peilin-ye wrote:
@yonghong-song Hi Yonghong, about my earlier question:
> However, I wonder how do I generate a sign-extending (sext), acquiring
> ATOMIC_LOAD in SelectionDAG?
After more digging, I found this in
`llvm/include/llvm/CodeGen/TargetLowering.h`:
```cpp
/// Returns how the platform's atomic operations are extended (ZERO_EXTEND,
/// SIGN_EXTEND, or ANY_EXTEND).
virtual ISD::NodeType getExtendForAtomicOps() const {
return ISD::ZERO_EXTEND;
}
```
If I override it in `class BPFTargetLowering`:
```diff
--- a/llvm/lib/Target/BPF/BPFISelLowering.h
+++ b/llvm/lib/Target/BPF/BPFISelLowering.h
@@ -46,6 +46,10 @@ public:
// with the given GlobalAddress is legal.
bool isOffsetFoldingLegal(const GlobalAddressSDNode *GA) const override;
+ ISD::NodeType getExtendForAtomicOps() const override {
+ return ISD::SIGN_EXTEND;
+ }
+
BPFTargetLowering::ConstraintType
getConstraintType(StringRef Constraint) const override;
```
I can then get a `sext` sub-word `AtomicLoad` SelectionDAG node:
```
Optimized legalized selection DAG: %bb.0 'bar:entry'
SelectionDAG has 8 nodes:
t0: ch,glue = EntryToken
t2: i64,ch = CopyFromReg t0, Register:i64 %0
t8: i32,ch = AtomicLoad<(load acquire (s8) from %ir.ptr), sext from i8> t0,
t2, demo.bpf.c:3:12
t12: i64 = sign_extend t8, demo.bpf.c:3:12
t6: ch,glue = CopyToReg t8:1, Register:i64 $r0, t12, demo.bpf.c:3:5
t7: ch = BPFISD::RET_GLUE t6, Register:i64 $r0, t6:1, demo.bpf.c:3:5
```
...but now I can't have `zext` :-)
- - -
Looks like we can't have both `sext` and `zext` sub-word atomic load, unless we
change `getExtendForAtomicOps()` and how it's used.
https://github.com/llvm/llvm-project/pull/108636
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