| Issue |
208778
|
| Summary |
[SCEV][LoopVectorize] Missed vectorization when trip count involves zext
|
| Labels |
new issue
|
| Assignees |
aleks-tmb
|
| Reporter |
aleks-tmb
|
Here are 2 reduced examples of missed vectorization because SCEV cannot prove loop deref safety: https://godbolt.org/z/TfKMe7vd8
A required condition for vectorization is proving that the accessed pointers are dereferenceable and properly aligned inside the loop (see `llvm::isDereferenceableAndAlignedInLoop`). When the deref size is known from an `llvm.assume`, the access size must be proven to be `ULE` to the known deref size.
- In the first case
```
assume(dereferenceable(p, zext(len + 8)));
if (len > 0)
for (u64 i = 0; i <= zext(len - 1); ++i)
if (p[8 + i] == 0) return;
```
LAA needs to prove: `AccessSize <=u KnownDerefSize`. After applying loop guards, the SCEV expressions are:
```
AccessSizeSCEV:
(9 + (zext i32 (-1 + (1 umax %len))<nsw> to i64))<nuw><nsw>
KnownSizeSCEV:
(8 + (1 umax (zext i32 %len to i64)))<nuw><nsw>
```
**SCEV doesn't move the -1 outside the zext**, which would allow it to fold with the outer 9 and make AccessSizeSCEV equal to KnownSizeSCEV.
Without this transformation, SCEV can't prove the predicate. After the fold, the predicate becomes trivially true.
- In the second case:
```
assume(dereferenceable(p, zext(len + 8)));
if (len > 0) {
u64 exit = zext(umin(A, len - 1));
if (exit > 1)
for (u64 i = 0; i <= exit; ++i)
if (p[8 + i] == 0) return;
}
```
After applying loop guards, the SCEV expressions are:
```
AccessSizeSCEV:
(9 + ((2 umax (zext i32 (-1 + %len)<nsw> to i64)) umin (2 umax (zext i32 %A to i64))))<nuw><nsw>
KnownSizeSCEV:
(8 + (1 umax (zext i32 %len to i64)))<nuw><nsw>
```
This case exposes a second issue in addition to SCEV not moving the -1 outside the zext. **The two SCEV expressions use different inferred ranges for `%len`**, which prevents SCEV from proving the ULE predicate.
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