[PATCH] D154261: [LV] Consider if scalar epilogue is required in getMaximizedVFForTarget.
Florian Hahn via Phabricator via llvm-commits
llvm-commits at lists.llvm.org
Tue Jul 4 07:41:06 PDT 2023
fhahn updated this revision to Diff 537095.
fhahn marked an inline comment as done.
fhahn added a comment.
In D154261#4471308 <https://reviews.llvm.org/D154261#4471308>, @Ayal wrote:
> In D154261#4468894 <https://reviews.llvm.org/D154261#4468894>, @fhahn wrote:
>
>> 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.
>
> Ah, the original issue of pr56319 was to ensure that the epilog loop leaves at-least one iteration for the scalar loop (or actually epilogVF iterations) rather than vectorizing all remaining iterations? An odd trip count of 39 will always end up with a last scalar iteration, due to main and epilog VF's being even. Perhaps a trip count of 48 with (forced?) main-loop VF of 32 and epilog-loop VF of 8 should ensure the epilog loop runs only once.
The original issue was that we simplified the branch in the main vector loop to exit in the first iteration (VF == vector TC) and because epilogue and main vector loops share the same VPlan, it was also simplified for the epilogue vector loop, but there where more than 1 vector iteration (fix was https://github.com/llvm/llvm-project/commit/0dddf04caba55a64f8534518d65311bdac05cf39).
Now, the test still checks that we don't simplify neither the main nor epilogue vector loop branch. But in this case, we could simplify both, because the epilogue vector loop only executes a single iteration; we should also have a variant where the epilogue vector loops executes multiple times (e.g. main VF=32, epilogue VF=8, TC = 49), but unfortunately my attempts so far weren't successful, because there's some code in `isMoreProfitable` that estimates the cost of the vector loop + scalar tail for the given VFs, which ignores possible epilogue vectorization( https://github.com/llvm/llvm-project/blob/main/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp#L5384). For all cases I tried, this leads to a small main VF ends up being chose (8 in that case) if there would be more than a single iteration of the epilogue vector loop.
Repository:
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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
@@ -283,12 +283,10 @@
ret void
}
-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:%.*]]
+define void @limit_main_loop_vf_to_avoid_dead_main_vector_loop(ptr noalias %src, ptr noalias %dst) {
+; CHECK-LABEL: @limit_main_loop_vf_to_avoid_dead_main_vector_loop(
+; 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
@@ -5279,6 +5279,13 @@
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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