<div dir="ltr">+ the new llvm-dev</div><div class="gmail_extra"><br><div class="gmail_quote">On Fri, Aug 7, 2015 at 11:30 AM, Jingyue Wu <span dir="ltr"><<a href="mailto:jingyue@google.com" target="_blank">jingyue@google.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div dir="ltr"><div>Hi folks,</div><div><br></div><div>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. </div><div><br></div><div>This sounds to me an overdue feature. I saw several discussion threads on that direction, but none of them really happened. </div><div><br></div>(1) <a href="http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20111010/129615.html" target="_blank">http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20111010/129615.html</a>. 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. <div><div><br></div><div>(2) <a href="http://lists.cs.uiuc.edu/pipermail/llvmdev/2013-August/064620.html" target="_blank">http://lists.cs.uiuc.edu/pipermail/llvmdev/2013-August/064620.html</a>. 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. <br></div><br>So, can we make BasicAA to consider address spaces? Specifically, I am proposing:</div><div>a) adding a new interface like TTI::addressSpacesAlias(unsigned, unsigned), and</div><div>b) adding a little piece of logic in BasicAA that reports "no alias" if address spaces don't alias. </div><div><br></div><div>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. </div><div><br></div><div>Any comments? </div><span class="HOEnZb"><font color="#888888"><div><br></div><div>Jingyue</div></font></span></div>
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