| Issue |
165354
|
| Summary |
[LoopUnroll][SCEV] Runtime unroll blocked by non‑computable exit block SCEV unless IV add has NUW
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
forandom
|
Hi folks,
I’m seeing a loop fail runtime unrolling because SCEV can’t compute the exiting block’s exit count unless the induction increment has NUW. I’d appreciate guidance on whether this is expected, and if SCEV/Unroll could rely on the predicated backedge count here.
Minimal IR:
```
define void @test(i32 %S, i32 %E, ptr %arr) {
entry:
br label %loop
loop:
%i = phi i32 [ %S, %entry ], [ %inc, %latch ]
%idx.ext = zext i32 %i to i64
%ptr = getelementptr i32, ptr %arr, i64 %idx.ext
%x = load i32, ptr %ptr, align 4
store i32 %x, ptr %ptr, align 4
%inc = add i32 %i, 1
%exit = icmp ult i32 %E, %inc
br i1 %exit, label %exitBB, label %latch
latch:
br label %loop
exitBB:
ret void
}
```
Command to reproduce:
```
opt -S -passes='loop-rotate,indvar,loop-simplify,print<scalar-evolution>,loop-unroll' -debug-_only_=loop-unroll -unroll-runtime -print-after-all a.ll
```
What I observe
SCEV without nuw
- %i is {%S,+,1}<%loop> with Exits: <<Unknown>>.
- SCEV reports “Unpredictable [symbolic] backedge‑taken count,” but does compute a predicated backedge‑taken count:
```
Predicated backedge-taken count:
((-1 * (zext i32 (1 + %S) to i64))<nsw>
+ ((zext i32 (1 + %S) to i64)
umax (1 + (zext i32 %E to i64))<nuw><nsw>))
Predicates:
{(1 + %S),+,1}<%loop> Added Flags: <nusw>
```
- LoopUnroll bails early due to non‑computable exiting block SCEV.
SCEV with nuw on %inc
- %i becomes {%S,+,1}<nuw><%loop> with Exits: (%S umax %E).
- Backedge‑taken count is ((-1 * %S) + (%S umax %E)).
- Runtime unrolling proceeds.
Questions
- SCEV reasoning: In this pattern, S/E are unknown at compile time and the exit is unsigned (icmp ult %E, %inc). Without nuw on the IV, wrap‑around can’t be excluded, so I understand why SCEV won’t produce an exit count. But SCEV does compute a predicated backedge count that assumes <nusw>. Would it be acceptable for the unroller to use that predicated count (with appropriate guards/epilogue) or is this considered unsafe/undesirable?
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