[PATCH] D155329: [TableGen][CodeEmitterGen] Add support for querying operand bit offsets

[Ilya Leoshkevich via Phabricator via llvm-commits]

iii added inline comments.


================
Comment at: llvm/utils/TableGen/CodeEmitterGen.cpp:216
+      BitOffsetCase += "        return " +
+                       utostr(BI->getNumBits() - beginInstBit - 1) + ";\n";
+      BitOffsetCaseEmitted = true;
----------------
craig.topper wrote:
> If I'm reading this right, this is the offset from the MSB of the encoding to the MSB of the field? Which works for SystemZ because the encoding bytes are emitted in big endian, but would not be useful for a little endian target?
You are right, I thought that reverseBitsForLittleEndianEncoding() would handle that, but it doesn't.
I've updated the patch and verified the update with PPC, which sets isLittleEndianEncoding.
For BCCTR (https://www.ibm.com/docs/en/aix/7.2?topic=set-bcctr-bcc-branch-conditional-count-register-instruction) we get:

```
      switch (OpNum) {
      case 0:
        // op: BI
        return 20;
      }
```

which matches the drawing and the encoding in practice:

```
       0: 20 1c 22 4c   bcctr 1, 2, 3

                               bit 20
                                 |
                                 v
BitV  00100000    00011100    00100010    01001100
Bit#  76543210    54321098    32109876    10987654
      +++++++-    ...--+++    +++-----    ------++
Field   528  LK      BH       BO   BI       19  BO
```


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D155329/new/

https://reviews.llvm.org/D155329