[cfe-dev] [llvm-dev] generate vectorized code

Rail Shafigulin via cfe-dev cfe-dev at lists.llvm.org
Fri Mar 18 13:37:46 PDT 2016

> I think you created a cycle, this is easy to do with SelectionDAG :)
> Basically SelecitonDAG will iterate until it does not see anything to
> change. So if you insert a transformation on a pattern A, that generates
> pattern B, while you have another transformation that matches B and
> generates somehow A, you run into an infinite loop.
> I'm doing a lot of guess work in trying to understand what is going on. I
> would really appreciate any help on this.
> Here is how I started with SelectionDAG:
> - small IR (bugpoint can help)
> - the magic flag: -debug
> - read the output of SelectionDAG debugging (especially with cycles)
> - matching the log to source code
> - single stepping in a debugger sometimes.
> Also: try to run your experiments with llc so you can easily tweak the
> input IR to SelectionDAG.
> --
> Mehdi

I ran a very simple test using llc and the following .ll file
target datalayout = "E-m:e-p:32:32-i64:32-f64:32-v64:32-v128:32-a:0:32-n32"
target triple = "esencia"

; Function Attrs: nounwind uwtable
define i32 @main() {
   %z = alloca <4 x i32>
   %a = alloca <4 x i32>
   %b = alloca <4 x i32>
   %a.l = load <4 x i32>* %a
   %b.l = load <4 x i32>* %b
   %z.l = add <4 x i32> %a.l, %b.l
   store <4 x i32> %z.l, <4 x i32>* %z
   ret i32 0

The test ran successfully (by successfully I mean genration of correct
assembly for my target) without any modifications to the code, i.e. I
didn't have to add any
  setOperationAction(ISD::BUILD_VECTOR,      MVT::v4i32, Expand);
  setOperationAction(ISD::EXTRACT_VECTOR_ELT,      MVT::v4i32, Expand);
  setOperationAction(ISD::VECTOR_SHUFFLE,      MVT::v4i32, Expand);

In other words I left the code as is.

However if I use a .c code and run it through clang, I don't see any vector
instructions. I'm puzzled. What am I doing wrong? There seems to be a step
missing, the one that will generate vectorized IR, but I can't seem to find
how to do it.

Any help on this is really appreciated.

Rail Shafigulin
Software Engineer
Esencia Technologies
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