[LLVMdev] c const

[Chris Lattner]

On Thu, 9 Aug 2007, Daniel Berlin wrote:
> This certainly doesn't occur in gcc mainline.
> In fact, I improved the error message, and added a error test to gcc
> just yesterday.

Yep, clang reports:

t.c:4:12: error: read-only variable is not assignable
     arr[0] = 1;
     ~~~~~~ ^
1 diagnostic generated.

so this is specific to llvm-gcc somehow.

-Chris

> On 8/9/07, Chris Lattner <sabre at nondot.org> wrote:
>> On Wed, 8 Aug 2007, Nikhil A. Patil wrote:
>>> I think I found a bug. I don't know if it's in upstream gcc or llvm-gcc4.
>>
>> Looks like a bug, please file a bugzilla entry.
>>
>> -Chris
>>
>>> int func()
>>> {
>>>    const int *arr;
>>>    arr[0] = 1;
>>> }
>>>
>>> $ llvm-gcc main.c -c; echo $?
>>> 0
>>>
>>> $ gcc main.c -c
>>> main.c: In function 'func':
>>> main.c:4: error: assignment of read-only location
>>>
>>> The difference disappears when arr[0] is replaced by *arr.
>>>
>>> (I tried the above with gcc 4.1.2, 3.4.6, 4.0.3. (I don't have access
>>> to 4.0.1, off which llvm-gcc seems to be derived.)
>>>
>>> nikhil
>>>
>>> On 08/08/2007, Chris Lattner <sabre at nondot.org> wrote:
>>>> This property isn't preserved on the llvm ir, because const can always
>>>> be cast away.  If you want mod information, then I suggest using the
>>>> aliasanalysis interface to get mod ref info for a call.
>>>>
>>>> -Chris
>>>>
>>>> http://nondot.org/sabre
>>>> http://llvm.org
>>>>
>>>> On Aug 8, 2007, at 12:07 AM, "Ryan M. Lefever" <lefever at crhc.uiuc.edu>
>>>> wrote:
>>>>
>>>>> How is c's const keyword translated when compiling c into llvm
>>>>> bytecode.
>>>>>  I'm specifically interested in const pointer function arguments.
>>>>> Consider a function declared as follows in c:
>>>>>
>>>>> void f(const int* arg);
>>>>>
>>>>> When I examine f in llvm bytecode, how can I tell that arg is a
>>>>> pointer,
>>>>> whose contents can only be read, not written.
>>>>>
>>>>> Regards,
>>>>> Ryan
>>>>> _______________________________________________
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>>
>> -Chris
>>
>> --
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-Chris

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