Extended Description

Test: Transforms/InstCombine/vec_demanded_elts.ll
Summary: gep(ptr, undef) isn't undef. It's a pointer based on ptr with an undef offset (a multiple of 4 in this case). A way to make this correct is to return <%base, %base>.

define <2 x > @​gep_all_lanes_undef( %base, i64 %idx) {
%basevec = insertelement <2 x *> undef, * %base, i32 0
%idxvec = insertelement <2 x i64> undef, i64 %idx, i32 1
%gep = gep <2 x *> %basevec, 4 x <2 x i64> %idxvec
ret <2 x *> %gep
}
=>
define <2 x > @​gep_all_lanes_undef( %base, i64 %idx) {
ret <2 x *> undef
}

Transformation doesn't verify!
ERROR: Value mismatch

Example:

• %base = pointer(non-local, block_id=1, offset=0)
  i64 %idx = #x0000000000080000 (524288)

Source:
<2 x *> %basevec = < pointer(non-local, block_id=1, offset=0), null >
<2 x i64> %idxvec = < undef, #x0000000000080000 (524288) >
<2 x *> %gep = < pointer(non-local, block_id=1, offset=0), pointer(non-local, block_id=0, offset=2097152) >

Target:
Source value: < pointer(non-local, block_id=1, offset=0), pointer(non-local, block_id=0, offset=2097152) >
Target value: < pointer(non-local, block_id=0, offset=16), null >

https://web.ist.utl.pt/nuno.lopes/alive2/index.php?hash=2009353267698970&test=Transforms%2FInstCombine%2Fvec_demanded_elts.ll