[llvm] [LV] Transform to handle exits in the scalar loop (PR #148626)

[David Sherwood via llvm-commits]

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@@ -262,6 +262,16 @@ struct VPlanTransforms {
                                          VPlan &Plan, VPBasicBlock *HeaderVPBB,
                                          VPBasicBlock *LatchVPBB);
 
+  /// Update \p Plan to check whether the next iteration of the vector loop
+  /// would exit (using any exit type) and if so branch to the scalar loop
+  /// instead. This requires identifying the recipes that form the conditions
+  /// for exiting, cloning them to the preheader, then adjusting both the
+  /// preheader recipes (to check the first vector iteration) and those in
+  /// the vector loop (to check the next vector iteration instead of the
+  /// current one). This can be used to avoid complex masking for state-changing
+  /// recipes (like stores).
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david-arm wrote:

Also, when I was at a similar stage doing the initial early exit work (under a flag) @fhahn asked me to put some documentation in llvm/docs/Vectorizers.rst too, along with a diagram (vplan-early-exit.png) showing the new vplan structure. I think it would be good to add something similar to be consistent. At the very least, the section called "Early Exit Vectorization" in llvm/docs/Vectorizers.rst needs updating I think.

https://github.com/llvm/llvm-project/pull/148626