1;
struct btf1 {
unsigned long x:1;
unsigned long y:3;
};
void func01(void)
{
src1 = 0x80000000; /* Condition (2) */
((struct btf1 *)&dst1)->y = 2; /* Conditions (3) and (4) */
dst1 = src1; /* Condition (5) */
}
--------------------------------------------------------------------
Source file 2: sample2.c
--------------------------------------------------------------------
unsigned long src2, dst2;
struct btf2 {
unsigned long x:1;
unsigned long y:3;
} *ptr2;
void initialize(void)
{
ptr2 = (struct btf2 *) &src2; /* Condition (4) */
}
void func02(void)
{
/* An initialize function executed before func02 done */
src2 = 0; /* Condition (2) */
ptr2->y = 2; /* Condition (3) */
dst2 = src2; /* Condition (5) */
}
--------------------------------------------------------------------
Source file 3: sample3.c
--------------------------------------------------------------------
long src3, dst3;
struct btf3 {
unsigned long x:1;
unsigned long y:3;
} *ptr3;
long func03(void)
{
src3 = 255; /* Condition (2) */
ptr3 = (struct btf3 *) &src3; /* Condition (4) */
ptr3->y = 2; /* Condition (3) */
return src3; /* Condition (5) */
}
--------------------------------------------------------------------
Source file 4: sample4.c
--------------------------------------------------------------------
long src4, dst4;
struct btf4 {
unsigned long x:1;
unsigned long y:3;
} *ptr4;
long func04(long arg4)
{
src4 = arg4; /* Condition (2) */
ptr4 = (struct btf4 *) &src4; /* Condition (4) */
ptr4->y = 2; /* Condition (3) */
return src4; /* Condition (5) */
}
--------------------------------------------------------------------
- Workaround
This problem can be circumvented in any of the following ways:
| (1) |
If the pointer in Condition (3) is the address of a variable,
create a union having two members of the original type of the variable and the type of the structure containing a bit field;
then replace the assignment in Condition (3) with the one to a member of the union.
Modification of source file 1:
--------------------------------------------------------------------
unsigned long dst1;
struct btf1 {
unsigned long x:1;
unsigned long y:3;
};
union { /* Replace src1 with a union */
unsigned long word;
struct btf1 bit;
} src1;
void func01(void)
{
src1.word = 0x80000000; /* Replace src1 with src1.word */
src1.bit.y = 2; /* Modify src1 by bitwise operation */
dst1 = src1.word; /* Replace src1 with src1.word */
}
--------------------------------------------------------------------
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| (2) |
When the variable referenced in Condition (5) is the right term of an assignment expression,
replace each of the left and right terms with an indirection reference using a pointer to a volatile object.
Modification of source file 2:
--------------------------------------------------------------------
unsigned long src2, dst2;
struct btf2 {
unsigned long x:1;
unsigned long y:3;
} *ptr2;
void initialize(void)
{
ptr2 = (struct btf2 *) &src2; /* Condition (4) */
}
void func02(void)
{
/* An initialize function executed before func02 done */
src2 = 0;
ptr2->y = 2;
/* Both terms changed to volatile objects */
*((volatile unsigned long*)&dst2) = *((volatile unsigned long*)&src2);
}
--------------------------------------------------------------------
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| (3) |
When -Ospace only or -Otime only is selected, use any of the optimizing options -O0, -O1, -O2, and -O3 at the same time.
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- Schedule of Fixing the Problem
We plan to fix this problem in our next release of the product.
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