[llvm-dev] Marking memory as immutable in LLVM IR
Philip Reames via llvm-dev
llvm-dev at lists.llvm.org
Sat Jul 15 13:18:33 PDT 2017
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
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.
p.s. If you haven't found it already, you might find
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