[llvm-dev] invariant.load metadata semantics

[Geoff Berry via llvm-dev]

I'm working on enhancing EarlyCSE to use MemorySSA and have come across 
the following issue due to differences in EarlyCSE and MemorySSA's 
handling of !invariant.load.  EarlyCSE will *not* currently optimize the 
following code by replacing %x2 with %x and removing the second load:

    B1:

       %x = load %p

       clobber()

    ...

    B2: // dominated by B1

       %x2 = load %p !invariant.load


Sanjoy (who added the !invariant.load support to EarlyCSE) and I 
discussed this, and I believe we are both in agreement that this 
optimization should be legal.  I'd like to make sure there is agreement 
on this and possibly clarify the LangRef wording on !invariant.load to 
make the legality of this transformation more clear.

Sanjoy suggested the following:

    Instead of "The existence of the !invariant.load metadata on the
    instruction tells the optimizer and code generator that the address
    operand to this load points to memory which can be assumed
    unchanged." we say "It is undefined behavior to invariant_load from
    a location that has been changed since it became dereferenceable". 
    In the current langref, I find "The existence" somewhat confusing,
    since it seems to imply that adding dead code can change the
    behavior of the program.

    I don't want to specify the semantics in a way that:

        int* ptr = ...
        int k0 = *ptr; // normal load
        clobber();
        int k1 = *ptr; // normal load

    has a different meaning than

        int* ptr = ...
        int k0 = *ptr; // normal load
        clobber();
        int k1 = *ptr; // normal load
        if (<always false>) {
          int k2 = *ptr; // !invariant load
        }

    That is, adding dead code should not change the behavior of the
    program -- the code guarded by (<always false>) should be able to have
    any amount of junk without breaking the program, since it does not
    actually execute.


Does this seem like a clearer wording of the intended semantics?

-- 
Geoff Berry
Employee of Qualcomm Datacenter Technologies, Inc.
  Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm Technologies, Inc.  Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.

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