[llvm] [SimplifyCFG] Improve range reducing for switches (PR #67882)
Yingwei Zheng via llvm-commits
llvm-commits at lists.llvm.org
Sat Sep 30 06:10:39 PDT 2023
https://github.com/dtcxzyw created https://github.com/llvm/llvm-project/pull/67882
This patch improves range reducing for switches when the default block is unreachable.
It will mask out the common high bits and move the largest hole to the back.
Example:
```
int f1(int x) {
switch (x) {
case 0: return 1;
case 1: return 2;
case 255: return 0;
default: __builtin_unreachable();
}
}
```
after range reduction:
```
int f1(int x) {
switch ((x + 1) & 255) {
case 1: return 1;
case 2: return 2;
case 0: return 0;
default: __builtin_unreachable();
}
}
```
Fixes #67842.
>From e4c4aac0dcdd3084e373043a4e2c5ff007a2d917 Mon Sep 17 00:00:00 2001
From: Yingwei Zheng <dtcxzyw2333 at gmail.com>
Date: Sat, 30 Sep 2023 21:02:14 +0800
Subject: [PATCH] [SimplifyCFG] Improve range reducing for switches
---
llvm/lib/Transforms/Utils/SimplifyCFG.cpp | 74 +++++++++-
.../Transforms/SimplifyCFG/rangereduce.ll | 139 ++++++++++++++++++
2 files changed, 210 insertions(+), 3 deletions(-)
diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index 35fead111aa9666..ef7a8d34b539662 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -80,6 +80,7 @@
#include <cstddef>
#include <cstdint>
#include <iterator>
+#include <limits>
#include <map>
#include <optional>
#include <set>
@@ -6748,6 +6749,71 @@ static bool SwitchToLookupTable(SwitchInst *SI, IRBuilder<> &Builder,
return true;
}
+/// Try to reduce the range of cases with an unreachable default.
+static bool
+ReduceSwitchRangeWithUnreachableDefault(SwitchInst *SI,
+ const SmallVectorImpl<int64_t> &Values,
+ uint64_t Base, IRBuilder<> &Builder) {
+ bool HasDefault =
+ !isa<UnreachableInst>(SI->getDefaultDest()->getFirstNonPHIOrDbg());
+ if (HasDefault)
+ return false;
+
+ // Try reducing the range to (idx + offset) & mask
+ // Mask out common high bits
+ uint64_t CommonOnes = std::numeric_limits<uint64_t>::max();
+ uint64_t CommonZeros = std::numeric_limits<uint64_t>::max();
+ for (auto &V : Values) {
+ CommonOnes &= (uint64_t)V;
+ CommonZeros &= ~(uint64_t)V;
+ }
+ uint64_t CommonBits = countl_one(CommonOnes | CommonZeros);
+ unsigned LowBits = 64 - CommonBits;
+ uint64_t Mask = (1ULL << LowBits) - 1;
+ if (Mask == std::numeric_limits<uint64_t>::max())
+ return false;
+ // Now we have some case values in the additive group Z/(2**k)Z.
+ // Find the largest hole in the group and move it to back.
+ uint64_t MaxHole = 0;
+ uint64_t BestOffset = 0;
+ for (unsigned I = 0; I < Values.size(); ++I) {
+ uint64_t Hole = ((uint64_t)Values[I] -
+ (uint64_t)(I == 0 ? Values.back() : Values[I - 1])) &
+ Mask;
+ if (Hole > MaxHole) {
+ MaxHole = Hole;
+ BestOffset = Mask - (uint64_t)Values[I] + 1;
+ }
+ }
+
+ SmallVector<int64_t, 4> NewValues;
+ for (auto &V : Values)
+ NewValues.push_back(
+ (((int64_t)(((uint64_t)V + BestOffset) & Mask)) << CommonBits) >>
+ CommonBits);
+
+ llvm::sort(NewValues);
+ if (!isSwitchDense(NewValues))
+ // Transform didn't create a dense switch.
+ return false;
+
+ auto *Ty = cast<IntegerType>(SI->getCondition()->getType());
+ APInt Offset(Ty->getBitWidth(), BestOffset - Base);
+ auto *Index = Builder.CreateAnd(
+ Builder.CreateAdd(SI->getCondition(), ConstantInt::get(Ty, Offset)),
+ Mask);
+ SI->replaceUsesOfWith(SI->getCondition(), Index);
+
+ for (auto Case : SI->cases()) {
+ auto *Orig = Case.getCaseValue();
+ auto CaseVal =
+ (Orig->getValue() + Offset).trunc(LowBits).sext(Ty->getBitWidth());
+ Case.setValue(cast<ConstantInt>(ConstantInt::get(Ty, CaseVal)));
+ }
+
+ return true;
+}
+
/// Try to transform a switch that has "holes" in it to a contiguous sequence
/// of cases.
///
@@ -6763,9 +6829,8 @@ static bool ReduceSwitchRange(SwitchInst *SI, IRBuilder<> &Builder,
if (CondTy->getIntegerBitWidth() > 64 ||
!DL.fitsInLegalInteger(CondTy->getIntegerBitWidth()))
return false;
- // Only bother with this optimization if there are more than 3 switch cases;
- // SDAG will only bother creating jump tables for 4 or more cases.
- if (SI->getNumCases() < 4)
+ // Ignore switches with less than three cases.
+ if (SI->getNumCases() < 3)
return false;
// This transform is agnostic to the signedness of the input or case values. We
@@ -6786,6 +6851,9 @@ static bool ReduceSwitchRange(SwitchInst *SI, IRBuilder<> &Builder,
for (auto &V : Values)
V -= (uint64_t)(Base);
+ if (ReduceSwitchRangeWithUnreachableDefault(SI, Values, Base, Builder))
+ return true;
+
// Now we have signed numbers that have been shifted so that, given enough
// precision, there are no negative values. Since the rest of the transform
// is bitwise only, we switch now to an unsigned representation.
diff --git a/llvm/test/Transforms/SimplifyCFG/rangereduce.ll b/llvm/test/Transforms/SimplifyCFG/rangereduce.ll
index b1a3802a2bb58b8..939c7b782d579da 100644
--- a/llvm/test/Transforms/SimplifyCFG/rangereduce.ll
+++ b/llvm/test/Transforms/SimplifyCFG/rangereduce.ll
@@ -315,3 +315,142 @@ three:
ret i32 99783
}
+define i8 @pr67842(i32 %0) {
+; CHECK-LABEL: @pr67842(
+; CHECK-NEXT: start:
+; CHECK-NEXT: [[TMP1:%.*]] = add i32 [[TMP0:%.*]], 1
+; CHECK-NEXT: [[TMP2:%.*]] = and i32 [[TMP1]], 255
+; CHECK-NEXT: [[SWITCH_IDX_CAST:%.*]] = trunc i32 [[TMP2]] to i8
+; CHECK-NEXT: [[SWITCH_OFFSET:%.*]] = add nsw i8 [[SWITCH_IDX_CAST]], -1
+; CHECK-NEXT: ret i8 [[SWITCH_OFFSET]]
+;
+start:
+ switch i32 %0, label %bb2 [
+ i32 0, label %bb5
+ i32 1, label %bb4
+ i32 255, label %bb1
+ ]
+
+bb2: ; preds = %start
+ unreachable
+
+bb4: ; preds = %start
+ br label %bb5
+
+bb1: ; preds = %start
+ br label %bb5
+
+bb5: ; preds = %start, %bb1, %bb4
+ %.0 = phi i8 [ -1, %bb1 ], [ 1, %bb4 ], [ 0, %start ]
+ ret i8 %.0
+}
+
+define i8 @reduce_masked_common_high_bits(i32 %0) {
+; CHECK-LABEL: @reduce_masked_common_high_bits(
+; CHECK-NEXT: start:
+; CHECK-NEXT: [[TMP1:%.*]] = add i32 [[TMP0:%.*]], -127
+; CHECK-NEXT: [[TMP2:%.*]] = and i32 [[TMP1]], 127
+; CHECK-NEXT: [[SWITCH_IDX_CAST:%.*]] = trunc i32 [[TMP2]] to i8
+; CHECK-NEXT: [[SWITCH_OFFSET:%.*]] = add nsw i8 [[SWITCH_IDX_CAST]], -1
+; CHECK-NEXT: ret i8 [[SWITCH_OFFSET]]
+;
+start:
+ switch i32 %0, label %bb2 [
+ i32 128, label %bb5
+ i32 129, label %bb4
+ i32 255, label %bb1
+ ]
+
+bb2: ; preds = %start
+ unreachable
+
+bb4: ; preds = %start
+ br label %bb5
+
+bb1: ; preds = %start
+ br label %bb5
+
+bb5: ; preds = %start, %bb1, %bb4
+ %.0 = phi i8 [ -1, %bb1 ], [ 1, %bb4 ], [ 0, %start ]
+ ret i8 %.0
+}
+
+define i8 @reduce_masked_common_high_bits_fail(i32 %0) {
+; CHECK-LABEL: @reduce_masked_common_high_bits_fail(
+; CHECK-NEXT: start:
+; CHECK-NEXT: switch i32 [[TMP0:%.*]], label [[BB2:%.*]] [
+; CHECK-NEXT: i32 128, label [[BB5:%.*]]
+; CHECK-NEXT: i32 129, label [[BB4:%.*]]
+; CHECK-NEXT: i32 511, label [[BB1:%.*]]
+; CHECK-NEXT: ]
+; CHECK: bb2:
+; CHECK-NEXT: unreachable
+; CHECK: bb4:
+; CHECK-NEXT: br label [[BB5]]
+; CHECK: bb1:
+; CHECK-NEXT: br label [[BB5]]
+; CHECK: bb5:
+; CHECK-NEXT: [[DOT0:%.*]] = phi i8 [ -1, [[BB1]] ], [ 1, [[BB4]] ], [ 0, [[START:%.*]] ]
+; CHECK-NEXT: ret i8 [[DOT0]]
+;
+start:
+ switch i32 %0, label %bb2 [
+ i32 128, label %bb5
+ i32 129, label %bb4
+ i32 511, label %bb1
+ ]
+
+bb2: ; preds = %start
+ unreachable
+
+bb4: ; preds = %start
+ br label %bb5
+
+bb1: ; preds = %start
+ br label %bb5
+
+bb5: ; preds = %start, %bb1, %bb4
+ %.0 = phi i8 [ -1, %bb1 ], [ 1, %bb4 ], [ 0, %start ]
+ ret i8 %.0
+}
+
+; Optimization shouldn't trigger; The default block is reachable.
+define i8 @reduce_masked_default_reachable(i32 %0) {
+; CHECK-LABEL: @reduce_masked_default_reachable(
+; CHECK-NEXT: start:
+; CHECK-NEXT: switch i32 [[TMP0:%.*]], label [[COMMON_RET:%.*]] [
+; CHECK-NEXT: i32 0, label [[BB5:%.*]]
+; CHECK-NEXT: i32 1, label [[BB4:%.*]]
+; CHECK-NEXT: i32 255, label [[BB1:%.*]]
+; CHECK-NEXT: ]
+; CHECK: common.ret:
+; CHECK-NEXT: [[COMMON_RET_OP:%.*]] = phi i8 [ [[DOT0:%.*]], [[BB5]] ], [ 24, [[START:%.*]] ]
+; CHECK-NEXT: ret i8 [[COMMON_RET_OP]]
+; CHECK: bb4:
+; CHECK-NEXT: br label [[BB5]]
+; CHECK: bb1:
+; CHECK-NEXT: br label [[BB5]]
+; CHECK: bb5:
+; CHECK-NEXT: [[DOT0]] = phi i8 [ -1, [[BB1]] ], [ 1, [[BB4]] ], [ 0, [[START]] ]
+; CHECK-NEXT: br label [[COMMON_RET]]
+;
+start:
+ switch i32 %0, label %bb2 [
+ i32 0, label %bb5
+ i32 1, label %bb4
+ i32 255, label %bb1
+ ]
+
+bb2: ; preds = %start
+ ret i8 24
+
+bb4: ; preds = %start
+ br label %bb5
+
+bb1: ; preds = %start
+ br label %bb5
+
+bb5: ; preds = %start, %bb1, %bb4
+ %.0 = phi i8 [ -1, %bb1 ], [ 1, %bb4 ], [ 0, %start ]
+ ret i8 %.0
+}
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