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RENESAS TOOL NEWS on August 1, 2004: RSO-M3T-CC32R-040801D
A Note on Using
C-Compiler Package M3T-CC32R
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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 reading out data in memory using pointers
- Versions Concerned
M3T-CC32R V.1.00 Release 1 through V.4.20 Release 1A
- Description
When several items of data in different addresses in memory are read using a pointer,
one or more of the addresses may be accessed incorrectly.
- 2.1 Conditions
- This problem occurs if the following four conditions are satisfied.
However, depending on how the registers are used for the processing in the
function that contains the statements meeting all the conditions,
this problem may not arise.
| (1) | Any of the optimizing options -O7, -O5, -O3, and -O1, or -Ospace
only or -Otime only is selected. |
| (2) | In the program exists a pointer pointing to an integer not
qualified as volatile. |
| (3) | In the program exists a series of processing including both
of the following, (a) and (b):
| (a) | At least one practice of increasing or decreasing the integer
pointed to by the pointer in (2) by a constant value |
| (b) | At least two practices of indirect references using the
pointer in (2); or indirect references using the pointer and
a constant; or references to subscripts of an array using
the pointer |
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| (4) | The series of processing in (3) includes none of the following:
| (a) | branches |
| (b) | writes to memory using pointers |
| (c) | function calls |
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- 2.2 Example
- Note that in this example the problem may not arise depending on
the contents of the omitted portions in the program.
--------------------------------------------------------------------
void func(unsigned char *ptr_top, unsigned int width)
{
unsigned int x,y;
unsigned int a,b,c;
unsigned char *ptr; /* Condition (2) */
for(y=0; y<width; y++){
ptr = ptr_top + y * width;
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
for(x=0 ; x<64 ; x+=2){
a = *ptr++; /* Conditions (3)-(a), -(b), and (4) */
b = *ptr++; /* Conditions (3)-(a), -(b), and (4) */
c = *ptr++; /* Conditions (3)-(a), -(b), and (4) */
. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ptr -= 3;
*ptr++ = (a << 4) | (b >> 4);
*ptr++ = (a << 4) | (c >> 4);
*ptr++ = c << 4;
}
}
}
--------------------------------------------------------------------
- Workaround
This problem can be circumvented in any of the following ways:
| (1) |
If you are using -O7, -O5, -O3, or -O1, change it to any of these,
-O6, -O4, -O2, and -O0.
If you want to use -Ospace or -Otime, use -O6, -O4, -O2, or -O0
at the same time. |
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| (2) |
Declare the pointer in 2.1-(2) to be volatile.
Circumvention of Condition 2
Original:
--------------------------------------
. . . . . . . . . . . . . . . . . . .
unsigned char *ptr;
. . . . . . . . . . . . . . . . . . .
--------------------------------------
Modified:
--------------------------------------------------------------
. . . . . . . . . . . . . . . . . . .
volatile unsigned char *ptr; /* Declared to be volatile */
. . . . . . . . . . . . . . . . . . .
--------------------------------------------------------------
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| (3) |
Disconnect the practices of increasing or decreasing an integer
value pointed to by a pointer and reads out memory using pointers.
Circumvention of Condition 4
Original:
--------------------------------------
void func( )
. . . . . . . . . . . . . . . . . .
for(x=0 ; x<64 ; x+=2){
a = *ptr++;
b = *ptr++;
c = *ptr++;
. . . . . . . . . . . . . . . . . .
--------------------------------------
ModifiedF
--------------------------------------------------------------
static void dummy(void) { return; } /* Dummy function declared */
void func( )
. . . . . . . . . . . . . . . . . .
for(x=0 ; x<64 ; x+=2){
a = *ptr; /* Value pointed to by a pointer
not increased or decreased */
b = *(ptr+1);
c = *(ptr+2);
dummy(); /* Dummy function inserted */
ptr += 3; /* Here values pointed to by a pointer
increased or decreased at a 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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