[PATCH] D15477: [X86][AVX] Only shuffle the lower half of vectors if the upper half is undefined

[Elena Demikhovsky via llvm-commits]

delena added inline comments.

================
Comment at: lib/Target/X86/X86ISelLowering.cpp:10367
@@ +10366,3 @@
+    int HalfIdx = (M / HalfNumElts);
+    M %= NumElts;
+
----------------
RKSimon wrote:
> delena wrote:
> > Let's assume that the original mask was:
> > 0, 1, 3, 3, 8, 8, 10, 11
> > You want to take V1-Lo and V2-Lo
> > the new mask should be
> > 0, 1, 3, 3, 4, 4, 6, 7
> > But M %= NumElts will not convert 8 to 4 and 10 to 6.
> I think this is for a v816 - then the modulo will create:
> 
> 0, 1, 3, 3, 0, 0, 2, 3
> 
> The lines below then offset the second half vector by halfnumelts:
> 
> 0, 1, 3, 3, 4, 4, 6, 7
Lets assume that original VT was v8i32. NumElts = 8.
M %= NumElts -?
may be 
M %= HalfNumElts ?


Repository:
  rL LLVM

http://reviews.llvm.org/D15477