[LLVMdev] c const

[Ryan M. Lefever]

I guess Bill Wendling cleared it up a little more for me.  Neither llvm 
nor gcc ignore const.  The type checking in each of their frontends 
makes sure that const is not violated.

The reason I was asking about const is as follows.  I was under the 
impression that const was part of c to aid the compiler with 
optimization and not just for type checking purposes.  As you previously 
pointed out, I could get mod info from the llvm alias analysis 
interface.  However, I was thinking why go through the expense of 
computing alias analysis and bother with the imprecision of alias 
analysis, if the programmer is going to tell you that the memory pointed 
to by a pointer argument is never written.

Chris Lattner wrote:
> I don't follow what you mean - gcc doesn't ignore const and llvm  
> doesn't deviate from gcc nor from the relevant language standards.   
> Note that if you declare a global as const that we do capture this in  
> the ir - what specifically do you want?  Please provide an example.
> 
> -Chris
> 
> http://nondot.org/sabre
> http://llvm.org
> 
> On Aug 14, 2007, at 11:58 PM, "Ryan M. Lefever"  
> <lefever at crhc.uiuc.edu> wrote:
> 
> 
>>I don't mean to be a pain, but I was thinking about this a bit more.
>>Does gcc ignore the const keyword?  If not, why has LLVM chosen to
>>deviate from gcc with respect to the const keyword?  If so, then why  
>>do
>>we bother using const in LLVM API code?  I'm just curious and wanted  
>>to
>>understand the thinking behind not preserving const.
>>
>>Thanks,
>>Ryan
>>
>>Chris Lattner 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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>>>>http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>>
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>>
>>-- 
>>Ryan M. Lefever  [http://www.crhc.uiuc.edu/~lefever/index.html]
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> 
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-- 
Ryan M. Lefever  [http://www.crhc.uiuc.edu/~lefever/index.html]