[llvm-dev] SLP example not being vectorized

[Adrien Guinet via llvm-dev]

On 11/28/19 6:45 PM, Sandoval Gonzalez, Leonardo via llvm-dev wrote:
> Hi,
> 
> I am new to llvm with a particular interested in the optimization area,
> specially on SLP. While working through the tutorial, I ran this example
> [1] with the hope to see SLP vectorization in action but for some
> reason, I do not see it on the LLVM assembly as seen below. Is there
> anything I am missing? I am using Clearlinux as build machine and this
> has clang version 9.0.0.
> 


If you're on Intel hardware, I'd say that AFAIK there is no vectorial
integer division instruction, so LLVM won't vectorize this code.
Moreover, you should specify which instruction set you want to use,
whether by specifying the CPU architecure with -march, or by activating
various instruction sets by "hand" (e.g. with -mavx2).

For instance, the SLP vectorizer will work here:

$ cat a.c
void foo(int a1, int a2, int b1, int b2, int *A) {
  A[0] = a1+a2;
  A[1] = b1+b2;
  A[2] = a1+b2;
  A[3] = a2+b1;
  A[4] = a2+a2;
  A[5] = b2+b2;
  A[6] = a2+b2;
  A[7] = a1+b1;
}

$ clang-9 -S -emit-llvm -O3 -march=native -o - a.c
define dso_local void @foo(i32, i32, i32, i32, i32* nocapture)
local_unnamed_addr #0 {
 [...]
  %20 = add nsw <8 x i32> %13, %19
  [...]
}