[PATCH] D79783: [LoopVectorize] Fallback to a scalar epilogue when TP fails

[Pierre van Houtryve via Phabricator via llvm-commits]

Pierre-vh marked an inline comment as done.
Pierre-vh added inline comments.


================
Comment at: llvm/lib/Transforms/Vectorize/LoopVectorize.cpp:5027
+                         "hint/switch and using a scalar epilogue instead.\n");
+    ScalarEpilogueStatus = CM_ScalarEpilogueAllowed;
+    return MaxVF;
----------------
Ayal wrote:
> Ayal wrote:
> > Pierre-vh wrote:
> > > Ayal wrote:
> > > > Why change ScalarEpilogueStatus?
> > > (Sorry, forgot to post this comment)
> > > It seems to be needed, my tests don't pass if unless I change it (they have conflicting ASM for both run lines, and the content is different)
> > Why change ScalarEpilogueStatus?
> Add `-check-prefix` to the FileCheck of one of the runs?
> https://llvm.org/docs/CommandGuide/FileCheck.html#cmdoption-filecheck-check-prefix
This is what's added when I don't change `ScalarEpilogueStatus`, and it's set to `ScalarEpilogueNotNeededUsePredicate`:

```
; CHECK-NEXT:    [[BROADCAST_SPLATINSERT:%.*]] = insertelement <4 x i32> undef, i32 [[OFFSET_IDX]], i32 0
; CHECK-NEXT:    [[BROADCAST_SPLAT:%.*]] = shufflevector <4 x i32> [[BROADCAST_SPLATINSERT]], <4 x i32> undef, <4 x i32> zeroinitializer
; CHECK-NEXT:    [[INDUCTION:%.*]] = add <4 x i32> [[BROADCAST_SPLAT]], <i32 0, i32 -1, i32 -2, i32 -3>
```

It's because of this in `InnerLoopVectorizer::widenIntOrFpInduction` in LoopVectorize.cpp:1920
```
  // All IV users are scalar instructions, so only emit a scalar IV, not a
  // vectorised IV. Except when we tail-fold, then the splat IV feeds the
  // predicate used by the masked loads/stores.
  if (!Cost->isScalarEpilogueAllowed()) // ScalarEpilogueStatus == CM_ScalarEpilogueAllowed
    CreateSplatIV(ScalarIV, Step);
```
I am guessing that this should be using `Cost->foldTailByMasking()` instead, which returns the `FoldTailByMasking` boolean instead of looking at `ScalarEpilogueStatus`.



CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D79783/new/

https://reviews.llvm.org/D79783