| Issue |
83430
|
| Summary |
[InstCombine] Missed optimization: fail to fold `(A >> C1) Pred C2` if `shr` is used multiple times
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
XChy
|
Alive2 proof: https://alive2.llvm.org/ce/z/4JKW9d
### Motivating example
```llvm
define i1 @src(i32 %a){
entry:
%div = ashr exact i32 %a, 3
call void @use(i32 %div)
%cmp = icmp slt i32 %div, 15
ret i1 %cmp
}
```
can be folded to:
```llvm
define i1 @tgt(i32 %a) {
%div = ashr exact i32 %a, 3
call void @use(i32 %div)
%cmp = icmp slt i32 %a, 120
ret i1 %cmp
}
```
This enable more optimizations for the other user of `shr` as I observe in benchmark.
Unsigned case is in alive2 proof.
### Real-world motivation
Signed case is derived from [z3/src/sat/smt/pb_solver.cpp](https://github.com/Z3Prover/z3/blob/master/src/sat/smt/pb_solver.cpp) (after O3 pipeline).
Unsigned case is derived from [z3/src/tactic/core/collect_occs.cpp](https://github.com/Z3Prover/z3/blob/master/src/tactic/core/collect_occs.cpp) (after O3 pipeline).
The example above is a reduced version. If you're interested in the original suboptimal IR and optimal IR, email me please.
**Let me know if you can confirm that it's an optimization opportunity, thanks.**
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