[PATCH] D114002: [LoopVectorize] Check the number of uses of an FAdd before classifying as ordered

[Kerry McLaughlin via Phabricator via llvm-commits]

kmclaughlin created this revision.
kmclaughlin added reviewers: david-arm, dmgreen, fhahn, skatkov, RosieSumpter.
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checkOrderedReductions looks for Phi nodes which can be classified as in-order,
meaning they can be vectorised without unsafe math. In order to vectorise the
reduction it should also be classified as in-loop by getReductionOpChain, which
checks that the reduction has two uses.

In this patch, a similar check is added to checkOrderedReductions so that we
now return false if there are more than two uses of the FAdd instruction.
This fixes PR52515.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D114002

Files:
  llvm/lib/Analysis/IVDescriptors.cpp
  llvm/test/Transforms/LoopVectorize/AArch64/strict-fadd.ll


Index: llvm/test/Transforms/LoopVectorize/AArch64/strict-fadd.ll
===================================================================
--- llvm/test/Transforms/LoopVectorize/AArch64/strict-fadd.ll
+++ llvm/test/Transforms/LoopVectorize/AArch64/strict-fadd.ll
@@ -931,6 +931,42 @@
   ret double %res
 }
 
+; We should not mark the fadd as an ordered reduction here as there are
+; more than 2 uses of the instruction
+define float @fadd_multiple_use(i64 %n) {
+; CHECK-ORDERED-LABEL: @fadd_multiple_use
+; CHECK-ORDERED-LABEL-NOT: vector.body
+
+; CHECK-UNORDERED-LABEL: @fadd_multiple_use
+; CHECK-UNORDERED-LABEL-NOT: vector.body
+
+; CHECK-NOT-VECTORIZED-LABEL: @fadd_multiple_use
+; CHECK-NOT-VECTORIZED-NOT: vector.body
+entry:
+  br label %for.body
+
+for.body:
+  %iv = phi i64 [ 0, %entry ], [ %iv.next2, %bb2 ]
+  %red = phi float [ 0.0, %entry ], [ %fadd, %bb2 ]
+  %phi1 = phi i64 [ 0, %entry ], [ %iv.next, %bb2 ]
+  %fadd = fadd float %red, 1.000000e+00
+  %iv.next = add nsw i64 %phi1, 1
+  %cmp = icmp ult i64 %iv, %n
+  br i1 %cmp, label %bb2, label %bb1
+
+bb1:
+  %phi2 = phi float [ %fadd, %for.body ]
+  ret float %phi2
+
+bb2:
+  %iv.next2 = add nuw nsw i64 %iv, 1
+  br i1 false, label %for.end, label %for.body
+
+for.end:
+  %phi3 = phi float [ %fadd, %bb2 ]
+  ret float %phi3
+}
+
 !0 = distinct !{!0, !5, !9, !11}
 !1 = distinct !{!1, !5, !10, !11}
 !2 = distinct !{!2, !6, !9, !11}
Index: llvm/lib/Analysis/IVDescriptors.cpp
===================================================================
--- llvm/lib/Analysis/IVDescriptors.cpp
+++ llvm/lib/Analysis/IVDescriptors.cpp
@@ -201,7 +201,10 @@
   if (Kind != RecurKind::FAdd)
     return false;
 
-  if (Exit->getOpcode() != Instruction::FAdd || Exit != ExactFPMathInst)
+  // Ensure the exit instruction is an FAdd, and that it only has one user
+  // other than the reduction PHI
+  if (Exit->getOpcode() != Instruction::FAdd || Exit->getNumUses() > 2 ||
+      Exit != ExactFPMathInst)
     return false;
 
   // The only pattern accepted is the one in which the reduction PHI


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