[LLVMdev] Add a new llvm intrinsic?

[Owen Anderson]

Hi Jeff,

Do you know if anything came of this?  I understand we may need to seek clarification to get a formal answer, particularly with respect to C, but it seems pretty clear to me that this is a significant QoI issue, both for C and CL.  LLVM is effectively hoisting a load above a thread-join.  This may or may not technically allowed in C, but it seems generally undesirable, and it’s extremely undesirable in CL where these kinds of thread joins are a fundamental of the programming model.

—Owen


On Aug 6, 2013, at 5:36 PM, Jeffrey Yasskin <jyasskin at googlers.com> wrote:

> Chandler pointed out another interpretation of C11/6.7.3.1, in which
> 'restrict' only addresses aliasing within a single thread. If that's
> the right interpretation, then it's a bug in LLVM that it moves
> noalias pointers across memory-ordering operations at all, and you
> still don't need a new fence, just a bug fix.
> 
> 6.7.3.1 says "During each execution of B, ...". "During" could either
> mean just within the same thread or within any segment of a thread
> that doesn't happen-before or happen-after B.
> 
> It's a defect in C that this is ambiguous. Anyone want to volunteer to
> send it to the committee? (I'll be happy to proofread, etc., just not
> be in charge of finding the right email target)
> 
> On Tue, Aug 6, 2013 at 5:01 PM, Jeffrey Yasskin <jyasskin at googlers.com> wrote:
>> This sounds a lot like the question at
>> http://lists.cs.uiuc.edu/pipermail/llvmdev/2013-July/064462.html. It
>> sounds like you have a pointer marked 'restrict', but it's actually
>> aliased in another thread. That would be undefined behavior even with
>> a stronger fence.
>> 
>> On Tue, Aug 6, 2013 at 4:56 PM, Guo, Xiaoyi <Xiaoyi.Guo at amd.com> wrote:
>>> Hi,
>>> 
>>> In OpenCL, the "barrier()" function, as well as various target specific memory fence intrinsics, should prevent loads/stores of the relevant address space from being moved across them.
>>> Kernel pointers with "restrict" attributes are implemented by marking the pointer "noalias" in the LLVMIR. However, in LLVM, "noalias" pointers are not affected by llvm memory fence instructions.
>>> 
>>> To make sure all loads/stores, including those accessing "restrict" pointers are not moved across the barrier/fence intrinsics, we have considered using customized alias analysis passes. However, we would like to move away from using customized passes and would like to use standard llvm mechanisms as much as possible.
>>> 
>>> What do people think about adding an llvm intrinsic, something like llvm.opencl.mem_fence(i32) (or named something that doesn't have opencl in the name, llvm.addrspace_fence?), which acts as a fence for a single given address space (assuming again that there's no problem with implementing these things as a series of different functions to get the full effect), and which prevents even noalias pointers from being moved across it?
>>> 
>>> Alternatively (possibly nicer) would be something that looks like the memset intrinsic, which can work for any address space.
>>> llvm.addrspace_fence.p1.p2(void)
>>> llvm.addrspace_fence.p1(void) ...
>>> 
>>> Thanks,
>>> Xiaoyi
>>> 
>>> 
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