[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:7814-7819
+  for (unsigned int i = 0; i < NumBitsPerElt; i++) {
+    SDValue Index = DAG.getConstant(RshrArr[i], dl, VT);
+    ConstantSDNode *IndexNode = cast<ConstantSDNode>(Index);
+    ConstantInt *CI =
+        const_cast<ConstantInt *>(IndexNode->getConstantIntValue());
+    Elts.push_back(CI);
----------------
dmgreen wrote:
> Do we need this loop, or can we create the array from the constant pool directly? The elements should be MVT::i8.
> ```
> auto *CA = ConstantDataArray::get(*DAG.getContext(), RshrArr);
> ```
If I directly use `RshrArr`, I get below table in assembly -

```
.LCPI0_0:
        .ascii  "\000\001\034\002\035\016\030\003\036\026\024\017\031\021\004\b\037\033\r\027\025\023\020\007\032\f\022\006\013\005\n\t"
```

instead of -

```
.LCPI0_0:
        .word   0                               ! 0x0
        .word   1                               ! 0x1
        .word   28                              ! 0x1c
        .word   2                               ! 0x2
        .word   29                              ! 0x1d
        .word   14                              ! 0xe
        .word   24                              ! 0x18
        .word   3                               ! 0x3
        .word   30                              ! 0x1e
        .word   22                              ! 0x16
        .word   20                              ! 0x14
        .word   15                              ! 0xf
        .word   25                              ! 0x19
        .word   17                              ! 0x11
        .word   4                               ! 0x4
        .word   8                               ! 0x8
        .word   31                              ! 0x1f
        .word   27                              ! 0x1b
        .word   13                              ! 0xd
        .word   23                              ! 0x17
        .word   21                              ! 0x15
        .word   19                              ! 0x13
        .word   16                              ! 0x10
        .word   7                               ! 0x7
        .word   26                              ! 0x1a
        .word   12                              ! 0xc
        .word   18                              ! 0x12
        .word   6                               ! 0x6
        .word   11                              ! 0xb
        .word   5                               ! 0x5
        .word   10                              ! 0xa
        .word   9                               ! 0x9
        .text
        .globl  f

```


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

https://reviews.llvm.org/D128911