[LLVMdev] c const

[Christopher Lamb]

On Aug 21, 2007, at 10:17 AM, Duncan Sands wrote:

> Hi Christopher,
>
>>> it looks like noalias might be very useful for Ada: for certain  
>>> types,
>>> such as array types, the language standard explicitly allows the
>>> compiler
>>> to pretend that a formal argument of that type is passed by-copy (=
>>> by-value)
>>> for alias analysis purposes, while actually passing it by-reference
>>> (= via a pointer).
>>> I'm not sure, but based on Chris's suggested implementation
>>> http://lists.cs.uiuc.edu/pipermail/llvmdev/2007-March/008323.html
>>> it seems that this may map exactly to "noalias".  There is a  
>>> subtlety
>>> though: the compiler may pretend that a copy was passed in, but it
>>> must
>>> correspond to a copy made at the moment of the function call, not
>>> later.
>>>
>>> In Chris's description
>>> " The semantics of 'noalias' are that they "define a new object" "
>>> it is not specified *when* the new object gets its value; Ada  
>>> requires
>>> the pretend "new object" to act as if it got its value at the  
>>> start of
>>> the function body, not later, as if a copy of the real object was  
>>> made
>>> at that point.
>>
>> I can't see how the semantics could be anything other than for the
>> "new object" that the noalias argument points to could be created at
>> any other time than the beginning of the function.

Please excuse this atrocious sentence. It's completely misleading (I  
don't drink coffee, but maybe I should start?)

It should read: I can't see how the semantics could be anything but  
for the "new object" to be created at the beginning of the function.

I merged the wrong parts of two sentences in my earlier email. Sorry  
for the grief!

--
Christopher Lamb



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