We sometimes see code like this:
%gep = getelementptr i32, i32* %a, <2 x i64> %splat %ext = extractelement <2 x i32*> %gep, i32 0
%gep = getelementptr i32, <4 x i32*> %a, i64 1 %ext = extractelement <4 x i32*> %gep, i32 0
where there is only one use of the GEP. In such cases it makes
sense to fold the two together such that we create a scalar GEP:
%ext = extractelement <2 x i64> %splat, i32 0 %gep = getelementptr i32, i32* %a, i64 %ext
%ext = extractelement <2 x i32*> %a, i32 0 %gep = getelementptr i32, i32* %ext, i64 1
This may create further folding opportunities as a result, i.e.
the extract of a splat vector can be completely eliminated. Also,
even for the general case where the vector operand is not a splat
it seems beneficial to create a scalar GEP and extract the scalar
element from the operand. Therefore, in this patch I've assumed
that a scalar GEP is always preferrable to a vector GEP and have
added code to unconditionally fold the extract + GEP.
I haven't added folds for the case when we have both a vector of
pointers and a vector of indices, since this would require
generating an additional extractelement operation.
Tests have been added here: