| Issue |
124067
|
| Summary |
[Tablegen] CodeEmitterGen::run in CodeEmitterGen.cpp doesn't grow Bitwidth
|
| Labels |
tablegen
|
| Assignees |
|
| Reporter |
farzonl
|
## Bug
If all instruction records are TargetOpcode or isPseudo we never grows BitWidth above zero.
## Why?
On line 487 BitWidth is set to 0.
https://github.com/llvm/llvm-project/blob/de209fa11b5455155228bcdba012b6074388b917/llvm/utils/TableGen/CodeEmitterGen.cpp#L487
We never get to line 506
https://github.com/llvm/llvm-project/blob/de209fa11b5455155228bcdba012b6074388b917/llvm/utils/TableGen/CodeEmitterGen.cpp#L506-L506
that could have set BitWidth Because of
https://github.com/llvm/llvm-project/blob/de209fa11b5455155228bcdba012b6074388b917/llvm/utils/TableGen/CodeEmitterGen.cpp#L490-L492
The reason why it is like this is because `TargetOpcode` do not have a `Inst` field.
## The problem
If we only have TargetOpcode or isPseudo then We never add any `UINT64_C(0)`
to `NEW_TARGETGenMCCodeEmitter.inc` in
```cpp
uint64_t DirectXMCCodeEmitter::getBinaryCodeForInstr(const MCInst &MI,
SmallVectorImpl<MCFixup> &Fixups,
const MCSubtargetInfo &STI) const {
static const uint64_t InstBits[] = {
```
That causes a compile error because `InstBits` is just full of commas with no data in the array.
## How to debug
```bash
lldb -- /mnt/DevDrive/projects/llvm_debug_build/bin/llvm-tblgen -gen-emitter -I /mnt/DevDrive/projects/llvm-project/llvm/lib/Target/NEW_TARGET -I/mnt/DevDrive/projects/llvm_debug_build/include -I/mnt/DevDrive/projects/llvm-project/llvm/include -I /mnt/DevDrive/projects/llvm-project/llvm/lib/Target /mnt/DevDrive/projects/llvm-project/llvm/lib/Target/NEW_TARGET/NEW_TARGET.td --write-if-changed -o /mnt/DevDrive/projects/llvm_debug_build/lib/Target/NEW_TARGET/NEW_TARGETGenMCCodeEmitter.inc -d /mnt/DevDrive/projects/llvm_debug_build/lib/Target/NEW_TARGET/NEW_TARGETGenMCCodeEmiter.inc
```
## Potential Fixes
### Fix 1
On line 487 Set BitWidth to 1.
https://github.com/llvm/llvm-project/blob/de209fa11b5455155228bcdba012b6074388b917/llvm/utils/TableGen/CodeEmitterGen.cpp#L487
### Fix 2
Maybe you are thinking what does it mean to have a target with only TargetOpcode. Well then at least Pseudo instructions need to set the Bit width
```cpp
if( R->getValueAsBit("isPseudo")) {
const BitsInit *BI = R->getValueAsBitsInit("Inst");
BitWidth = std::max(BitWidth, BI->getNumBits());
}
```
Of the Two I think fix one is more correct. a 0 bitwidth doesn't make any sense and could cause problems for others. I don't know if 1 for the bitwidth makes sense though.
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