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<div class="moz-cite-prefix">On 8/14/12 9:31 AM, Baozeng wrote:<br>
</div>
<blockquote
cite="mid:CAP=GMUHeCuWdQTUiL2LTGQJy+2jGHu0KOT=dyNsJ92O=mbDpnw@mail.gmail.com"
type="cite">
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Hello all,<br>
I found that the alignment for stack value has no limitation, but
for global value, it has a limitation.<br>
</blockquote>
<br>
So here's what I found:<br>
<br>
1) We already know from Baozeng's experiments that LLVM 3.0 and
Clang 3.0 do not align globals properly when the alignment is large.<br>
<br>
2) LLVM 3.1 will properly align globals with alignment up to 0x10000
(which is larger than the 8192 byte alignment given in Baozeng's
original example). However, Clang 3.1 will assert out because it
uses an unsigned short for representing alignments of lvalues.
Changing the alignment class member to unsigned long magically fixes
everything, and the code will align variables up to 0x10000
properly.<br>
<br>
3) LLVM/Clang mainline have the same behavior as LLVM 3.1.<br>
<br>
Baozeng, for your Baggy Bounds Checking work, the upgrade to LLVM
mainline that SAFECode is currently undergoing will just fix the
problem that you're seeing. You and I can discuss separately
whether to make the LLVM mainline switch for BBC during GSoC.<br>
<br>
As for the bug in Clang, I'll file a bug report.<br>
<br>
-- John T.<br>
<br>
<br>
<blockquote
cite="mid:CAP=GMUHeCuWdQTUiL2LTGQJy+2jGHu0KOT=dyNsJ92O=mbDpnw@mail.gmail.com"
type="cite"><br>
Here is an example:<br>
<br>
#include <stdio.h><br>
#include <stdlib.h><br>
<br>
char x[4000] __attribute__((aligned(4096)));<br>
<br>
int<br>
main (int argc, char ** argv) {<br>
char y[4000] __attribute__((aligned(4096)));<br>
printf("x is %p\n", x);<br>
printf("y is %p\n", y);<br>
<br>
return 0;<br>
}<br>
<br>
After compiled with clang, the result is:<br>
x is 0x804b000<br>
y is 0xbf9d8000<br>
<br>
They are both aligned to be 4096 as we expected.<br>
<br>
Then we change the example as the following:<br>
<br>
char x[4000] __attribute__((aligned(8192)));<br>
<br>
int<br>
main (int argc, char ** argv) {<br>
char y[4000] __attribute__((aligned(8192)));<br>
printf("x is %p\n", x);<br>
printf("y is %p\n", y);<br>
<br>
return 0;<br>
}<br>
<br>
The result is :<br>
x is 0x804d000<br>
y is 0xbffd2000<br>
<br>
We can see that the stack value y is aligned to be 8192, but the
global value x is not!<br>
<br>
My target OS is 32-bit Linux. Anyone can explain this? or is this
a bug of clang? <br>
<br>
<br>
<br>
<br>
-- <br>
Best Regards,<br>
Baozeng Ding<br>
OSTG,NFS,ISCAS<br>
<br>
<br>
<br>
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