[llvm-dev] Attribute for a invariant function ?

[Nat! via llvm-dev]

I have a read-only lookup table, which takes integer arguments. 
Optimally I would like to indicate in the C source, that the function is 
invariant (?), so that the superflous calls can be optimized away.

```
extern void  *lookup( int a);

void   foo( void)
{
    void   *a, *b;

    a = lookup( 0x12345678);
    bar( a);

    b = lookup( 0x12345678);  // b guaranteed to be the same as a, no 
call to (slow) lookup needed
    baz( b);
}
```

But I don't really find anything in clang or llvm in terms of function 
attributes. The best I could come up with is in the llvm code to use 
attributes "argmemonly=true" readonly=true". But still two calls did 
happen. (I am on 3.7)

Ciao
    Nat!

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