[PATCH] D128911: Emit table lookup from TargetLowering::expandCTTZ()

[Shubham Narlawar via Phabricator via llvm-commits]

gsocshubham added inline comments.


================
Comment at: llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp:7870
+    SDValue CPIdx = DAG.getConstantPool(CA, getPointerTy(TD),
+					TD.getPrefTypeAlign(Elts[0]->getType()));
+    Align Alignment = cast<ConstantPoolSDNode>(CPIdx)->getAlign();
----------------
barannikov88 wrote:
> You should use the alignment requirement of the array (i.e. CA), not of its element. They may differ.
>From the assembly dump of SPARC/cttz.ll, I am not sure whether to use array element alignment or array alignment?

If I use array alignment `CA`, I get below assembly as compared to `SPARC/cttz.ll` assembly if array element alignment is used. What do you think? Should I update from `CPIdx` to `CA`?

```
f:                                      ! @f
        .cfi_startproc
! %bb.0:                                ! %entry
        mov     %o0, %o1
        cmp %o0, 0
        be      .LBB0_2
        mov     %g0, %o0
! %bb.1:                                ! %entry
        sub %o0, %o1, %o0
        and %o1, %o0, %o0
        sethi 122669, %o1
        or %o1, 305, %o1
        smul %o0, %o1, %o0
        srl %o0, 27, %o0
        sethi %hi(.LCPI0_0), %o1
        add %o1, %lo(.LCPI0_0), %o1
        add %o1, %o0, %o2
        ldub [%o2+2], %o3
        ldub [%o2+3], %o4
        ldub [%o1+%o0], %o0
        ldub [%o2+1], %o1
        sll %o3, 8, %o2
        or %o2, %o4, %o2
        sll %o0, 8, %o0
        or %o0, %o1, %o0
        sll %o0, 16, %o0
        or %o0, %o2, %o0
```


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D128911/new/

https://reviews.llvm.org/D128911