[Bug target/21387] Unaligned writes on MIPSEL systems after typecast

[maarten at contemplated dot nl]

--- Additional Comments From maarten at contemplated dot nl  2005-05-05 
11:56 ---
(In reply to comment #2)

Reading your reply further, I understand that the behavior I observere is
correct and related to the fact that the 'int32_t' type is assumed to be 
aligned. 

It is not a bug then, but merely a bit of a problem for me. Is there a typecast
to tell the compiler that the adress references an unaligned integer, i.e.
something like int32_unaligned_t?

-- 
   What|Removed |Added

 Status|WAITING |SUSPENDED


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21387

[Bug target/21387] Unaligned writes on MIPSEL systems after typecast

[maarten at contemplated dot nl]

--- Additional Comments From maarten at contemplated dot nl  2005-05-05 
11:49 ---
(In reply to comment #2)

You are completely right, the code above merely demonstrates what happens when
one writes to an illegal address. The correct version,

*((int32_t *) &a.unaligned_int32) = 0x123456;

does work. Sorry, result of 'compressing' the problem.

My 'unaligned write' problem is, however, still there. I stumbled over it when
trying to compile code where copying of 16 bytes (in a char array) was
implemented as two reads and writes of int64_t. The problem occurs when
typecasting char arrays to integer types, see modified example:

1: 

#include 

int main(int argc, char *argv[]) {
char x[100];

*((int32_t *) ((char *) (x))) = 0x123456;   // ok
*((int32_t *) ((char *) (x+4))) = 0x123456; // ok
*((int32_t *) ((char *) (x+1))) = 0x123456; // fails

return(0);
}

2: (more related to original)

#include 

#pragma pack(1)
typedef struct  {
int8_t  byte;   // removing this byte, code executes
char unaligned_int32[4];
} test_unaligned;
#pragma pack()

int main(int argc, char *argv[]) {
test_unaligned a;

*((int32_t *) a.unaligned_int32) = 0x123456;// fails
 
return(0);
}



-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21387

[Bug c++/21387] New: Unaligned writes on MIPSEL systems after typecast

[maarten at contemplated dot nl]

See code below. After type-casting, the compiler incorrectly assumes that
'a.unaligned_int' is aligned on a word boundary. On a MIPSEL system (here a
Cobalt RaQ 2) a 'sw' assembler instruction is generated to store the value in
memory, resulting in a 'Segmentation fault'. Changing the 'sw' instruction to
'swl', the problem is solved.


#include 

#pragma pack(1)
typedef struct  {
int8_t  byte;
int32_t unaligned_int;
} test_unaligned;
#pragma pack()

int main(int argc, char *argv[]) {
test_unaligned a;

*((int32_t *) a.unaligned_int) = 0x123456;

return(0);
}

-- 
   Summary: Unaligned writes on MIPSEL systems after typecast
   Product: gcc
   Version: 3.3.3
Status: UNCONFIRMED
  Severity: normal
  Priority: P2
 Component: c++
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: maarten at contemplated dot nl
CC: gcc-bugs at gcc dot gnu dot org
 GCC build triplet: mipsel-netbsd2.0
  GCC host triplet: mipsel-netbsd2.0
GCC target triplet: mipsel-netbsd2.0


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21387