Re: Problems migrating to gcc 4.4.3&eabi - apparently a gcc bug

[Andrew Haley <aph@redhat.com>]

On 03/09/2010 06:54 PM, Thomas Martitz wrote:
> Am 09.03.2010 19:36, schrieb Andrew Haley:
>> That does not surprise me.  I think you're seeing a problem that is
>> caused by something elsewhere in your program.  I'm guessing that
>> there may be a bad prototype or somesuch.
>>
>> I think you need to strip down your sources until you find something.
>>
>> Maybe you should try -save-temps and have a look at the actual
>> preprocessd source.  Maybe some bastard has done
>>
>> #define int long
>>
>> or something evil like that!
>>
>> Andrew.
>>    
> 
> No, I know our codebase pretty well. This is not the problem. Not that
> int or long matters, anyway.
> 
> In the mean time we found a test case:
> 
> ----
> void foo(int last, char * block);
> 
> void bar(void)
> {
>     struct {
>         char * __attribute__((aligned(8))) member;
>     } s;
> 
>     foo(0,s.member);
> }
> ----
> 
> compiled with arm-elf-eabi-gcc -c test.c
> 
> This example exposes the problem.
> 
> We found the problem is related to struct addressing and the aligned
> attribute.
> 
> - normal stack variables work
> - struct members work with __attribute__((aligned(4)))
> - struct members with __attribute__((aligned(X))) where X >= 8 *do not*
> work.
> 
> Look at the assembly output for this very example. block is passed in
> r2, while it's supposed to be passed in r1.
> 
> Our temporary "fix" is to make it "void foo(int last, volatile char *
> block);" [notice the volatile keyword] and it works as well (block
> passed in r1).
> 
> This is definitely a gcc bug. The generated call is dependent on the
> parameter passed. The callee can't know about this. And it also happens
> with -O0.

Yeah, it's a bug.  Mind you, it's pretty bizarre code: I'm not surprised
we never came across this before.

Why are you doing this, anyway?  I would have expected

char __attribute__((aligned(8))) *member;

for a pointer to an 8-aligned buffer...

Andrew.