[LV] Ignore user-specified interleave count when unsafe. (#153009)

When an VF is specified via a loop hint, it will be clamped to a safe
VF or ignored if it is found to be unsafe. This is not the case for
user-specified interleave counts, which can lead to loops such as
the following with a memory dependence being vectorised with
interleaving:

```
#pragma clang loop interleave_count(4)
for (int i = 4; i < LEN; i++)
    b[i] = b[i - 4] + a[i];
```

According to [1], loop hints are ignored if they are not safe to apply.

This patch adds a check to prevent vectorisation with interleaving if
isSafeForAnyVectorWidth() returns false. This is already checked in
selectInterleaveCount().

[1]
https://llvm.org/docs/LangRef.html#llvm-loop-vectorize-and-llvm-loop-interleave

1 files changed, 11 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
index adf27bed3d74..d2c100c9adbf 100644
--- a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
+++ b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
@@ -9860,6 +9860,8 @@ bool LoopVectorizePass::processLoop(Loop *L) {
   // Get user vectorization factor and interleave count.
   ElementCount UserVF = Hints.getWidth();
   unsigned UserIC = Hints.getInterleave();
+  if (UserIC > 1 && !LVL.isSafeForAnyVectorWidth())
+    UserIC = 1;
 
   // Plan how to best vectorize.
   LVP.plan(UserVF, UserIC);
@@ -9924,7 +9926,15 @@ bool LoopVectorizePass::processLoop(Loop *L) {
     VectorizeLoop = false;
   }
 
-  if (!LVP.hasPlanWithVF(VF.Width) && UserIC > 1) {
+  if (UserIC == 1 && Hints.getInterleave() > 1) {
+    assert(!LVL.isSafeForAnyVectorWidth() &&
+           "UserIC should only be ignored due to unsafe dependencies");
+    LLVM_DEBUG(dbgs() << "LV: Ignoring user-specified interleave count.\n");
+    IntDiagMsg = {"InterleavingUnsafe",
+                  "Ignoring user-specified interleave count due to possibly "
+                  "unsafe dependencies in the loop."};
+    InterleaveLoop = false;
+  } else if (!LVP.hasPlanWithVF(VF.Width) && UserIC > 1) {
     // Tell the user interleaving was avoided up-front, despite being explicitly
     // requested.
     LLVM_DEBUG(dbgs() << "LV: Ignoring UserIC, because vectorization and "