[llvm] [LV] Provide utility routine to find uncounted exit recipes (PR #152530)

[David Sherwood via llvm-commits]

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@@ -138,3 +138,103 @@ VPBasicBlock *vputils::getFirstLoopHeader(VPlan &Plan, VPDominatorTree &VPDT) {
   });
   return I == DepthFirst.end() ? nullptr : cast<VPBasicBlock>(*I);
 }
+
+std::optional<VPValue *> vputils::getRecipesForUncountedExit(
+    VPlan &Plan, SmallVectorImpl<VPRecipeBase *> &Recipes,
+    SmallVectorImpl<VPReplicateRecipe *> &GEPs) {
+  using namespace llvm::VPlanPatternMatch;
+  // Given a vplan like the following (just including the recipes contributing
+  // to loop control exiting here, not the actual work), we're looking to match
+  // the recipes contributing to the uncounted exit condition comparison
+  // (here, vp<%4>) back to the canonical induction for the vector body so that
+  // we can copy them to a preheader and rotate the address in the loop to the
+  // next vector iteration.
+  //
+  // VPlan ' for UF>=1' {
+  // Live-in vp<%0> = VF
+  // Live-in ir<64> = original trip-count
+  //
+  // entry:
+  // Successor(s): preheader, vector.ph
+  //
+  // vector.ph:
+  // Successor(s): vector loop
+  //
+  // <x1> vector loop: {
+  //   vector.body:
+  //     EMIT vp<%2> = CANONICAL-INDUCTION ir<0>
+  //     vp<%3> = SCALAR-STEPS vp<%2>, ir<1>, vp<%0>
+  //     CLONE ir<%ee.addr> = getelementptr ir<0>, vp<%3>
+  //     WIDEN ir<%ee.load> = load ir<%ee.addr>
+  //     WIDEN vp<%4> = icmp eq ir<%ee.load>, ir<0>
+  //     EMIT vp<%5> = any-of vp<%4>
+  //     EMIT vp<%6> = add vp<%2>, vp<%0>
+  //     EMIT vp<%7> = icmp eq vp<%6>, ir<64>
+  //     EMIT vp<%8> = or vp<%5>, vp<%7>
+  //     EMIT branch-on-cond vp<%8>
+  //   No successors
+  // }
+  // Successor(s): middle.block
----------------
david-arm wrote:

The IR above suggests we've already called `handleUncountableEarlyExit` and so this probably should be `// Successor(s): middle.split`

https://github.com/llvm/llvm-project/pull/152530