| Issue |
61458
|
| Summary |
`reassociate` creates loop dependent variables and defeats `licm`
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
qcolombet
|
I stumbled on a case where the reassociate pass is actively making the generated code worse.
In a nutshell, reassociate turns loop invariant computations into loop variant ones and licm is not able to hoist them back.
I haven't dug why, maybe reassociate is tweaked to form cse'able expressions / reduce the total number of instructions but the overall runtime is going to be worse in this case.
To reproduce:
```
opt -S -passes='reassociate' repro_reassoc_defeats_licm.ll -o -
```
Result:
Before reassociate we have:
- 2 loop invariant adds
- 1 loop variant shift
- 3 loop variant adds
After reassociate we have:
- 0 loop invariant adds
- 1 loop variant shift
- 5 loop variant adds (of which 1 is cse'able)
So accounting for `cse` (`-passes=reassociate,early-cse`), we would trade 2 loop invariant adds + 3 loop variant adds with 4 loop variant adds.
Before:
```
define void @reassoc_defeats_licm(i64 %inv1, i64 %inv2, i64 %inv3) {
bb:
%inv4 = add nuw nsw i64 %inv1, %inv2
%inv5 = add nuw nsw i64 %inv2, %inv3
br label %bb214
bb214: ; preds = %bb214, %bb
%iv1 = phi i64 [ %iv2, %bb214 ], [ 0, %bb ]
%iv2 = phi i64 [ %iv2_plus_1, %bb214 ], [ 1, %bb ]
%loop_dependent = shl nuw nsw i64 %iv1, 13
%loop_dependent2 = add nsw i64 %inv4, %loop_dependent
call void @keep_alive(i64 %loop_dependent2)
%loop_dependent3 = add i64 %inv5, %loop_dependent
call void @keep_alive(i64 %loop_dependent3)
%iv2_plus_1 = add i64 %iv2, 1
br label %bb214
}
declare void @keep_alive(i64)
```
After (with cse):
```
define void @reassoc_defeats_licm(i64 %inv1, i64 %inv2, i64 %inv3) {
bb:
br label %bb214
bb214: ; preds = %bb214, %bb
%iv1 = phi i64 [ %iv2, %bb214 ], [ 0, %bb ]
%iv2 = phi i64 [ %iv2_plus_1, %bb214 ], [ 1, %bb ]
%loop_dependent = shl nuw nsw i64 %iv1, 13
%inv4 = add i64 %loop_dependent, %inv2
%loop_dependent2 = add i64 %inv4, %inv1
call void @keep_alive(i64 %loop_dependent2)
%loop_dependent3 = add i64 %inv4, %inv3
call void @keep_alive(i64 %loop_dependent3)
%iv2_plus_1 = add i64 %iv2, 1
br label %bb214
}
declare void @keep_alive(i64)
```
[repro_reassoc_defeats_licm.ll.txt](https://github.com/llvm/llvm-project/files/10994658/repro_reassoc_defeats_licm.ll.txt)
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