[LLVMdev] Reviving the new LLVM concurrency model

Jianzhou Zhao jianzhou at seas.upenn.edu
Sun Jul 31 12:49:49 PDT 2011

I noticed the patch was already merged into the current LLVM language
reference manual with new memory instructions, fence, cmpxchg and
atomicrmw. Will the instructions be available in LLVM 3.0?

On Mon, Jul 18, 2011 at 8:22 PM, Eli Friedman <eli.friedman at gmail.com> wrote:
> There was some discussion a while back about adding a C++0x-style
> memory model and atomics for LLVM a while back
> (http://thread.gmane.org/gmane.comp.compilers.llvm.devel/31295), but
> it got stalled.  I'm going to try and restart progress on it.
> Attached are two patches; the first adds a section to LangRef with
> just the memory model, without directly changing the documentation or
> implementation of atomic intrinsics.  This mostly comes from
> https://docs.google.com/View?docID=ddb4mhxz_22dz5g98dd&revision=_latest,
> but it's been modified a bit, so please look at the attached version.
> The second fixes the one place I know of where LLVM violates that
> proposed memory model.
> I would appreciate any reviews (primarily for the LangRef bits; I'm
> reasonably confident the patch to LICM is correct).
> There was some discussion about allowing targets to implement
> unaligned stores by widening, but I've left that out.  No current
> backend uses such an implementation, so there isn't a substantial
> benefit, and allowing it would break some current optimizations (like
> transforming a memset to an unaligned store).  See also the FIXME in
> the LangRef patch about architectures without a proper single-byte
> store instruction.
> -Eli
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