Hi, trying to come up with a testcase we figured out what the problem could be.
When the optimizer is on and memcpy sees that it is copying a struct
with double words in it, it will assume that the struct starts on an 8
byte boundary and use double word loads and stores. This is a safe
assumption, as gcc will always ensure that structs containing doubles
start on an 8 byte boundary when the memory is mallocced.
However we managed to trick gcc by mallocing a large chunk of memory
and then assigning a pointer to a user data (unsigned int user[0])
without first ensuring that the user data was 8 byte aligned. Since
the structure does contain a double, this resulted in a crash in
memcopy.
The fix for this was to inform the compiler that this "void" pointer
should be 8 byte aligned by changing the "unsigned int user[0]" to a
"unsigned long long user[0]". This will cause gcc to pad this entry
out to ensure that it starts on an 8 byte boundary.
Does this make sense?
Alex
On 8/9/07, Alex Gonzalez <[EMAIL PROTECTED]> wrote:
> Hi,
>
> I'll try to come up with a short test.
>
> I have narrowed it a bit more. The PVAR structure contains a long long
> variable ( with a sizeof 8 and an alignof 8 for my architecture). If I
> take out the long long variable, the compiler uses sdl instructions
> instead of sd and the exception doesn't happen.
>
> Also, if I do
>
> static void varcopy(void *pvar1, void *pvar2)
>
> the compiler uses sdl and avoids the crash.
>
> I am compiling for n32 ABI, so the register size is 64bits.
>
> Any ideas?
>
> On 8/9/07, David Daney <[EMAIL PROTECTED]> wrote:
> > Alex Gonzalez wrote:
> > > Hi,
> > >
> > > I am seeing an address error exception caused by the gcc optimizer -O1.
> > >
> > > I have narrowed it down to the following function:
> > >
> > > static void varcopy(PVAR *pvar1, PVAR *pvar2) {
> > > memcpy(pvar1,pvar2,sizeof(PVAR));
> > > }
> > >
> > > Being the sizeof(PVAR) 160 bytes.
> > >
> > > The exception is caused on an sd instruction when the input is not
> > > aligned on a doubleword boundary.
> > >
> > > I was under the assumption that the compiler made sure that it doesn't
> > > store a doubleword that is not aligned on a doubleword boundary. Is
> > > this a bug in the optimizer?
> > >
> > > I am using a gcc mips64 cross-compiler,
> > >
> > > mips64-linux-gnu-gcc (GCC) 3.3-mips64linux-031001
> > >
> > > Has anyone experienced this problem before?
> > >
> > In order to investigate we would need a self contained test case (i.e.
> > the definition of PVAR must be included). Also it would be nice if you
> > could try it on a current version of GCC (4.2.1 perhaps).
> >
> > David Daney
> >
>
