[llvm] [AArch64] Runtime-unroll small load/store loops for Apple Silicon CPUs. (PR #118317)
Florian Hahn via llvm-commits
llvm-commits at lists.llvm.org
Tue Dec 3 02:51:11 PST 2024
https://github.com/fhahn updated https://github.com/llvm/llvm-project/pull/118317
>From 3c83402cacb274ef4228c79f90c26431142d9f47 Mon Sep 17 00:00:00 2001
From: Florian Hahn <flo at fhahn.com>
Date: Mon, 2 Dec 2024 12:29:00 +0000
Subject: [PATCH 1/2] [AArch64] Runtime-unroll small load/store loops for Apple
Silicon CPUs.
Add initial heuristics to selectively enable runtime unrolling for loops
where doing so is expected to be highly beneficial on Apple Silicon
CPUs.
To start with, we try to runtime-unroll small, single block loops, if they
have load/store dependencies, to expose more parallel memory access streams [1]
and to improve instruction delivery [2].
We also explicitly avoid runtime-unrolling for loop structures that may
limit the expected gains from runtime unrolling. Such loops include
loops with complex control flow (aren't innermost loops, have multiple
exits, have a large number of blocks), trip count expansion is
expensive and are expected to execute a small number of iterations.
Note that the heuristics here may be overly conservative and we err on the
side of avoiding runtime unrolling rather than unroll excessively. They are
all subject to further refinement.
[1] 4.6.10 in Apple Silicon CPU Optimization Guide
[2] 4.4.4 in Apple Silicon CPU Optimization Guide
(Depends on https://github.com/llvm/llvm-project/pull/118316 for TTI
changes, which are included in this PR for now)
---
.../AArch64/AArch64TargetTransformInfo.cpp | 90 +++++++++++++++++++
.../LoopUnroll/AArch64/apple-unrolling.ll | 82 ++++++++++++++++-
2 files changed, 168 insertions(+), 4 deletions(-)
diff --git a/llvm/lib/Target/AArch64/AArch64TargetTransformInfo.cpp b/llvm/lib/Target/AArch64/AArch64TargetTransformInfo.cpp
index 5b333d33cffd52..035854b8a40293 100644
--- a/llvm/lib/Target/AArch64/AArch64TargetTransformInfo.cpp
+++ b/llvm/lib/Target/AArch64/AArch64TargetTransformInfo.cpp
@@ -3989,6 +3989,90 @@ getFalkorUnrollingPreferences(Loop *L, ScalarEvolution &SE,
}
}
+/// For Apple CPUs, we want to runtime-unroll loops to make better use if the
+/// OOO engine's wide instruction window and various predictors.
+static void
+getAppleRuntimeUnrollPreferences(Loop *L, ScalarEvolution &SE,
+ TargetTransformInfo::UnrollingPreferences &UP,
+ AArch64TTIImpl &TTI) {
+ // Limit loops with structure that is highly likely to benefit from runtime
+ // unrolling; that is we exclude outer loops, loops with multiple exits and
+ // many blocks (i.e. likely with complex control flow). Note that the
+ // heuristics here may be overly conservative and we err on the side of
+ // avoiding runtime unrolling rather than unroll excessively. They are all
+ // subject to further refinement.
+ if (!L->isInnermost() || !L->getExitBlock() || L->getNumBlocks() > 8)
+ return;
+
+ const SCEV *BTC = SE.getBackedgeTakenCount(L);
+ if (isa<SCEVConstant>(BTC) || isa<SCEVCouldNotCompute>(BTC) ||
+ (SE.getSmallConstantMaxTripCount(L) > 0 &&
+ SE.getSmallConstantMaxTripCount(L) <= 32))
+ return;
+ if (findStringMetadataForLoop(L, "llvm.loop.isvectorized"))
+ return;
+
+ int64_t Size = 0;
+ for (auto *BB : L->getBlocks()) {
+ for (auto &I : *BB) {
+ if (!isa<IntrinsicInst>(&I) && isa<CallBase>(&I))
+ return;
+ SmallVector<const Value *, 4> Operands(I.operand_values());
+ Size +=
+ *TTI.getInstructionCost(&I, Operands, TTI::TCK_CodeSize).getValue();
+ }
+ }
+
+ // Limit to loops with trip counts that are cheap to expand.
+ UP.SCEVExpansionBudget = 1;
+
+ // Try to unroll small, single block loops, if they have load/store
+ // dependencies, to expose more parallel memory access streams.
+ if (L->getHeader() != L->getLoopLatch() || Size > 8)
+ return;
+
+ SmallPtrSet<const SCEV *, 8> LoadPtrs;
+ SmallPtrSet<const SCEV *, 8> StorePtrs;
+ SmallPtrSet<Value *, 8> LoadedValues;
+ SmallVector<StoreInst *> Stores;
+ for (auto *BB : L->blocks()) {
+ for (auto &I : *BB) {
+ Value *Ptr = getLoadStorePointerOperand(&I);
+ if (!Ptr)
+ continue;
+ const SCEV *PtrSCEV = SE.getSCEV(Ptr);
+ if (SE.isLoopInvariant(PtrSCEV, L))
+ continue;
+ if (isa<LoadInst>(&I)) {
+ LoadPtrs.insert(PtrSCEV);
+ LoadedValues.insert(&I);
+ } else {
+ Stores.push_back(cast<StoreInst>(&I));
+ StorePtrs.insert(PtrSCEV);
+ }
+ }
+ }
+
+ // Try to find an unroll count that maximizes the use of the instruction
+ // window.
+ unsigned UC = std::max(16ll / Size, 2ll);
+ unsigned BestUC = 0;
+ while (UC <= 8 && UC * Size <= 48) {
+ if ((UC * Size % 16) == 0 || (BestUC * Size % 16) < (UC * Size % 16) % 16) {
+ BestUC = UC;
+ }
+ UC++;
+ }
+
+ if (BestUC == 0 || none_of(Stores, [&LoadedValues](StoreInst *SI) {
+ return LoadedValues.contains(SI->getOperand(0));
+ }))
+ return;
+
+ UP.Runtime = true;
+ UP.DefaultUnrollRuntimeCount = BestUC;
+}
+
void AArch64TTIImpl::getUnrollingPreferences(Loop *L, ScalarEvolution &SE,
TTI::UnrollingPreferences &UP,
OptimizationRemarkEmitter *ORE) {
@@ -4010,6 +4094,12 @@ void AArch64TTIImpl::getUnrollingPreferences(Loop *L, ScalarEvolution &SE,
EnableFalkorHWPFUnrollFix)
getFalkorUnrollingPreferences(L, SE, UP);
+ if (ST->getProcFamily() == AArch64Subtarget::AppleA14 ||
+ ST->getProcFamily() == AArch64Subtarget::AppleA15 ||
+ ST->getProcFamily() == AArch64Subtarget::AppleA16 ||
+ ST->getProcFamily() == AArch64Subtarget::AppleM4)
+ getAppleRuntimeUnrollPreferences(L, SE, UP, *this);
+
// Scan the loop: don't unroll loops with calls as this could prevent
// inlining. Don't unroll vector loops either, as they don't benefit much from
// unrolling.
diff --git a/llvm/test/Transforms/LoopUnroll/AArch64/apple-unrolling.ll b/llvm/test/Transforms/LoopUnroll/AArch64/apple-unrolling.ll
index deacec795fb03a..d27d5e74e28f2e 100644
--- a/llvm/test/Transforms/LoopUnroll/AArch64/apple-unrolling.ll
+++ b/llvm/test/Transforms/LoopUnroll/AArch64/apple-unrolling.ll
@@ -12,17 +12,91 @@ define void @small_load_store_loop(ptr %src, ptr %dst, i64 %N, i64 %scale) {
; APPLE-LABEL: define void @small_load_store_loop(
; APPLE-SAME: ptr [[SRC:%.*]], ptr [[DST:%.*]], i64 [[N:%.*]], i64 [[SCALE:%.*]]) #[[ATTR0:[0-9]+]] {
; APPLE-NEXT: [[ENTRY:.*]]:
+; APPLE-NEXT: [[TMP0:%.*]] = add i64 [[N]], -1
+; APPLE-NEXT: [[XTRAITER:%.*]] = and i64 [[N]], 7
+; APPLE-NEXT: [[TMP1:%.*]] = icmp ult i64 [[TMP0]], 7
+; APPLE-NEXT: br i1 [[TMP1]], label %[[EXIT_UNR_LCSSA:.*]], label %[[ENTRY_NEW:.*]]
+; APPLE: [[ENTRY_NEW]]:
+; APPLE-NEXT: [[UNROLL_ITER:%.*]] = sub i64 [[N]], [[XTRAITER]]
; APPLE-NEXT: br label %[[LOOP:.*]]
; APPLE: [[LOOP]]:
-; APPLE-NEXT: [[IV_EPIL:%.*]] = phi i64 [ 0, %[[ENTRY]] ], [ [[IV_NEXT_EPIL:%.*]], %[[LOOP]] ]
+; APPLE-NEXT: [[IV_EPIL:%.*]] = phi i64 [ 0, %[[ENTRY_NEW]] ], [ [[IV_NEXT_7:%.*]], %[[LOOP]] ]
+; APPLE-NEXT: [[NITER:%.*]] = phi i64 [ 0, %[[ENTRY_NEW]] ], [ [[NITER_NEXT_7:%.*]], %[[LOOP]] ]
; APPLE-NEXT: [[SCALED_IV_EPIL:%.*]] = mul nuw nsw i64 [[IV_EPIL]], [[SCALE]]
; APPLE-NEXT: [[GEP_SRC_EPIL:%.*]] = getelementptr inbounds float, ptr [[SRC]], i64 [[SCALED_IV_EPIL]]
; APPLE-NEXT: [[L_EPIL:%.*]] = load float, ptr [[GEP_SRC_EPIL]], align 4
; APPLE-NEXT: [[GEP_DST_EPIL:%.*]] = getelementptr inbounds float, ptr [[DST]], i64 [[IV_EPIL]]
; APPLE-NEXT: store float [[L_EPIL]], ptr [[GEP_DST_EPIL]], align 4
-; APPLE-NEXT: [[IV_NEXT_EPIL]] = add nuw nsw i64 [[IV_EPIL]], 1
-; APPLE-NEXT: [[EC_EPIL:%.*]] = icmp eq i64 [[IV_NEXT_EPIL]], [[N]]
-; APPLE-NEXT: br i1 [[EC_EPIL]], label %[[EXIT:.*]], label %[[LOOP]]
+; APPLE-NEXT: [[IV_NEXT_EPIL:%.*]] = add nuw nsw i64 [[IV_EPIL]], 1
+; APPLE-NEXT: [[SCALED_IV_1:%.*]] = mul nuw nsw i64 [[IV_NEXT_EPIL]], [[SCALE]]
+; APPLE-NEXT: [[GEP_SRC_1:%.*]] = getelementptr inbounds float, ptr [[SRC]], i64 [[SCALED_IV_1]]
+; APPLE-NEXT: [[L_1:%.*]] = load float, ptr [[GEP_SRC_1]], align 4
+; APPLE-NEXT: [[GEP_DST_1:%.*]] = getelementptr inbounds float, ptr [[DST]], i64 [[IV_NEXT_EPIL]]
+; APPLE-NEXT: store float [[L_1]], ptr [[GEP_DST_1]], align 4
+; APPLE-NEXT: [[IV_NEXT_1:%.*]] = add nuw nsw i64 [[IV_EPIL]], 2
+; APPLE-NEXT: [[SCALED_IV_2:%.*]] = mul nuw nsw i64 [[IV_NEXT_1]], [[SCALE]]
+; APPLE-NEXT: [[GEP_SRC_2:%.*]] = getelementptr inbounds float, ptr [[SRC]], i64 [[SCALED_IV_2]]
+; APPLE-NEXT: [[L_2:%.*]] = load float, ptr [[GEP_SRC_2]], align 4
+; APPLE-NEXT: [[GEP_DST_2:%.*]] = getelementptr inbounds float, ptr [[DST]], i64 [[IV_NEXT_1]]
+; APPLE-NEXT: store float [[L_2]], ptr [[GEP_DST_2]], align 4
+; APPLE-NEXT: [[IV_NEXT_2:%.*]] = add nuw nsw i64 [[IV_EPIL]], 3
+; APPLE-NEXT: [[SCALED_IV_3:%.*]] = mul nuw nsw i64 [[IV_NEXT_2]], [[SCALE]]
+; APPLE-NEXT: [[GEP_SRC_3:%.*]] = getelementptr inbounds float, ptr [[SRC]], i64 [[SCALED_IV_3]]
+; APPLE-NEXT: [[L_3:%.*]] = load float, ptr [[GEP_SRC_3]], align 4
+; APPLE-NEXT: [[GEP_DST_3:%.*]] = getelementptr inbounds float, ptr [[DST]], i64 [[IV_NEXT_2]]
+; APPLE-NEXT: store float [[L_3]], ptr [[GEP_DST_3]], align 4
+; APPLE-NEXT: [[IV_NEXT_3:%.*]] = add nuw nsw i64 [[IV_EPIL]], 4
+; APPLE-NEXT: [[SCALED_IV_4:%.*]] = mul nuw nsw i64 [[IV_NEXT_3]], [[SCALE]]
+; APPLE-NEXT: [[GEP_SRC_4:%.*]] = getelementptr inbounds float, ptr [[SRC]], i64 [[SCALED_IV_4]]
+; APPLE-NEXT: [[L_4:%.*]] = load float, ptr [[GEP_SRC_4]], align 4
+; APPLE-NEXT: [[GEP_DST_4:%.*]] = getelementptr inbounds float, ptr [[DST]], i64 [[IV_NEXT_3]]
+; APPLE-NEXT: store float [[L_4]], ptr [[GEP_DST_4]], align 4
+; APPLE-NEXT: [[IV_NEXT_4:%.*]] = add nuw nsw i64 [[IV_EPIL]], 5
+; APPLE-NEXT: [[SCALED_IV_5:%.*]] = mul nuw nsw i64 [[IV_NEXT_4]], [[SCALE]]
+; APPLE-NEXT: [[GEP_SRC_5:%.*]] = getelementptr inbounds float, ptr [[SRC]], i64 [[SCALED_IV_5]]
+; APPLE-NEXT: [[L_5:%.*]] = load float, ptr [[GEP_SRC_5]], align 4
+; APPLE-NEXT: [[GEP_DST_5:%.*]] = getelementptr inbounds float, ptr [[DST]], i64 [[IV_NEXT_4]]
+; APPLE-NEXT: store float [[L_5]], ptr [[GEP_DST_5]], align 4
+; APPLE-NEXT: [[IV_NEXT_5:%.*]] = add nuw nsw i64 [[IV_EPIL]], 6
+; APPLE-NEXT: [[SCALED_IV_6:%.*]] = mul nuw nsw i64 [[IV_NEXT_5]], [[SCALE]]
+; APPLE-NEXT: [[GEP_SRC_6:%.*]] = getelementptr inbounds float, ptr [[SRC]], i64 [[SCALED_IV_6]]
+; APPLE-NEXT: [[L_6:%.*]] = load float, ptr [[GEP_SRC_6]], align 4
+; APPLE-NEXT: [[GEP_DST_6:%.*]] = getelementptr inbounds float, ptr [[DST]], i64 [[IV_NEXT_5]]
+; APPLE-NEXT: store float [[L_6]], ptr [[GEP_DST_6]], align 4
+; APPLE-NEXT: [[IV_NEXT_6:%.*]] = add nuw nsw i64 [[IV_EPIL]], 7
+; APPLE-NEXT: [[SCALED_IV_7:%.*]] = mul nuw nsw i64 [[IV_NEXT_6]], [[SCALE]]
+; APPLE-NEXT: [[GEP_SRC_7:%.*]] = getelementptr inbounds float, ptr [[SRC]], i64 [[SCALED_IV_7]]
+; APPLE-NEXT: [[L_7:%.*]] = load float, ptr [[GEP_SRC_7]], align 4
+; APPLE-NEXT: [[GEP_DST_7:%.*]] = getelementptr inbounds float, ptr [[DST]], i64 [[IV_NEXT_6]]
+; APPLE-NEXT: store float [[L_7]], ptr [[GEP_DST_7]], align 4
+; APPLE-NEXT: [[IV_NEXT_7]] = add nuw nsw i64 [[IV_EPIL]], 8
+; APPLE-NEXT: [[NITER_NEXT_7]] = add i64 [[NITER]], 8
+; APPLE-NEXT: [[NITER_NCMP_7:%.*]] = icmp eq i64 [[NITER_NEXT_7]], [[UNROLL_ITER]]
+; APPLE-NEXT: br i1 [[NITER_NCMP_7]], label %[[EXIT_UNR_LCSSA_LOOPEXIT:.*]], label %[[LOOP]]
+; APPLE: [[EXIT_UNR_LCSSA_LOOPEXIT]]:
+; APPLE-NEXT: [[IV_UNR_PH:%.*]] = phi i64 [ [[IV_NEXT_7]], %[[LOOP]] ]
+; APPLE-NEXT: br label %[[EXIT_UNR_LCSSA]]
+; APPLE: [[EXIT_UNR_LCSSA]]:
+; APPLE-NEXT: [[IV_UNR:%.*]] = phi i64 [ 0, %[[ENTRY]] ], [ [[IV_UNR_PH]], %[[EXIT_UNR_LCSSA_LOOPEXIT]] ]
+; APPLE-NEXT: [[LCMP_MOD:%.*]] = icmp ne i64 [[XTRAITER]], 0
+; APPLE-NEXT: br i1 [[LCMP_MOD]], label %[[LOOP_EPIL_PREHEADER:.*]], label %[[EXIT:.*]]
+; APPLE: [[LOOP_EPIL_PREHEADER]]:
+; APPLE-NEXT: br label %[[LOOP_EPIL:.*]]
+; APPLE: [[LOOP_EPIL]]:
+; APPLE-NEXT: [[IV_EPIL1:%.*]] = phi i64 [ [[IV_UNR]], %[[LOOP_EPIL_PREHEADER]] ], [ [[IV_NEXT_EPIL1:%.*]], %[[LOOP_EPIL]] ]
+; APPLE-NEXT: [[EPIL_ITER:%.*]] = phi i64 [ 0, %[[LOOP_EPIL_PREHEADER]] ], [ [[EPIL_ITER_NEXT:%.*]], %[[LOOP_EPIL]] ]
+; APPLE-NEXT: [[SCALED_IV_EPIL1:%.*]] = mul nuw nsw i64 [[IV_EPIL1]], [[SCALE]]
+; APPLE-NEXT: [[GEP_SRC_EPIL1:%.*]] = getelementptr inbounds float, ptr [[SRC]], i64 [[SCALED_IV_EPIL1]]
+; APPLE-NEXT: [[L_EPIL1:%.*]] = load float, ptr [[GEP_SRC_EPIL1]], align 4
+; APPLE-NEXT: [[GEP_DST_EPIL1:%.*]] = getelementptr inbounds float, ptr [[DST]], i64 [[IV_EPIL1]]
+; APPLE-NEXT: store float [[L_EPIL1]], ptr [[GEP_DST_EPIL1]], align 4
+; APPLE-NEXT: [[IV_NEXT_EPIL1]] = add nuw nsw i64 [[IV_EPIL1]], 1
+; APPLE-NEXT: [[EC_EPIL:%.*]] = icmp eq i64 [[IV_NEXT_EPIL1]], [[N]]
+; APPLE-NEXT: [[EPIL_ITER_NEXT]] = add i64 [[EPIL_ITER]], 1
+; APPLE-NEXT: [[EPIL_ITER_CMP:%.*]] = icmp ne i64 [[EPIL_ITER_NEXT]], [[XTRAITER]]
+; APPLE-NEXT: br i1 [[EPIL_ITER_CMP]], label %[[LOOP_EPIL]], label %[[EXIT_EPILOG_LCSSA:.*]], !llvm.loop [[LOOP0:![0-9]+]]
+; APPLE: [[EXIT_EPILOG_LCSSA]]:
+; APPLE-NEXT: br label %[[EXIT]]
; APPLE: [[EXIT]]:
; APPLE-NEXT: ret void
;
>From 2c29c0db5be4030a013fba2d2080f7751ded8733 Mon Sep 17 00:00:00 2001
From: Florian Hahn <flo at fhahn.com>
Date: Tue, 3 Dec 2024 10:48:50 +0000
Subject: [PATCH 2/2] !fixup address latest comments, thanks!
---
.../AArch64/AArch64TargetTransformInfo.cpp | 41 ++++++++++++-------
1 file changed, 27 insertions(+), 14 deletions(-)
diff --git a/llvm/lib/Target/AArch64/AArch64TargetTransformInfo.cpp b/llvm/lib/Target/AArch64/AArch64TargetTransformInfo.cpp
index 035854b8a40293..988677265e63a9 100644
--- a/llvm/lib/Target/AArch64/AArch64TargetTransformInfo.cpp
+++ b/llvm/lib/Target/AArch64/AArch64TargetTransformInfo.cpp
@@ -4054,17 +4054,24 @@ getAppleRuntimeUnrollPreferences(Loop *L, ScalarEvolution &SE,
}
// Try to find an unroll count that maximizes the use of the instruction
- // window.
- unsigned UC = std::max(16ll / Size, 2ll);
- unsigned BestUC = 0;
- while (UC <= 8 && UC * Size <= 48) {
- if ((UC * Size % 16) == 0 || (BestUC * Size % 16) < (UC * Size % 16) % 16) {
+ // window, i.e. trying to fetch as many instructions per cycle as possible.
+ unsigned MaxInstsPerLine = 16;
+ unsigned UC = 1;
+ unsigned BestUC = 1;
+ unsigned SizeWithBestUC = BestUC * Size;
+ while (UC <= 8) {
+ unsigned SizeWithUC = UC * Size;
+ if (SizeWithUC > 48)
+ break;
+ if ((SizeWithUC % MaxInstsPerLine) == 0 ||
+ (SizeWithBestUC % MaxInstsPerLine) < (SizeWithUC % MaxInstsPerLine)) {
BestUC = UC;
+ SizeWithBestUC = BestUC * Size;
}
UC++;
}
- if (BestUC == 0 || none_of(Stores, [&LoadedValues](StoreInst *SI) {
+ if (BestUC == 1 || none_of(Stores, [&LoadedValues](StoreInst *SI) {
return LoadedValues.contains(SI->getOperand(0));
}))
return;
@@ -4090,15 +4097,21 @@ void AArch64TTIImpl::getUnrollingPreferences(Loop *L, ScalarEvolution &SE,
// Disable partial & runtime unrolling on -Os.
UP.PartialOptSizeThreshold = 0;
- if (ST->getProcFamily() == AArch64Subtarget::Falkor &&
- EnableFalkorHWPFUnrollFix)
- getFalkorUnrollingPreferences(L, SE, UP);
-
- if (ST->getProcFamily() == AArch64Subtarget::AppleA14 ||
- ST->getProcFamily() == AArch64Subtarget::AppleA15 ||
- ST->getProcFamily() == AArch64Subtarget::AppleA16 ||
- ST->getProcFamily() == AArch64Subtarget::AppleM4)
+ // Apply subtarget-specific unrolling preferences.
+ switch (ST->getProcFamily()) {
+ case AArch64Subtarget::AppleA14:
+ case AArch64Subtarget::AppleA15:
+ case AArch64Subtarget::AppleA16:
+ case AArch64Subtarget::AppleM4:
getAppleRuntimeUnrollPreferences(L, SE, UP, *this);
+ break;
+ case AArch64Subtarget::Falkor:
+ if (EnableFalkorHWPFUnrollFix)
+ getFalkorUnrollingPreferences(L, SE, UP);
+ break;
+ default:
+ break;
+ }
// Scan the loop: don't unroll loops with calls as this could prevent
// inlining. Don't unroll vector loops either, as they don't benefit much from
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