[LAA] Be more precise on different store sizes #122318
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@llvm/pr-subscribers-llvm-analysis @llvm/pr-subscribers-llvm-transforms Author: Ramkumar Ramachandra (artagnon) ChangesThe HasSameSize checks, which are triggered on different store sizes, in MemDepChecker::isDependent are ad-hoc and imprecise, leading to spurious dependencies and runtime-checks. Identify that the exact scenario in which to bail out is unequal store sizes when dependence distance is zero, and check precisely this condition in Full diff: https://github.com/llvm/llvm-project/pull/122318.diff 4 Files Affected:
diff --git a/llvm/lib/Analysis/LoopAccessAnalysis.cpp b/llvm/lib/Analysis/LoopAccessAnalysis.cpp
index 38e9145826c08e..90a9de8bf1a601 100644
--- a/llvm/lib/Analysis/LoopAccessAnalysis.cpp
+++ b/llvm/lib/Analysis/LoopAccessAnalysis.cpp
@@ -1990,10 +1990,17 @@ MemoryDepChecker::getDependenceDistanceStrideAndSize(
}
uint64_t TypeByteSize = DL.getTypeAllocSize(ATy);
- bool HasSameSize =
- DL.getTypeStoreSizeInBits(ATy) == DL.getTypeStoreSizeInBits(BTy);
- if (!HasSameSize)
- TypeByteSize = 0;
+
+ // When the distance is zero, we're reading/writing the same memory location:
+ // check that the store sizes are equal. Otherwise, fail with an unknown
+ // dependence for which we should not generate runtime checks.
+ TypeSize AStoreSz = DL.getTypeStoreSizeInBits(ATy),
+ BStoreSz = DL.getTypeStoreSizeInBits(BTy);
+ if (Dist->isZero() && AStoreSz != BStoreSz) {
+ LLVM_DEBUG(
+ dbgs() << "LAA: zero dependence distance but different type sizes\n");
+ return Dependence::Unknown;
+ }
StrideAPtrInt = std::abs(StrideAPtrInt);
StrideBPtrInt = std::abs(StrideBPtrInt);
@@ -2029,7 +2036,6 @@ MemoryDepChecker::isDependent(const MemAccessInfo &A, unsigned AIdx,
auto &[Dist, MaxStride, CommonStride, ShouldRetryWithRuntimeCheck,
TypeByteSize, AIsWrite, BIsWrite] =
std::get<DepDistanceStrideAndSizeInfo>(Res);
- bool HasSameSize = TypeByteSize > 0;
if (isa<SCEVCouldNotCompute>(Dist)) {
// TODO: Relax requirement that there is a common unscaled stride to retry
@@ -2047,9 +2053,9 @@ MemoryDepChecker::isDependent(const MemAccessInfo &A, unsigned AIdx,
// upper bound of the number of iterations), the accesses are independet, i.e.
// they are far enough appart that accesses won't access the same location
// across all loop ierations.
- if (HasSameSize && isSafeDependenceDistance(
- DL, SE, *(PSE.getSymbolicMaxBackedgeTakenCount()),
- *Dist, MaxStride, TypeByteSize))
+ if (isSafeDependenceDistance(DL, SE,
+ *(PSE.getSymbolicMaxBackedgeTakenCount()), *Dist,
+ MaxStride, TypeByteSize))
return Dependence::NoDep;
const SCEVConstant *ConstDist = dyn_cast<SCEVConstant>(Dist);
@@ -2060,7 +2066,7 @@ MemoryDepChecker::isDependent(const MemAccessInfo &A, unsigned AIdx,
// If the distance between accesses and their strides are known constants,
// check whether the accesses interlace each other.
- if (Distance > 0 && CommonStride && CommonStride > 1 && HasSameSize &&
+ if (Distance > 0 && CommonStride && CommonStride > 1 &&
areStridedAccessesIndependent(Distance, *CommonStride, TypeByteSize)) {
LLVM_DEBUG(dbgs() << "LAA: Strided accesses are independent\n");
return Dependence::NoDep;
@@ -2074,15 +2080,9 @@ MemoryDepChecker::isDependent(const MemAccessInfo &A, unsigned AIdx,
// Negative distances are not plausible dependencies.
if (SE.isKnownNonPositive(Dist)) {
- if (SE.isKnownNonNegative(Dist)) {
- if (HasSameSize) {
- // Write to the same location with the same size.
- return Dependence::Forward;
- }
- LLVM_DEBUG(dbgs() << "LAA: possibly zero dependence difference but "
- "different type sizes\n");
- return Dependence::Unknown;
- }
+ if (SE.isKnownNonNegative(Dist))
+ // Write to the same location with the same size.
+ return Dependence::Forward;
bool IsTrueDataDependence = (AIsWrite && !BIsWrite);
// Check if the first access writes to a location that is read in a later
@@ -2102,8 +2102,7 @@ MemoryDepChecker::isDependent(const MemAccessInfo &A, unsigned AIdx,
FoundNonConstantDistanceDependence |= ShouldRetryWithRuntimeCheck;
return Dependence::Unknown;
}
- if (!HasSameSize ||
- couldPreventStoreLoadForward(
+ if (couldPreventStoreLoadForward(
ConstDist->getAPInt().abs().getZExtValue(), TypeByteSize)) {
LLVM_DEBUG(
dbgs() << "LAA: Forward but may prevent st->ld forwarding\n");
@@ -2134,12 +2133,6 @@ MemoryDepChecker::isDependent(const MemAccessInfo &A, unsigned AIdx,
FoundNonConstantDistanceDependence |= ShouldRetryWithRuntimeCheck;
}
- if (!HasSameSize) {
- LLVM_DEBUG(dbgs() << "LAA: ReadWrite-Write positive dependency with "
- "different type sizes\n");
- return Dependence::Unknown;
- }
-
if (!CommonStride)
return Dependence::Unknown;
diff --git a/llvm/test/Analysis/LoopAccessAnalysis/forward-loop-carried.ll b/llvm/test/Analysis/LoopAccessAnalysis/forward-loop-carried.ll
index adfd19923e921c..7837c20f003e24 100644
--- a/llvm/test/Analysis/LoopAccessAnalysis/forward-loop-carried.ll
+++ b/llvm/test/Analysis/LoopAccessAnalysis/forward-loop-carried.ll
@@ -70,10 +70,6 @@ define void @forward_different_access_sizes(ptr readnone %end, ptr %start) {
; CHECK-NEXT: store i32 0, ptr %gep.2, align 4 ->
; CHECK-NEXT: %l = load i24, ptr %gep.1, align 1
; CHECK-EMPTY:
-; CHECK-NEXT: Forward:
-; CHECK-NEXT: store i32 0, ptr %gep.2, align 4 ->
-; CHECK-NEXT: store i24 %l, ptr %ptr.iv, align 1
-; CHECK-EMPTY:
; CHECK-NEXT: Run-time memory checks:
; CHECK-NEXT: Grouped accesses:
; CHECK-EMPTY:
diff --git a/llvm/test/Analysis/LoopAccessAnalysis/positive-dependence-distance-different-access-sizes.ll b/llvm/test/Analysis/LoopAccessAnalysis/positive-dependence-distance-different-access-sizes.ll
index 08e0bae7f05bac..c43b072b30a8ea 100644
--- a/llvm/test/Analysis/LoopAccessAnalysis/positive-dependence-distance-different-access-sizes.ll
+++ b/llvm/test/Analysis/LoopAccessAnalysis/positive-dependence-distance-different-access-sizes.ll
@@ -3,26 +3,13 @@
target datalayout = "e-m:o-i64:64-i128:128-n32:64-S128"
-; TODO: No runtime checks should be needed, as the distance between accesses
-; is large enough to need runtime checks.
define void @test_distance_positive_independent_via_trip_count(ptr %A) {
; CHECK-LABEL: 'test_distance_positive_independent_via_trip_count'
; CHECK-NEXT: loop:
-; CHECK-NEXT: Memory dependences are safe with run-time checks
+; CHECK-NEXT: Memory dependences are safe
; CHECK-NEXT: Dependences:
; CHECK-NEXT: Run-time memory checks:
-; CHECK-NEXT: Check 0:
-; CHECK-NEXT: Comparing group ([[GRP1:0x[0-9a-f]+]]):
-; CHECK-NEXT: %gep.A.400 = getelementptr inbounds i32, ptr %A.400, i64 %iv
-; CHECK-NEXT: Against group ([[GRP2:0x[0-9a-f]+]]):
-; CHECK-NEXT: %gep.A = getelementptr inbounds i8, ptr %A, i64 %iv
; CHECK-NEXT: Grouped accesses:
-; CHECK-NEXT: Group [[GRP1]]:
-; CHECK-NEXT: (Low: (400 + %A)<nuw> High: (804 + %A))
-; CHECK-NEXT: Member: {(400 + %A)<nuw>,+,4}<nuw><%loop>
-; CHECK-NEXT: Group [[GRP2]]:
-; CHECK-NEXT: (Low: %A High: (101 + %A))
-; CHECK-NEXT: Member: {%A,+,1}<nuw><%loop>
; CHECK-EMPTY:
; CHECK-NEXT: Non vectorizable stores to invariant address were not found in loop.
; CHECK-NEXT: SCEV assumptions:
@@ -57,15 +44,15 @@ define void @test_distance_positive_backwards(ptr %A) {
; CHECK-NEXT: Dependences:
; CHECK-NEXT: Run-time memory checks:
; CHECK-NEXT: Check 0:
-; CHECK-NEXT: Comparing group ([[GRP3:0x[0-9a-f]+]]):
+; CHECK-NEXT: Comparing group ([[GRP1:0x[0-9a-f]+]]):
; CHECK-NEXT: %gep.A.400 = getelementptr inbounds i32, ptr %A.1, i64 %iv
-; CHECK-NEXT: Against group ([[GRP4:0x[0-9a-f]+]]):
+; CHECK-NEXT: Against group ([[GRP2:0x[0-9a-f]+]]):
; CHECK-NEXT: %gep.A = getelementptr inbounds i8, ptr %A, i64 %iv
; CHECK-NEXT: Grouped accesses:
-; CHECK-NEXT: Group [[GRP3]]:
+; CHECK-NEXT: Group [[GRP1]]:
; CHECK-NEXT: (Low: (1 + %A)<nuw> High: (405 + %A))
; CHECK-NEXT: Member: {(1 + %A)<nuw>,+,4}<nuw><%loop>
-; CHECK-NEXT: Group [[GRP4]]:
+; CHECK-NEXT: Group [[GRP2]]:
; CHECK-NEXT: (Low: %A High: (101 + %A))
; CHECK-NEXT: Member: {%A,+,1}<nuw><%loop>
; CHECK-EMPTY:
@@ -100,15 +87,15 @@ define void @test_distance_positive_via_assume(ptr %A, i64 %off) {
; CHECK-NEXT: Dependences:
; CHECK-NEXT: Run-time memory checks:
; CHECK-NEXT: Check 0:
-; CHECK-NEXT: Comparing group ([[GRP5:0x[0-9a-f]+]]):
+; CHECK-NEXT: Comparing group ([[GRP3:0x[0-9a-f]+]]):
; CHECK-NEXT: %gep.A.400 = getelementptr inbounds i32, ptr %A.off, i64 %iv
-; CHECK-NEXT: Against group ([[GRP6:0x[0-9a-f]+]]):
+; CHECK-NEXT: Against group ([[GRP4:0x[0-9a-f]+]]):
; CHECK-NEXT: %gep.A = getelementptr inbounds i8, ptr %A, i64 %iv
; CHECK-NEXT: Grouped accesses:
-; CHECK-NEXT: Group [[GRP5]]:
+; CHECK-NEXT: Group [[GRP3]]:
; CHECK-NEXT: (Low: (%off + %A) High: (404 + %off + %A))
; CHECK-NEXT: Member: {(%off + %A),+,4}<nw><%loop>
-; CHECK-NEXT: Group [[GRP6]]:
+; CHECK-NEXT: Group [[GRP4]]:
; CHECK-NEXT: (Low: %A High: (101 + %A))
; CHECK-NEXT: Member: {%A,+,1}<nuw><%loop>
; CHECK-EMPTY:
diff --git a/llvm/test/Transforms/LoopVectorize/AArch64/interleave-allocsize-not-equal-typesize.ll b/llvm/test/Transforms/LoopVectorize/AArch64/interleave-allocsize-not-equal-typesize.ll
index bd77f9779b680d..6451aa96b04da4 100644
--- a/llvm/test/Transforms/LoopVectorize/AArch64/interleave-allocsize-not-equal-typesize.ll
+++ b/llvm/test/Transforms/LoopVectorize/AArch64/interleave-allocsize-not-equal-typesize.ll
@@ -35,10 +35,10 @@ define void @pr58722_load_interleave_group(ptr %src, ptr %dst) {
; CHECK-NEXT: [[TMP10:%.*]] = getelementptr inbounds i32, ptr [[TMP5]], i64 1
; CHECK-NEXT: [[TMP11:%.*]] = getelementptr inbounds i32, ptr [[TMP6]], i64 1
; CHECK-NEXT: [[TMP12:%.*]] = getelementptr inbounds i32, ptr [[TMP7]], i64 1
-; CHECK-NEXT: [[TMP13:%.*]] = load i24, ptr [[TMP9]], align 4, !alias.scope !0
-; CHECK-NEXT: [[TMP14:%.*]] = load i24, ptr [[TMP10]], align 4, !alias.scope !0
-; CHECK-NEXT: [[TMP15:%.*]] = load i24, ptr [[TMP11]], align 4, !alias.scope !0
-; CHECK-NEXT: [[TMP16:%.*]] = load i24, ptr [[TMP12]], align 4, !alias.scope !0
+; CHECK-NEXT: [[TMP13:%.*]] = load i24, ptr [[TMP9]], align 4, !alias.scope [[META0:![0-9]+]]
+; CHECK-NEXT: [[TMP14:%.*]] = load i24, ptr [[TMP10]], align 4, !alias.scope [[META0]]
+; CHECK-NEXT: [[TMP15:%.*]] = load i24, ptr [[TMP11]], align 4, !alias.scope [[META0]]
+; CHECK-NEXT: [[TMP16:%.*]] = load i24, ptr [[TMP12]], align 4, !alias.scope [[META0]]
; CHECK-NEXT: [[TMP17:%.*]] = insertelement <4 x i24> poison, i24 [[TMP13]], i32 0
; CHECK-NEXT: [[TMP18:%.*]] = insertelement <4 x i24> [[TMP17]], i24 [[TMP14]], i32 1
; CHECK-NEXT: [[TMP19:%.*]] = insertelement <4 x i24> [[TMP18]], i24 [[TMP15]], i32 2
@@ -47,7 +47,7 @@ define void @pr58722_load_interleave_group(ptr %src, ptr %dst) {
; CHECK-NEXT: [[TMP22:%.*]] = add <4 x i32> [[STRIDED_VEC]], [[TMP21]]
; CHECK-NEXT: [[TMP23:%.*]] = getelementptr inbounds i32, ptr [[DST]], i64 [[TMP0]]
; CHECK-NEXT: [[TMP24:%.*]] = getelementptr inbounds i32, ptr [[TMP23]], i32 0
-; CHECK-NEXT: store <4 x i32> [[TMP22]], ptr [[TMP24]], align 4, !alias.scope !3, !noalias !0
+; CHECK-NEXT: store <4 x i32> [[TMP22]], ptr [[TMP24]], align 4, !alias.scope [[META3:![0-9]+]], !noalias [[META0]]
; CHECK-NEXT: [[INDEX_NEXT]] = add nuw i64 [[INDEX]], 4
; CHECK-NEXT: [[TMP25:%.*]] = icmp eq i64 [[INDEX_NEXT]], 10000
; CHECK-NEXT: br i1 [[TMP25]], label [[MIDDLE_BLOCK:%.*]], label [[VECTOR_BODY]], !llvm.loop [[LOOP5:![0-9]+]]
@@ -96,17 +96,42 @@ exit:
define void @pr58722_store_interleave_group(ptr %src, ptr %dst) {
; CHECK-LABEL: @pr58722_store_interleave_group(
; 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:
+; CHECK-NEXT: [[INDEX:%.*]] = phi i32 [ 0, [[VECTOR_PH]] ], [ [[INDEX_NEXT:%.*]], [[VECTOR_BODY]] ]
+; CHECK-NEXT: [[OFFSET_IDX:%.*]] = mul i32 [[INDEX]], 2
+; CHECK-NEXT: [[TMP0:%.*]] = add i32 [[OFFSET_IDX]], 0
+; CHECK-NEXT: [[TMP1:%.*]] = add i32 [[OFFSET_IDX]], 2
+; CHECK-NEXT: [[TMP2:%.*]] = getelementptr inbounds i64, ptr [[SRC:%.*]], i32 [[TMP0]]
+; CHECK-NEXT: [[TMP3:%.*]] = getelementptr inbounds i64, ptr [[SRC]], i32 [[TMP1]]
+; CHECK-NEXT: store i32 [[TMP0]], ptr [[TMP2]], align 4
+; CHECK-NEXT: store i32 [[TMP1]], ptr [[TMP3]], align 4
+; CHECK-NEXT: [[TMP4:%.*]] = getelementptr inbounds i64, ptr [[TMP2]], i64 1
+; CHECK-NEXT: [[TMP5:%.*]] = getelementptr inbounds i64, ptr [[TMP3]], i64 1
+; CHECK-NEXT: [[TMP6:%.*]] = trunc i32 [[TMP0]] to i24
+; CHECK-NEXT: [[TMP7:%.*]] = trunc i32 [[TMP1]] to i24
+; CHECK-NEXT: store i24 [[TMP6]], ptr [[TMP4]], align 4
+; CHECK-NEXT: store i24 [[TMP7]], ptr [[TMP5]], align 4
+; CHECK-NEXT: [[INDEX_NEXT]] = add nuw i32 [[INDEX]], 2
+; CHECK-NEXT: [[TMP8:%.*]] = icmp eq i32 [[INDEX_NEXT]], 5000
+; CHECK-NEXT: br i1 [[TMP8]], label [[MIDDLE_BLOCK:%.*]], label [[VECTOR_BODY]], !llvm.loop [[LOOP9:![0-9]+]]
+; CHECK: middle.block:
+; CHECK-NEXT: br i1 false, label [[EXIT:%.*]], label [[SCALAR_PH]]
+; CHECK: scalar.ph:
+; CHECK-NEXT: [[BC_RESUME_VAL:%.*]] = phi i32 [ 10000, [[MIDDLE_BLOCK]] ], [ 0, [[ENTRY:%.*]] ]
; CHECK-NEXT: br label [[LOOP:%.*]]
; CHECK: loop:
-; CHECK-NEXT: [[IV:%.*]] = phi i32 [ 0, [[ENTRY:%.*]] ], [ [[IV_NEXT:%.*]], [[LOOP]] ]
-; CHECK-NEXT: [[GEP_IV:%.*]] = getelementptr inbounds i64, ptr [[SRC:%.*]], i32 [[IV]]
+; CHECK-NEXT: [[IV:%.*]] = phi i32 [ [[BC_RESUME_VAL]], [[SCALAR_PH]] ], [ [[IV_NEXT:%.*]], [[LOOP]] ]
+; CHECK-NEXT: [[GEP_IV:%.*]] = getelementptr inbounds i64, ptr [[SRC]], i32 [[IV]]
; CHECK-NEXT: store i32 [[IV]], ptr [[GEP_IV]], align 4
; CHECK-NEXT: [[GEP:%.*]] = getelementptr inbounds i64, ptr [[GEP_IV]], i64 1
; CHECK-NEXT: [[TRUNC_IV:%.*]] = trunc i32 [[IV]] to i24
; CHECK-NEXT: store i24 [[TRUNC_IV]], ptr [[GEP]], align 4
; CHECK-NEXT: [[IV_NEXT]] = add i32 [[IV]], 2
; CHECK-NEXT: [[CMP:%.*]] = icmp eq i32 [[IV]], 10000
-; CHECK-NEXT: br i1 [[CMP]], label [[EXIT:%.*]], label [[LOOP]]
+; CHECK-NEXT: br i1 [[CMP]], label [[EXIT]], label [[LOOP]], !llvm.loop [[LOOP10:![0-9]+]]
; CHECK: exit:
; CHECK-NEXT: ret void
;
|
fhahn
reviewed
Jan 9, 2025
| // dependence for which we should not generate runtime checks. | ||
| TypeSize AStoreSz = DL.getTypeStoreSize(ATy), | ||
| BStoreSz = DL.getTypeStoreSize(BTy); | ||
| if (Dist->isZero() && AStoreSz != BStoreSz) { |
There was a problem hiding this comment.
I think it would be good to be more conservative here and check something like !SE.isKnownNonZero(Dist), as Dist could be zero even if it is not a zero constant?
There was a problem hiding this comment.
So I think this is wrong, and results in test regressions, because if the distance is not constant, we should retry with runtime checks. We're handling the scenario where we know that the distance is zero and hence know that there is a read/write with different store sizes: this is the condition on which we should bail, and not generate any runtime checks. If the store sizes are different and the distance is possibly zero, I think this existing code handles the case fine already:
bool IsTrueDataDependence = (AIsWrite && !BIsWrite);
// Check if the first access writes to a location that is read in a later
// iteration, where the distance between them is not a multiple of a vector
// factor and relatively small.
//
// NOTE: There is no need to update MaxSafeVectorWidthInBits after call to
// couldPreventStoreLoadForward, even if it changed MinDepDistBytes, since a
// forward dependency will allow vectorization using any width.
if (IsTrueDataDependence && EnableForwardingConflictDetection) {
if (!ConstDist) {
// TODO: FoundNonConstantDistanceDependence is used as a necessary
// condition to consider retrying with runtime checks. Historically, we
// did not set it when strides were different but there is no inherent
// reason to.
FoundNonConstantDistanceDependence |= ShouldRetryWithRuntimeCheck;
return Dependence::Unknown;
}
if (couldPreventStoreLoadForward(
ConstDist->getAPInt().abs().getZExtValue(), TypeByteSize)) {
LLVM_DEBUG(
dbgs() << "LAA: Forward but may prevent st->ld forwarding\n");
return Dependence::ForwardButPreventsForwarding;
}
}There was a problem hiding this comment.
I think these hunks in the patch are correct, and that we were being overly conservative earlier. See also: positive LV test change.
@@ -2102,8 +2110,7 @@ MemoryDepChecker::isDependent(const MemAccessInfo &A, unsigned AIdx,
FoundNonConstantDistanceDependence |= ShouldRetryWithRuntimeCheck;
return Dependence::Unknown;
}
- if (!HasSameSize ||
- couldPreventStoreLoadForward(
+ if (couldPreventStoreLoadForward(
ConstDist->getAPInt().abs().getZExtValue(), TypeByteSize)) {
LLVM_DEBUG(
dbgs() << "LAA: Forward but may prevent st->ld forwarding\n");
@@ -2134,12 +2141,6 @@ MemoryDepChecker::isDependent(const MemAccessInfo &A, unsigned AIdx,
FoundNonConstantDistanceDependence |= ShouldRetryWithRuntimeCheck;
}
- if (!HasSameSize) {
- LLVM_DEBUG(dbgs() << "LAA: ReadWrite-Write positive dependency with "
- "different type sizes\n");
- return Dependence::Unknown;
- }
-The HasSameSize checks, which are triggered on different store sizes, in MemDepChecker::isDependent are ad-hoc and imprecise, leading to spurious dependencies and runtime-checks. Identify that the exact scenario in which to bail out is unequal store sizes when dependence distance is zero, and check precisely this condition in MemDepChecker::getDependenceDistanceAndSize, eliminating all the ad-hoc checks in isDependent and making LoopAccessAnalysis more precise.
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Gentle ping. |
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The HasSameSize checks, which are triggered on different store sizes, in MemDepChecker::isDependent are ad-hoc and imprecise, leading to spurious dependencies, always-false runtime-checks, and failed vectorization. Identify that the exact scenario in which to bail out is unequal store sizes when dependence distance is zero, and check precisely this condition in
MemDepChecker::getDependenceDistanceAndSize, eliminating all the ad-hoc checks in isDependent and making LoopAccessAnalysis more precise.