Prazek added inline comments.
================
Comment at: lib/CodeGen/CGBlocks.cpp:1318
+ CGM.getModule().getMDKindID("invariant.load"),
+ llvm::MDNode::get(getLLVMContext(), None));
+
----------------
rjmccall wrote:
> yaxunl wrote:
> > rjmccall wrote:
> > > OpenCL blocks are still potentially function-local, right? I don't think
> > > you're allowed to put `invariant.load` on something that's visibly
> > > initialized, even if it's visibly initialized to the same thing every
> > > time. The problem is that `invariant.load` could allow the load to be
> > > hoisted above the initialization.
> > >
> > > If you can solve that problem, you can make this non-OpenCL-specific.
> > It seems that invariant.load implies the pointer is invariant in the whole
> > module, disregarding the store to it, which is not suitable for this case.
> >
> > In this case what I need is that after the first store to the pointer,
> > every load does not change.
> >
> > It seems invariant.group can be used for this.
> Hmm. I guess `invariant.group` does what you need because LLVM will probably
> unify the GEPs to extract the function-pointer field if they appear in the
> same function, and there are no launderings of that pointer.
>
> CC'ing Piotr to get his opinion about whether this is a correct use of
> `invariant.group`. Piotr, we have a struct-typed alloca that we initialize
> in a probably-dominating position. A pointer to the struct can be passed off
> to other contexts which we can't necessarily analyze, blocking normal
> optimization; however, we know that this struct can (mostly) not be legally
> modified after initialization. We're marking one of the initializing stores
> as well as the load of the corresponding field when it's used. These two
> places are completely separate and will perform their own GEPs, but because
> we don't emit any `launder`s on the struct, LLVM will likely unify the GEPs
> if they appear in the same function, allowing `invariant.group`-based
> optimizations to apply. Does this seem reasonable?
Yep, seems totally reasonable. Sorry for such a late response, I just have open
reviews and saw that.
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D53295/new/
https://reviews.llvm.org/D53295
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