[llvm] 082a004 - [VPlan] Allow building a VPlan to may fail.

[Florian Hahn via llvm-commits]

Author: Florian Hahn
Date: 2023-04-11T15:41:18+01:00
New Revision: 082a004690eb40d62287d98ec5c5b1de9d7bcf92

URL: https://github.com/llvm/llvm-project/commit/082a004690eb40d62287d98ec5c5b1de9d7bcf92
DIFF: https://github.com/llvm/llvm-project/commit/082a004690eb40d62287d98ec5c5b1de9d7bcf92.diff

LOG: [VPlan] Allow building a VPlan to may fail.

Update the planning code constructing VPlan to allow building VPlans to
fail. This allows us to gradually shift some legality checks to VPlan
construction. The first candidate is checking if all users of
first-order recurrence phis can be sunk past the recipe computing the
previous value.

The new functionality will be used by D142886 which is approved and will
be landed shortly.

Reviewed By: Ayal

Differential Revision: https://reviews.llvm.org/D142885

Added: 
    

Modified: 
    llvm/lib/Transforms/Vectorize/LoopVectorizationPlanner.h
    llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

Removed: 
    


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diff  --git a/llvm/lib/Transforms/Vectorize/LoopVectorizationPlanner.h b/llvm/lib/Transforms/Vectorize/LoopVectorizationPlanner.h
index 8d9a7ffc6bfa7..317e31107ce22 100644
--- a/llvm/lib/Transforms/Vectorize/LoopVectorizationPlanner.h
+++ b/llvm/lib/Transforms/Vectorize/LoopVectorizationPlanner.h
@@ -350,9 +350,12 @@ class LoopVectorizationPlanner {
 
   /// Build a VPlan using VPRecipes according to the information gather by
   /// Legal. This method is only used for the legacy inner loop vectorizer.
-  VPlanPtr
-  buildVPlanWithVPRecipes(VFRange &Range,
-                          SmallPtrSetImpl<Instruction *> &DeadInstructions);
+  /// \p Range's largest included VF is restricted to the maximum VF the
+  /// returned VPlan is valid for. If no VPlan can be built for the input range,
+  /// set the largest included VF to the maximum VF for which no plan could be
+  /// built.
+  std::optional<VPlanPtr> tryToBuildVPlanWithVPRecipes(
+      VFRange &Range, SmallPtrSetImpl<Instruction *> &DeadInstructions);
 
   /// Build VPlans for power-of-2 VF's between \p MinVF and \p MaxVF inclusive,
   /// according to the information gathered by Legal when it checked if it is

diff  --git a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
index 2b7f1f7f5cd82..0081478270fe9 100644
--- a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
+++ b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
@@ -7589,6 +7589,12 @@ LoopVectorizationPlanner::plan(ElementCount UserVF, unsigned UserIC) {
       LLVM_DEBUG(dbgs() << "LV: Using user VF " << UserVF << ".\n");
       CM.collectInLoopReductions();
       buildVPlansWithVPRecipes(UserVF, UserVF);
+      if (!hasPlanWithVF(UserVF)) {
+        LLVM_DEBUG(dbgs() << "LV: No VPlan could be built for " << UserVF
+                          << ".\n");
+        return std::nullopt;
+      }
+
       LLVM_DEBUG(printPlans(dbgs()));
       return {{UserVF, 0, 0}};
     } else
@@ -7626,6 +7632,11 @@ LoopVectorizationPlanner::plan(ElementCount UserVF, unsigned UserIC) {
   // Select the optimal vectorization factor.
   VectorizationFactor VF = CM.selectVectorizationFactor(VFCandidates);
   assert((VF.Width.isScalar() || VF.ScalarCost > 0) && "when vectorizing, the scalar cost must be non-zero.");
+  if (!hasPlanWithVF(VF.Width)) {
+    LLVM_DEBUG(dbgs() << "LV: No VPlan could be built for " << VF.Width
+                      << ".\n");
+    return std::nullopt;
+  }
   return VF;
 }
 
@@ -8722,7 +8733,8 @@ void LoopVectorizationPlanner::buildVPlansWithVPRecipes(ElementCount MinVF,
   auto MaxVFTimes2 = MaxVF * 2;
   for (ElementCount VF = MinVF; ElementCount::isKnownLT(VF, MaxVFTimes2);) {
     VFRange SubRange = {VF, MaxVFTimes2};
-    VPlans.push_back(buildVPlanWithVPRecipes(SubRange, DeadInstructions));
+    if (auto Plan = tryToBuildVPlanWithVPRecipes(SubRange, DeadInstructions))
+      VPlans.push_back(std::move(*Plan));
     VF = SubRange.End;
   }
 }
@@ -8853,7 +8865,7 @@ static void addUsersInExitBlock(VPBasicBlock *HeaderVPBB,
   }
 }
 
-VPlanPtr LoopVectorizationPlanner::buildVPlanWithVPRecipes(
+std::optional<VPlanPtr> LoopVectorizationPlanner::tryToBuildVPlanWithVPRecipes(
     VFRange &Range, SmallPtrSetImpl<Instruction *> &DeadInstructions) {
 
   SmallPtrSet<const InterleaveGroup<Instruction> *, 1> InterleaveGroups;