================
@@ -79,6 +79,35 @@ struct CIRRecordLowering final {
cir::ArrayType::get(cirGenTypes.convertTypeForMem(field->getType()),
0),
cir::RecordMemberKind::BitField);
}
+
+ /// An access unit can be narrower than the types its bit-fields were
+ /// declared with. In `struct { int a : 4; int b : 11; int c : 17; }` all
+ /// three share one 4-byte unit, but `c` is declared `int`, so the declared
+ /// data runs 15 bits past the unit:
+ ///
+ /// |-a-|----b-----|-------c--------|
+ /// |--------- 4-byte unit ---------|--- extent ---|
+ /// |------ c declared as int ------|
+ /// 0 4 15 32 47 bits
----------------
adams381 wrote:
That example explains the concept but not the motivation. Consider this example
```c
typedef struct { long long x : 32; } Wide;
typedef struct { int x : 32; } __attribute__((aligned(8))) Narrow;
```
`Wide` needs: `i64`
`Narrow` needs: `i32`
Currently we generate
```mlir
!rec_Narrow = !cir.struct<"Narrow" {bitfield !u32i, pad !cir.array<!u8i x 4>}>
!rec_Wide = !cir.struct<"Wide" {bitfield !u32i, bitfield
!cir.array<!cir.array<!u8i x 4> x 0>, pad !cir.array<!u8i x 4>}>
```
Without the extent member, the two types are identical.
https://github.com/llvm/llvm-project/pull/220069
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