[cfe-dev] global alignment

[John Criswell]

On 8/14/12 12:19 PM, John McCall wrote:
> On Aug 14, 2012, at 7:31 AM, Baozeng wrote:
>> Hello all,
>> I found that the alignment for stack value has no limitation, but for global value, it has a limitation.

Baozen, with what version of LLVM did you do the following test? Was it 
LLVM mainline?

>>
>> Here is an example:
>>
>> #include <stdio.h>
>> #include <stdlib.h>
>>
>> char x[4000] __attribute__((aligned(4096)));
>>
>> int
>> main (int argc, char ** argv) {
>>    char y[4000] __attribute__((aligned(4096)));
>>    printf("x is %p\n", x);
>>    printf("y is %p\n", y);
>>    
>>    return 0;
>> }
>>
>> After compiled with clang, the result is:
>> x is 0x804b000
>> y is 0xbf9d8000
>>
>> They are both aligned to be 4096 as we expected.
>>
>> Then we change the example as the following:
>>
>> char x[4000] __attribute__((aligned(8192)));
>>
>> int
>> main (int argc, char ** argv) {
>>    char y[4000] __attribute__((aligned(8192)));
>>    printf("x is %p\n", x);
>>    printf("y is %p\n", y);
>>    
>>    return 0;
>> }
>>
>> The result is :
>> x is 0x804d000
>> y is 0xbffd2000
>>
>> We can see that the stack value y is aligned to be 8192, but the global value x is not!
>>
>> My target OS is 32-bit Linux. Anyone can explain this? or is this a bug of clang?
> It is very unlikely to be a frontend bug, although I suppose it's possible that it's a backend problem.
>
> It is much more likely that your environment just doesn't support aligning symbols at granularities larger than a page.  The page size on x86-32 is 4K because that's what the hardware supports (or 4M if you're using PSE, which Linux does support, but it's not the default, and I don't think it'd solve your problem).

I don't think that's the issue.  I've had problems aligning global 
variables on a 4096 byte boundary with Clang/LLVM 3.0.  Using 
hand-written assembly code to force the alignment fixed the issue.

Additionally, OS page alignment has nothing to do with symbol 
alignment.  Symbol alignment is done by the loader/linker, and from what 
I've seen, ELF places no restriction on alignment size (other than it 
being a 32-bit or 64-bit value).

I suspect what we're seeing is a bug in the backend; we'd like to know 
(if possible) if someone is aware of the limitation.

>
> The stack example works because it has to dynamically realign anyway, i.e. it's just %ing the stack pointer, which has no inherent limitations.  Note that this is quite expensive and is leaving a potentially huge gap on your stack.
>
> I'll admit to being curious as to why you actually need larger-than-page-size alignment.

Baggy Bounds Checking: 
http://static.usenix.org/event/sec09/tech/full_papers/sec09_memory.pdf

-- John T.