[llvm-dev] Marking memory as immutable in LLVM IR

[Philip Reames via llvm-dev]

On 07/14/2017 10:15 AM, Przemek Leśniak via llvm-dev wrote:
> Hello LLVM devs,
>
> I am working on LLVM backend for Chapel compiler. I'd like to use 
> 'llvm.invariant.start' and constant type-based alias analysis metadata 
> (tbaa). I have read documentation and looked how clang uses 
> 'llvm.invariant.start' in code generation and it's still not clear to 
> me how to use both of these correctly. The problem I have is when 
> should I use any of them and how are they really different.
>
> Here is one possible example:
>
> void f(int x, int z)
> {
>    const int y = x+g(z);
>    //...
> }
>
> One of the ways to compile above code is to:
>
> %y = alloca i32
> ; ... perform y computation here and store result into %y_tmp
> store i32 %y_tmp, i32* %y
> ; ... continue execution
>
> From now on we have two choices on what to do with it.
>
> 1. I can either use `llvm.invariant.start` on %y to mark that %y is 
> never going to change. In this case, I'm unsure whether I should 
> unmark %y with 'llvm.invariant.end' after I'm done executing.
If you have a location which *never* changes (even before the point of 
use, *no* change can ever be visible to the compiler), then use 
!invariant.load.  If you have a location which never changes *after a 
given point* use invariant.start (without an end).  If you have a 
*region* in which a value is known not to change, then use a start/end 
pair.  Note that it's safest to use these only when the *backing memory 
location* is actually unchanging during the lifetime specified.  The 
rules for reading through one assumed constant pointer while another 
writes to it get hairy quickly; I don't actually even remember what all 
the cornercases are.

> 2. I can mark store and subsequent loads using constant tbaa metadata.
I don't remember if we treat TBAA is-constant as being control 
dependent.  If we don't, then this use is invalid.  You should check.
>
> Now here are few questions I have:
> 1. Should I go with 1, or with 2 in this case? If I have to go with 1, 
> should I unmark memory with llvm.invariant.end after function is done 
> executing?
In your example, the abstract memory location (alloca) will never become 
non-constant.  invariant.start w/out end would be fine here.
> 2. In general, when should I use tbaa const and llvm.invariant? I can 
> think of: global constants, local loop constants, local if constants, 
> constant arguments in function.
This depends greatly on the exact semantics of your source language 
which I don't know and thus can't answer your question.  Please see the 
general advice above.
> 3. Which optimizations does llvm.invariant.start and tbaa impact? How 
> can I possibly check that I've added this metadata correctly and it 
> indeed helps? Possibly by seeing that some optimization occured with 
> new information.
Most memory optimizations.  I know that EarlyCSE and GVN have some 
support, I haven't checked recently to see how complete this is.

Philip

p.s. If you haven't found it already, you might find 
http://llvm.org/docs/Frontend/PerformanceTips.html useful.