[llvm-dev] if-conversion

[RCU via llvm-dev]

   Hi.
     I'm trying to vectorize the following piece of code with Loop Vectorizer (from LLVM 
distribution Nov 2015), but no vectorization takes place:
         int *Test(int *res, int *c, int *d, int *p) {
             int i;

             for (i = 0; i < 16; i++) {
                 //res[i] = (p[i] == 0) ? c[i] : d[i];
                 res[i] = (p[i] == 0) ? res[i] : res[i] + d[i];
             }

             return NULL;
         }

     It seems the problem is the if conversion, which is not working on this simple 
example, although I guess it should.
     Is it a problem of low profitability given the loads?

     Could you please tell me if there is any way to make If conversion work on programs 
like this.


     Below is an older message from 2013 about the status of if conversion from LLVM at 
that date (from http://lists.llvm.org/pipermail/llvm-dev/2013-November/067427.html or 
https://groups.google.com/forum/#!msg/llvm-dev/FlDGnqSGbR8/YZUN3h4IzA8J ).
 > Hi Rob,
 >
 > I chose to answer on the list since from time to time people come back
 > to this.
 >
 > That said, I did implement the generic variant of if-conversion that is
 > called "control-flow to data-flow conversion" as a basis for SIMD
 > vectorization. Essentially, the conversion removes all control flow
 > except for loop back edges and replaces it by masks and blend (select)
 > operations.
 >
 > Details on the algorithm can be found in our paper on "Whole-Function
 > Vectorization" (CGO 2011). The old, LLVM-based implementation of the
 > algorithm is still online at github I believe. A completely rewritten
 > one will be released along with submission of my PhD thesis at the end
 > of the year.
 >
 > That said, if you are only looking for if-conversion of code with
 > limited complexity (e.g. no side effects, no loops), it is really simple:
 > - Compute masks for every block (entry mask: disjunction of incoming
 > masks, exit masks: conjunctions of entry mask and (negated) condition).
 > - Replace each phi by a select that uses the entry mask of the
 > corresponding block.
 > - Order blocks topologically by data dependencies, remove outgoing
 > edges, create unconditional branches from each block to the next in the
 > list.
 >
 > Cheers,
 > Ralf
 >
 >
 > On 06/11/13 19:50, RobBishop wrote:
 >  > Hi all,
 >  >
 >  > Sorry to dig up an old thread but I wondered what the status of
 >  > if-conversion in LLVM is. Has any work been done towards handling this as a
 >  > transform pass on the IR?
 >  >
 >  > I'm looking to implement an if-conversion pass and wanted to ensure that I'm
 >  > not duplicating work. Is this something that others would also find useful?
 >  >
 >  > Rob
 >  >
 >  >
 >  >
 >  > --
 >  > View this message in context:
 > http://llvm.1065342.n5.nabble.com/if-conversion-tp2349p62937.html
 >  > Sent from the LLVM - Dev mailing list archive at Nabble.com.
 >  > _______________________________________________
 >  > LLVM Developers mailing list
 >  > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu
 >  > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev


  Thank you,
     Alex