| Issue |
143989
|
| Summary |
powerpc64le: power9 codgen bug when generating setb instructions
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
pmur
|
The rust frontend ran into a bug when running tests on ppc64le with pgo enabled on [centos](https://kojihub.stream.centos.org/kojifiles/vol/koji02/packages/rust/1.86.0/1.el10/data/logs/ppc64le/build.log). The compilation of the compiler's `rustc_errors` crate with `-mcpu=power9` created a bug where a boolean value is clobbered, resulting in incorrect json values.
I was able to extract the llvm input from rust, and run `llvm-reduce` to produce a reasonably sized reproducer using the upstream llvm repo.
The following codegen is suspect:
```
andi. 3, 4, 1
setb 3, 2
cmpldi 4, 0
std 28, 80(1) # 8-byte Folded Spill
stw 3, 60(1)
```
Notably, `cr2` in the `setb` above has no live value (I am assuming this is following the ELFv2 ABI).
After the `machine-cp` pass, a copy from `cr0gt` to `cr2lt` goes missing. Likewise, the other bits of `cr2` appear to be undefined. Furthermore, `setb` potentially examines all bits of the input `cr`, so moving one bit is not sufficient for well-defined behavior.
Attached is the reduced llvm input [reduced.ll.txt](https://github.com/user-attachments/files/20717223/reduced.ll.txt). Likewise, I assembled it via:
```
llc -march=ppc64le -mcpu=pwr9 reduced.ll.ltxt
```
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