[llvm-dev] [RFC] BasicAA considers address spaces?
Philip Reames via llvm-dev
llvm-dev at lists.llvm.org
Fri Aug 7 11:56:53 PDT 2015
+1 to the general idea of a target dependent hook for describing
aliasing of address spaces. I have no opinion on the particular
implementation strategy proposed.
On 08/07/2015 11:35 AM, Jingyue Wu via llvm-dev wrote:
> + the new llvm-dev
> On Fri, Aug 7, 2015 at 11:30 AM, Jingyue Wu <jingyue at google.com
> <mailto:jingyue at google.com>> wrote:
> Hi folks,
> Unsurprisingly, leveraging the fact that certain address spaces
> don't alias can significantly improve alias analysis precision and
> enhance (observably 2x performance gain) load/store optimizations
> such as LICM and DSE.
> This sounds to me an overdue feature. I saw several discussion
> threads on that direction, but none of them really happened.
> Justin Holewinski proposed to add an address-space alias analysis
> that treats pointers in different address spaces not aliasing.
> This patch got shot down because, in some targets, address spaces
> may alias. For example, in CUDA+NVPTX, addrspace(0) aliases everyone.
> http://lists.cs.uiuc.edu/pipermail/llvmdev/2013-August/064620.html. Michele
> Scandale proposed address space extensions in IR metadata which
> TBAA could then leverage to prove non-aliasing. This approach
> didn't fly either because address spaces are target-specific. The
> front end doesn't know enough to decide aliasing.
> So, can we make BasicAA to consider address spaces? Specifically,
> I am proposing:
> a) adding a new interface like TTI::addressSpacesAlias(unsigned,
> unsigned), and
> b) adding a little piece of logic in BasicAA that reports "no
> alias" if address spaces don't alias.
> This approach addresses the issue brought up in (2) because TTI
> can see the entire codegen. It also resolves the issue that shut
> down (1) because it allows address spaces to alias in a
> target-defined way. Actually, John Criswell did mention in that
> thread the idea of embedding alias info in TargetData. Now that we
> have TTI, it seems a better place to hold target-specific alias
> info than DataLayout.
> Any comments?
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org http://llvm.cs.uiuc.edu
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