On 5/9/12 2:01 AM, Dan Gohman wrote:> > On May 7, 2012, at 11:11 PM, Ralf Karrenberg<Chareos at gmx.de> wrote: > >> Hi Dan, >> >> I am using it to store results of a vectorization analysis. A BasicBlock has certain properties in this context, e.g. we mark control flow that may never diverge in different instances ("threads" if you think in terms of CUDA) of the same function by marking the corresponding blocks. This information is later used when linearizing the function (control flow to data flow conversion). I'll be happy to give you more detail on this if you want to :). > > Why are you using metadata to store the results of an analysis? > LLVM has infrastructure for running analysis passes and making > their information available to other passes.The analysis is only one way to supply the necessary information to the vectorizer - it could also be generated by a front-end directly. This is a very likely use-case for data-parallel languages that have specific constructs like "uniform"/"varying" (e.g. RenderMan in graphics). Metadata is the perfect thing to store this kind of information.>> I could imagine there are other things that could make use of this, or am I wrong with that? > > There are surely many things it could be used for. Interesting > questions include whether or not there are other ways to > achieve those things, and whether making a basic block be > something which can carry special semantics is a concept that > makes sense within the rest of the system.That is why I brought this up here for discussion with those of you people that know more about the implications that such functionality would have. Cheers, Ralf
On May 8, 2012, at 11:37 PM, Ralf Karrenberg wrote:> On 5/9/12 2:01 AM, Dan Gohman wrote: >> >> On May 7, 2012, at 11:11 PM, Ralf Karrenberg<Chareos at gmx.de> wrote: >> >>> Hi Dan, >>> >>> I am using it to store results of a vectorization analysis. A BasicBlock has certain properties in this context, e.g. we mark control flow that may never diverge in different instances ("threads" if you think in terms of CUDA) of the same function by marking the corresponding blocks. This information is later used when linearizing the function (control flow to data flow conversion). I'll be happy to give you more detail on this if you want to :). >> >> Why are you using metadata to store the results of an analysis? >> LLVM has infrastructure for running analysis passes and making >> their information available to other passes. > > The analysis is only one way to supply the necessary information to the vectorizer - it could also be generated by a front-end directly. This is a very likely use-case for data-parallel languages that have specific constructs like "uniform"/"varying" (e.g. RenderMan in graphics). Metadata is the perfect thing to store this kind of information.I'm not a graphics expert, but my understanding of "uniform/varying" is that it's really a property of values, the values which determine which basic blocks are executed, rather than of the basic blocks themselves. Is that true? Dan
On 5/9/12 5:08 PM, Dan Gohman wrote:> > On May 8, 2012, at 11:37 PM, Ralf Karrenberg wrote: > >> On 5/9/12 2:01 AM, Dan Gohman wrote: >>> >>> On May 7, 2012, at 11:11 PM, Ralf Karrenberg<Chareos at gmx.de> wrote: >>> >>>> Hi Dan, >>>> >>>> I am using it to store results of a vectorization analysis. A BasicBlock has certain properties in this context, e.g. we mark control flow that may never diverge in different instances ("threads" if you think in terms of CUDA) of the same function by marking the corresponding blocks. This information is later used when linearizing the function (control flow to data flow conversion). I'll be happy to give you more detail on this if you want to :). >>> >>> Why are you using metadata to store the results of an analysis? >>> LLVM has infrastructure for running analysis passes and making >>> their information available to other passes. >> >> The analysis is only one way to supply the necessary information to the vectorizer - it could also be generated by a front-end directly. This is a very likely use-case for data-parallel languages that have specific constructs like "uniform"/"varying" (e.g. RenderMan in graphics). Metadata is the perfect thing to store this kind of information. > > I'm not a graphics expert, but my understanding of "uniform/varying" is that it's really > a property of values, the values which determine which basic blocks are executed, rather > than of the basic blocks themselves. Is that true?That is true, sorry for that confusing sentence. When vectorizing an entire function to exploit data-level parallelism, normally the whole function is linearized during control-flow to data-flow conversion. Our analysis [1] is able to determines cases in which parts of the CFG can be excluded from linearization ("non-divergent" blocks). The basis for this is indeed the uniform/varying property of branch conditions, but the criterion is block-specific and thus naturally attaches to basic blocks. Also, the block properties influence the uniform/varying property of values (e.g. a phi with uniform incoming values that is in a block where paths from a varying branch join). Best, Ralf [1] http://dx.doi.org/10.1007/978-3-642-28652-0_1