[PATCH] D154261: [LV] Consider if scalar epilogue is required in getMaximizedVFForTarget.

Mon Jul 3 09:50:59 PDT 2023

fhahn updated this revision to Diff 536809.
fhahn added a comment.

Rebase on top of e561edaaa56c9a8818d546774b141dead7224b50 <https://reviews.llvm.org/rGe561edaaa56c9a8818d546774b141dead7224b50> which updates the test in pr56319-vector-exit-cond-optimization-epilogue-vectorization.ll so it keeps testing for the original issue.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D154261/new/

https://reviews.llvm.org/D154261

Files:
  llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
  llvm/test/Transforms/LoopVectorize/X86/limit-vf-by-tripcount.ll


Index: llvm/test/Transforms/LoopVectorize/X86/limit-vf-by-tripcount.ll
===================================================================

--- llvm/test/Transforms/LoopVectorize/X86/limit-vf-by-tripcount.ll
+++ llvm/test/Transforms/LoopVectorize/X86/limit-vf-by-tripcount.ll
@@ -285,10 +285,8 @@
 
 define void @limit_main_loop_vf_to_avoid_epilogue_vectorization(ptr noalias %src, ptr noalias %dst) {
 ; CHECK-LABEL: @limit_main_loop_vf_to_avoid_epilogue_vectorization(
-; CHECK-NEXT:  iter.check:
-; CHECK-NEXT:    br i1 false, label [[VEC_EPILOG_SCALAR_PH:%.*]], label [[VECTOR_MAIN_LOOP_ITER_CHECK:%.*]]
-; CHECK:       vector.main.loop.iter.check:
-; CHECK-NEXT:    br i1 true, label [[VEC_EPILOG_PH:%.*]], label [[VECTOR_PH:%.*]]
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br i1 false, label [[SCALAR_PH:%.*]], label [[VECTOR_PH:%.*]]
 ; CHECK:       vector.ph:
 ; CHECK-NEXT:    br label [[VECTOR_BODY:%.*]]
 ; CHECK:       vector.body:
@@ -296,48 +294,28 @@
 ; CHECK-NEXT:    [[TMP0:%.*]] = add i64 [[INDEX]], 0
 ; CHECK-NEXT:    [[TMP1:%.*]] = getelementptr inbounds [3 x i8], ptr [[SRC:%.*]], i64 [[TMP0]], i64 0
 ; CHECK-NEXT:    [[TMP2:%.*]] = getelementptr inbounds i8, ptr [[TMP1]], i32 0
-; CHECK-NEXT:    [[WIDE_VEC:%.*]] = load <96 x i8>, ptr [[TMP2]], align 1
-; CHECK-NEXT:    [[STRIDED_VEC:%.*]] = shufflevector <96 x i8> [[WIDE_VEC]], <96 x i8> poison, <32 x i32> <i32 0, i32 3, i32 6, i32 9, i32 12, i32 15, i32 18, i32 21, i32 24, i32 27, i32 30, i32 33, i32 36, i32 39, i32 42, i32 45, i32 48, i32 51, i32 54, i32 57, i32 60, i32 63, i32 66, i32 69, i32 72, i32 75, i32 78, i32 81, i32 84, i32 87, i32 90, i32 93>
+; CHECK-NEXT:    [[WIDE_VEC:%.*]] = load <24 x i8>, ptr [[TMP2]], align 1
+; CHECK-NEXT:    [[STRIDED_VEC:%.*]] = shufflevector <24 x i8> [[WIDE_VEC]], <24 x i8> poison, <8 x i32> <i32 0, i32 3, i32 6, i32 9, i32 12, i32 15, i32 18, i32 21>
 ; CHECK-NEXT:    [[TMP3:%.*]] = getelementptr inbounds i8, ptr [[DST:%.*]], i64 [[TMP0]]
 ; CHECK-NEXT:    [[TMP4:%.*]] = getelementptr inbounds i8, ptr [[TMP3]], i32 0
-; CHECK-NEXT:    store <32 x i8> [[STRIDED_VEC]], ptr [[TMP4]], align 1
-; CHECK-NEXT:    [[INDEX_NEXT]] = add nuw i64 [[INDEX]], 32
-; CHECK-NEXT:    [[TMP5:%.*]] = icmp eq i64 [[INDEX_NEXT]], 0
+; CHECK-NEXT:    store <8 x i8> [[STRIDED_VEC]], ptr [[TMP4]], align 1
+; CHECK-NEXT:    [[INDEX_NEXT]] = add nuw i64 [[INDEX]], 8
+; CHECK-NEXT:    [[TMP5:%.*]] = icmp eq i64 [[INDEX_NEXT]], 24
 ; CHECK-NEXT:    br i1 [[TMP5]], label [[MIDDLE_BLOCK:%.*]], label [[VECTOR_BODY]], !llvm.loop [[LOOP13:![0-9]+]]
 ; CHECK:       middle.block:
-; CHECK-NEXT:    br label [[VEC_EPILOG_ITER_CHECK:%.*]]
-; CHECK:       vec.epilog.iter.check:
-; CHECK-NEXT:    br i1 false, label [[VEC_EPILOG_SCALAR_PH]], label [[VEC_EPILOG_PH]]
-; CHECK:       vec.epilog.ph:
-; CHECK-NEXT:    [[VEC_EPILOG_RESUME_VAL:%.*]] = phi i64 [ 0, [[VEC_EPILOG_ITER_CHECK]] ], [ 0, [[VECTOR_MAIN_LOOP_ITER_CHECK]] ]
-; CHECK-NEXT:    br label [[VEC_EPILOG_VECTOR_BODY:%.*]]
-; CHECK:       vec.epilog.vector.body:
-; CHECK-NEXT:    [[INDEX1:%.*]] = phi i64 [ [[VEC_EPILOG_RESUME_VAL]], [[VEC_EPILOG_PH]] ], [ [[INDEX_NEXT4:%.*]], [[VEC_EPILOG_VECTOR_BODY]] ]
-; CHECK-NEXT:    [[TMP6:%.*]] = add i64 [[INDEX1]], 0
-; CHECK-NEXT:    [[TMP7:%.*]] = getelementptr inbounds [3 x i8], ptr [[SRC]], i64 [[TMP6]], i64 0
-; CHECK-NEXT:    [[TMP8:%.*]] = getelementptr inbounds i8, ptr [[TMP7]], i32 0
-; CHECK-NEXT:    [[WIDE_VEC2:%.*]] = load <24 x i8>, ptr [[TMP8]], align 1
-; CHECK-NEXT:    [[STRIDED_VEC3:%.*]] = shufflevector <24 x i8> [[WIDE_VEC2]], <24 x i8> poison, <8 x i32> <i32 0, i32 3, i32 6, i32 9, i32 12, i32 15, i32 18, i32 21>
-; CHECK-NEXT:    [[TMP9:%.*]] = getelementptr inbounds i8, ptr [[DST]], i64 [[TMP6]]
-; CHECK-NEXT:    [[TMP10:%.*]] = getelementptr inbounds i8, ptr [[TMP9]], i32 0
-; CHECK-NEXT:    store <8 x i8> [[STRIDED_VEC3]], ptr [[TMP10]], align 1
-; CHECK-NEXT:    [[INDEX_NEXT4]] = add nuw i64 [[INDEX1]], 8
-; CHECK-NEXT:    [[TMP11:%.*]] = icmp eq i64 [[INDEX_NEXT4]], 24
-; CHECK-NEXT:    br i1 [[TMP11]], label [[VEC_EPILOG_MIDDLE_BLOCK:%.*]], label [[VEC_EPILOG_VECTOR_BODY]], !llvm.loop [[LOOP14:![0-9]+]]
-; CHECK:       vec.epilog.middle.block:
-; CHECK-NEXT:    br label [[VEC_EPILOG_SCALAR_PH]]
-; CHECK:       vec.epilog.scalar.ph:
-; CHECK-NEXT:    [[BC_RESUME_VAL:%.*]] = phi i64 [ 24, [[VEC_EPILOG_MIDDLE_BLOCK]] ], [ 0, [[VEC_EPILOG_ITER_CHECK]] ], [ 0, [[ITER_CHECK:%.*]] ]
+; CHECK-NEXT:    br label [[SCALAR_PH]]
+; CHECK:       scalar.ph:
+; CHECK-NEXT:    [[BC_RESUME_VAL:%.*]] = phi i64 [ 24, [[MIDDLE_BLOCK]] ], [ 0, [[ENTRY:%.*]] ]
 ; CHECK-NEXT:    br label [[LOOP:%.*]]
 ; CHECK:       loop:
-; CHECK-NEXT:    [[IV:%.*]] = phi i64 [ [[BC_RESUME_VAL]], [[VEC_EPILOG_SCALAR_PH]] ], [ [[IV_NEXT:%.*]], [[LOOP]] ]
+; CHECK-NEXT:    [[IV:%.*]] = phi i64 [ [[BC_RESUME_VAL]], [[SCALAR_PH]] ], [ [[IV_NEXT:%.*]], [[LOOP]] ]
 ; CHECK-NEXT:    [[GEP_SRC:%.*]] = getelementptr inbounds [3 x i8], ptr [[SRC]], i64 [[IV]], i64 0
 ; CHECK-NEXT:    [[L:%.*]] = load i8, ptr [[GEP_SRC]], align 1
 ; CHECK-NEXT:    [[GEP_DST:%.*]] = getelementptr inbounds i8, ptr [[DST]], i64 [[IV]]
 ; CHECK-NEXT:    store i8 [[L]], ptr [[GEP_DST]], align 1
 ; CHECK-NEXT:    [[IV_NEXT]] = add nuw nsw i64 [[IV]], 1
 ; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i64 [[IV_NEXT]], 32
-; CHECK-NEXT:    br i1 [[CMP]], label [[EXIT:%.*]], label [[LOOP]], !llvm.loop [[LOOP15:![0-9]+]]
+; CHECK-NEXT:    br i1 [[CMP]], label [[EXIT:%.*]], label [[LOOP]], !llvm.loop [[LOOP14:![0-9]+]]
 ; CHECK:       exit:
 ; CHECK-NEXT:    ret void
 ;
Index: llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
===================================================================
--- llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
+++ llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
@@ -5308,6 +5308,13 @@
     auto Min = Attr.getVScaleRangeMin();
     WidestRegisterMinEC *= Min;
   }
+
+  // When a scalar epilogue is required, at least one iteration of the scalar
+  // loop has to execute. Adjust ConstTripCount accordingly to avoid picking a
+  // max VF that results in a dead vector loop.
+  if (ConstTripCount > 0 && requiresScalarEpilogue(true))
+    ConstTripCount -= 1;
+
   if (ConstTripCount && ConstTripCount <= WidestRegisterMinEC &&
       (!FoldTailByMasking || isPowerOf2_32(ConstTripCount))) {
     // If loop trip count (TC) is known at compile time there is no point in


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