[llvm] [SimplifyCFG] Allow some switch optimizations early in the pipeline (PR #158242)
Nikita Popov via llvm-commits
llvm-commits at lists.llvm.org
Fri Sep 12 03:26:30 PDT 2025
https://github.com/nikic updated https://github.com/llvm/llvm-project/pull/158242
>From 5aa1a479a68e2168b30364400a14861c7a327ef4 Mon Sep 17 00:00:00 2001
From: Nikita Popov <npopov at redhat.com>
Date: Fri, 12 Sep 2025 10:11:36 +0200
Subject: [PATCH 1/3] [SimplifyCFG] Allow some switch optimizations early in
the pipeline
While we do not want to form actual lookup tables early, we do
want to perform some optimizations, as they may enable inlining
of the much simpler form.
Builds on https://github.com/llvm/llvm-project/pull/156477, which
originally included this change as well.
---
llvm/lib/Transforms/Utils/SimplifyCFG.cpp | 24 ++++--
.../switch-transformations-no-lut.ll | 74 ++++---------------
2 files changed, 29 insertions(+), 69 deletions(-)
diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index 850e57e6b0b14..20c394b00a0ef 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -6477,6 +6477,9 @@ class SwitchReplacement {
/// Return true if the replacement is a lookup table.
bool isLookupTable();
+ /// Return true if the replacement is a bit map.
+ bool isBitMap();
+
private:
// Depending on the switch, there are different alternatives.
enum {
@@ -6767,6 +6770,8 @@ Constant *SwitchReplacement::getDefaultValue() { return DefaultValue; }
bool SwitchReplacement::isLookupTable() { return Kind == LookupTableKind; }
+bool SwitchReplacement::isBitMap() { return Kind == BitMapKind; }
+
static bool isSwitchDense(uint64_t NumCases, uint64_t CaseRange) {
// 40% is the default density for building a jump table in optsize/minsize
// mode. See also TargetLoweringBase::isSuitableForJumpTable(), which this
@@ -6932,7 +6937,8 @@ static void reuseTableCompare(
/// lookup tables.
static bool simplifySwitchLookup(SwitchInst *SI, IRBuilder<> &Builder,
DomTreeUpdater *DTU, const DataLayout &DL,
- const TargetTransformInfo &TTI) {
+ const TargetTransformInfo &TTI,
+ bool ConvertSwitchToLookupTable) {
assert(SI->getNumCases() > 1 && "Degenerate switch?");
BasicBlock *BB = SI->getParent();
@@ -7097,6 +7103,8 @@ static bool simplifySwitchLookup(SwitchInst *SI, IRBuilder<> &Builder,
bool AnyLookupTables = any_of(
PhiToReplacementMap, [](auto &KV) { return KV.second.isLookupTable(); });
+ bool AnyBitMaps = any_of(PhiToReplacementMap,
+ [](auto &KV) { return KV.second.isBitMap(); });
// A few conditions prevent the generation of lookup tables:
// 1. The target does not support lookup tables.
@@ -7109,6 +7117,11 @@ static bool simplifySwitchLookup(SwitchInst *SI, IRBuilder<> &Builder,
Fn->getFnAttribute("no-jump-tables").getValueAsBool()))
return false;
+ // In the early optimization pipeline, disable formation of lookup tables
+ // and bit maps, as they may inhibit further optimization.
+ if (!ConvertSwitchToLookupTable && (AnyLookupTables || AnyBitMaps))
+ return false;
+
Builder.SetInsertPoint(SI);
// TableIndex is the switch condition - TableIndexOffset if we don't
// use the condition directly
@@ -7749,13 +7762,8 @@ bool SimplifyCFGOpt::simplifySwitch(SwitchInst *SI, IRBuilder<> &Builder) {
if (Options.ForwardSwitchCondToPhi && forwardSwitchConditionToPHI(SI))
return requestResimplify();
- // The conversion from switch to lookup tables results in difficult-to-analyze
- // code and makes pruning branches much harder. This is a problem if the
- // switch expression itself can still be restricted as a result of inlining or
- // CVP. Therefore, only apply this transformation during late stages of the
- // optimisation pipeline.
- if (Options.ConvertSwitchToLookupTable &&
- simplifySwitchLookup(SI, Builder, DTU, DL, TTI))
+ if (simplifySwitchLookup(SI, Builder, DTU, DL, TTI,
+ Options.ConvertSwitchToLookupTable))
return requestResimplify();
if (simplifySwitchOfPowersOfTwo(SI, Builder, DL, TTI))
diff --git a/llvm/test/Transforms/SimplifyCFG/switch-transformations-no-lut.ll b/llvm/test/Transforms/SimplifyCFG/switch-transformations-no-lut.ll
index c9063d3ec26ff..01200136b6d40 100644
--- a/llvm/test/Transforms/SimplifyCFG/switch-transformations-no-lut.ll
+++ b/llvm/test/Transforms/SimplifyCFG/switch-transformations-no-lut.ll
@@ -7,23 +7,11 @@ target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
define i32 @linear_transform_with_default(i32 %x) {
; OPTNOLUT-LABEL: define i32 @linear_transform_with_default(
; OPTNOLUT-SAME: i32 [[X:%.*]]) {
-; OPTNOLUT-NEXT: [[ENTRY:.*]]:
-; OPTNOLUT-NEXT: switch i32 [[X]], label %[[END:.*]] [
-; OPTNOLUT-NEXT: i32 0, label %[[CASE0:.*]]
-; OPTNOLUT-NEXT: i32 1, label %[[CASE1:.*]]
-; OPTNOLUT-NEXT: i32 2, label %[[CASE2:.*]]
-; OPTNOLUT-NEXT: i32 3, label %[[CASE3:.*]]
-; OPTNOLUT-NEXT: ]
-; OPTNOLUT: [[CASE0]]:
-; OPTNOLUT-NEXT: br label %[[END]]
-; OPTNOLUT: [[CASE1]]:
-; OPTNOLUT-NEXT: br label %[[END]]
-; OPTNOLUT: [[CASE2]]:
-; OPTNOLUT-NEXT: br label %[[END]]
-; OPTNOLUT: [[CASE3]]:
-; OPTNOLUT-NEXT: br label %[[END]]
-; OPTNOLUT: [[END]]:
-; OPTNOLUT-NEXT: [[IDX:%.*]] = phi i32 [ 1, %[[CASE0]] ], [ 4, %[[CASE1]] ], [ 7, %[[CASE2]] ], [ 10, %[[CASE3]] ], [ 13, %[[ENTRY]] ]
+; OPTNOLUT-NEXT: [[ENTRY:.*:]]
+; OPTNOLUT-NEXT: [[TMP0:%.*]] = icmp ult i32 [[X]], 4
+; OPTNOLUT-NEXT: [[SWITCH_IDX_MULT:%.*]] = mul nsw i32 [[X]], 3
+; OPTNOLUT-NEXT: [[SWITCH_OFFSET:%.*]] = add nsw i32 [[SWITCH_IDX_MULT]], 1
+; OPTNOLUT-NEXT: [[IDX:%.*]] = select i1 [[TMP0]], i32 [[SWITCH_OFFSET]], i32 13
; OPTNOLUT-NEXT: ret i32 [[IDX]]
;
; TTINOLUT-LABEL: define i32 @linear_transform_with_default(
@@ -138,26 +126,8 @@ end:
define i32 @linear_transform_no_default(i32 %x) {
; OPTNOLUT-LABEL: define i32 @linear_transform_no_default(
; OPTNOLUT-SAME: i32 [[X:%.*]]) {
-; OPTNOLUT-NEXT: [[ENTRY:.*]]:
-; OPTNOLUT-NEXT: switch i32 [[X]], label %[[DEFAULT:.*]] [
-; OPTNOLUT-NEXT: i32 0, label %[[END:.*]]
-; OPTNOLUT-NEXT: i32 1, label %[[CASE1:.*]]
-; OPTNOLUT-NEXT: i32 2, label %[[CASE2:.*]]
-; OPTNOLUT-NEXT: i32 3, label %[[CASE3:.*]]
-; OPTNOLUT-NEXT: i32 4, label %[[CASE4:.*]]
-; OPTNOLUT-NEXT: ]
-; OPTNOLUT: [[CASE1]]:
-; OPTNOLUT-NEXT: br label %[[END]]
-; OPTNOLUT: [[CASE2]]:
-; OPTNOLUT-NEXT: br label %[[END]]
-; OPTNOLUT: [[CASE3]]:
-; OPTNOLUT-NEXT: br label %[[END]]
-; OPTNOLUT: [[CASE4]]:
-; OPTNOLUT-NEXT: br label %[[END]]
-; OPTNOLUT: [[DEFAULT]]:
-; OPTNOLUT-NEXT: unreachable
-; OPTNOLUT: [[END]]:
-; OPTNOLUT-NEXT: [[SWITCH_IDX_MULT:%.*]] = phi i32 [ 3, %[[CASE1]] ], [ 6, %[[CASE2]] ], [ 9, %[[CASE3]] ], [ 12, %[[CASE4]] ], [ 0, %[[ENTRY]] ]
+; OPTNOLUT-NEXT: [[ENTRY:.*:]]
+; OPTNOLUT-NEXT: [[SWITCH_IDX_MULT:%.*]] = mul nsw i32 [[X]], 3
; OPTNOLUT-NEXT: ret i32 [[SWITCH_IDX_MULT]]
;
; TTINOLUT-LABEL: define i32 @linear_transform_no_default(
@@ -350,18 +320,9 @@ end:
define i32 @single_value_withdefault(i32 %x) {
; OPTNOLUT-LABEL: define i32 @single_value_withdefault(
; OPTNOLUT-SAME: i32 [[X:%.*]]) {
-; OPTNOLUT-NEXT: [[ENTRY:.*]]:
-; OPTNOLUT-NEXT: switch i32 [[X]], label %[[DEFAULT:.*]] [
-; OPTNOLUT-NEXT: i32 0, label %[[END:.*]]
-; OPTNOLUT-NEXT: i32 1, label %[[END]]
-; OPTNOLUT-NEXT: i32 2, label %[[END]]
-; OPTNOLUT-NEXT: i32 3, label %[[END]]
-; OPTNOLUT-NEXT: i32 4, label %[[END]]
-; OPTNOLUT-NEXT: ]
-; OPTNOLUT: [[DEFAULT]]:
-; OPTNOLUT-NEXT: br label %[[END]]
-; OPTNOLUT: [[END]]:
-; OPTNOLUT-NEXT: [[DOT:%.*]] = phi i32 [ 3, %[[DEFAULT]] ], [ 2, %[[ENTRY]] ], [ 2, %[[ENTRY]] ], [ 2, %[[ENTRY]] ], [ 2, %[[ENTRY]] ], [ 2, %[[ENTRY]] ]
+; OPTNOLUT-NEXT: [[ENTRY:.*:]]
+; OPTNOLUT-NEXT: [[TMP0:%.*]] = icmp ult i32 [[X]], 5
+; OPTNOLUT-NEXT: [[DOT:%.*]] = select i1 [[TMP0]], i32 2, i32 3
; OPTNOLUT-NEXT: ret i32 [[DOT]]
;
; TTINOLUT-LABEL: define i32 @single_value_withdefault(
@@ -401,18 +362,9 @@ end:
define i32 @single_value_no_jump_tables(i32 %x) "no-jump-tables"="true" {
; OPTNOLUT-LABEL: define i32 @single_value_no_jump_tables(
; OPTNOLUT-SAME: i32 [[X:%.*]]) #[[ATTR0:[0-9]+]] {
-; OPTNOLUT-NEXT: [[ENTRY:.*]]:
-; OPTNOLUT-NEXT: switch i32 [[X]], label %[[DEFAULT:.*]] [
-; OPTNOLUT-NEXT: i32 0, label %[[END:.*]]
-; OPTNOLUT-NEXT: i32 1, label %[[END]]
-; OPTNOLUT-NEXT: i32 2, label %[[END]]
-; OPTNOLUT-NEXT: i32 3, label %[[END]]
-; OPTNOLUT-NEXT: i32 4, label %[[END]]
-; OPTNOLUT-NEXT: ]
-; OPTNOLUT: [[DEFAULT]]:
-; OPTNOLUT-NEXT: br label %[[END]]
-; OPTNOLUT: [[END]]:
-; OPTNOLUT-NEXT: [[IDX:%.*]] = phi i32 [ 3, %[[DEFAULT]] ], [ 2, %[[ENTRY]] ], [ 2, %[[ENTRY]] ], [ 2, %[[ENTRY]] ], [ 2, %[[ENTRY]] ], [ 2, %[[ENTRY]] ]
+; OPTNOLUT-NEXT: [[ENTRY:.*:]]
+; OPTNOLUT-NEXT: [[TMP0:%.*]] = icmp ult i32 [[X]], 5
+; OPTNOLUT-NEXT: [[IDX:%.*]] = select i1 [[TMP0]], i32 2, i32 3
; OPTNOLUT-NEXT: ret i32 [[IDX]]
;
; TTINOLUT-LABEL: define i32 @single_value_no_jump_tables(
>From 3712bec962f4379437bf525d294887fdbeb239d9 Mon Sep 17 00:00:00 2001
From: Nikita Popov <npopov at redhat.com>
Date: Fri, 12 Sep 2025 12:25:06 +0200
Subject: [PATCH 2/3] Add test with mask
---
.../switch-transformations-no-lut.ll | 56 +++++++++++++++++++
1 file changed, 56 insertions(+)
diff --git a/llvm/test/Transforms/SimplifyCFG/switch-transformations-no-lut.ll b/llvm/test/Transforms/SimplifyCFG/switch-transformations-no-lut.ll
index 01200136b6d40..fc2c8065a85a0 100644
--- a/llvm/test/Transforms/SimplifyCFG/switch-transformations-no-lut.ll
+++ b/llvm/test/Transforms/SimplifyCFG/switch-transformations-no-lut.ll
@@ -401,6 +401,62 @@ end:
ret i32 %idx
}
+define i1 @single_value_with_mask(i32 %x) {
+; OPTNOLUT-LABEL: define i1 @single_value_with_mask(
+; OPTNOLUT-SAME: i32 [[X:%.*]]) {
+; OPTNOLUT-NEXT: [[ENTRY:.*:]]
+; OPTNOLUT-NEXT: [[SWITCH_TABLEIDX:%.*]] = sub i32 [[X]], 16
+; OPTNOLUT-NEXT: [[TMP0:%.*]] = icmp ult i32 [[SWITCH_TABLEIDX]], 33
+; OPTNOLUT-NEXT: br i1 [[TMP0]], label %[[SWITCH_HOLE_CHECK:.*]], label %[[DEFAULT:.*]]
+; OPTNOLUT: [[DEFAULT]]:
+; OPTNOLUT-NEXT: [[CMP:%.*]] = icmp eq i32 [[X]], 80
+; OPTNOLUT-NEXT: [[SEL:%.*]] = select i1 [[CMP]], i1 false, i1 true
+; OPTNOLUT-NEXT: br label %[[END:.*]]
+; OPTNOLUT: [[SWITCH_HOLE_CHECK]]:
+; OPTNOLUT-NEXT: [[SWITCH_MASKINDEX:%.*]] = zext i32 [[SWITCH_TABLEIDX]] to i64
+; OPTNOLUT-NEXT: [[SWITCH_SHIFTED:%.*]] = lshr i64 4294967333, [[SWITCH_MASKINDEX]]
+; OPTNOLUT-NEXT: [[SWITCH_LOBIT:%.*]] = trunc i64 [[SWITCH_SHIFTED]] to i1
+; OPTNOLUT-NEXT: br i1 [[SWITCH_LOBIT]], label %[[END]], label %[[DEFAULT]]
+; OPTNOLUT: [[END]]:
+; OPTNOLUT-NEXT: [[RES:%.*]] = phi i1 [ [[SEL]], %[[DEFAULT]] ], [ false, %[[SWITCH_HOLE_CHECK]] ]
+; OPTNOLUT-NEXT: ret i1 [[RES]]
+;
+; TTINOLUT-LABEL: define i1 @single_value_with_mask(
+; TTINOLUT-SAME: i32 [[X:%.*]]) {
+; TTINOLUT-NEXT: [[ENTRY:.*]]:
+; TTINOLUT-NEXT: [[SWITCH_TABLEIDX:%.*]] = sub i32 [[X]], 16
+; TTINOLUT-NEXT: [[TMP0:%.*]] = icmp ult i32 [[SWITCH_TABLEIDX]], 33
+; TTINOLUT-NEXT: [[SWITCH_MASKINDEX:%.*]] = zext i32 [[SWITCH_TABLEIDX]] to i64
+; TTINOLUT-NEXT: [[SWITCH_SHIFTED:%.*]] = lshr i64 4294967333, [[SWITCH_MASKINDEX]]
+; TTINOLUT-NEXT: [[SWITCH_LOBIT:%.*]] = trunc i64 [[SWITCH_SHIFTED]] to i1
+; TTINOLUT-NEXT: [[OR_COND:%.*]] = select i1 [[TMP0]], i1 [[SWITCH_LOBIT]], i1 false
+; TTINOLUT-NEXT: br i1 [[OR_COND]], label %[[END:.*]], label %[[DEFAULT:.*]]
+; TTINOLUT: [[DEFAULT]]:
+; TTINOLUT-NEXT: [[CMP:%.*]] = icmp eq i32 [[X]], 80
+; TTINOLUT-NEXT: [[SEL:%.*]] = select i1 [[CMP]], i1 false, i1 true
+; TTINOLUT-NEXT: br label %[[END]]
+; TTINOLUT: [[END]]:
+; TTINOLUT-NEXT: [[RES:%.*]] = phi i1 [ [[SEL]], %[[DEFAULT]] ], [ false, %[[ENTRY]] ]
+; TTINOLUT-NEXT: ret i1 [[RES]]
+;
+entry:
+ switch i32 %x, label %default [
+ i32 18, label %end
+ i32 21, label %end
+ i32 48, label %end
+ i32 16, label %end
+ ]
+
+default:
+ %cmp = icmp eq i32 %x, 80
+ %sel = select i1 %cmp, i1 false, i1 true
+ br label %end
+
+end:
+ %res = phi i1 [ false, %entry ], [ false, %entry ], [ false, %entry ], [ false, %entry ], [ %sel, %default ]
+ ret i1 %res
+}
+
define i32 @lookup_table(i32 %x) {
; OPTNOLUT-LABEL: define i32 @lookup_table(
; OPTNOLUT-SAME: i32 [[X:%.*]]) {
>From 131d850926e8a82dde71562dc8f218c6497900b2 Mon Sep 17 00:00:00 2001
From: Nikita Popov <npopov at redhat.com>
Date: Fri, 12 Sep 2025 12:26:01 +0200
Subject: [PATCH 3/3] Don't create masks early
---
llvm/lib/Transforms/Utils/SimplifyCFG.cpp | 7 ++++---
.../switch-transformations-no-lut.ll | 20 +++++++++----------
2 files changed, 13 insertions(+), 14 deletions(-)
diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index 20c394b00a0ef..acf9cd455d63e 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -7117,9 +7117,10 @@ static bool simplifySwitchLookup(SwitchInst *SI, IRBuilder<> &Builder,
Fn->getFnAttribute("no-jump-tables").getValueAsBool()))
return false;
- // In the early optimization pipeline, disable formation of lookup tables
- // and bit maps, as they may inhibit further optimization.
- if (!ConvertSwitchToLookupTable && (AnyLookupTables || AnyBitMaps))
+ // In the early optimization pipeline, disable formation of lookup tables,
+ // bit maps and mask checks, as they may inhibit further optimization.
+ if (!ConvertSwitchToLookupTable &&
+ (AnyLookupTables || AnyBitMaps || NeedMask))
return false;
Builder.SetInsertPoint(SI);
diff --git a/llvm/test/Transforms/SimplifyCFG/switch-transformations-no-lut.ll b/llvm/test/Transforms/SimplifyCFG/switch-transformations-no-lut.ll
index fc2c8065a85a0..df5df57c3f242 100644
--- a/llvm/test/Transforms/SimplifyCFG/switch-transformations-no-lut.ll
+++ b/llvm/test/Transforms/SimplifyCFG/switch-transformations-no-lut.ll
@@ -404,21 +404,19 @@ end:
define i1 @single_value_with_mask(i32 %x) {
; OPTNOLUT-LABEL: define i1 @single_value_with_mask(
; OPTNOLUT-SAME: i32 [[X:%.*]]) {
-; OPTNOLUT-NEXT: [[ENTRY:.*:]]
-; OPTNOLUT-NEXT: [[SWITCH_TABLEIDX:%.*]] = sub i32 [[X]], 16
-; OPTNOLUT-NEXT: [[TMP0:%.*]] = icmp ult i32 [[SWITCH_TABLEIDX]], 33
-; OPTNOLUT-NEXT: br i1 [[TMP0]], label %[[SWITCH_HOLE_CHECK:.*]], label %[[DEFAULT:.*]]
+; OPTNOLUT-NEXT: [[ENTRY:.*]]:
+; OPTNOLUT-NEXT: switch i32 [[X]], label %[[DEFAULT:.*]] [
+; OPTNOLUT-NEXT: i32 18, label %[[END:.*]]
+; OPTNOLUT-NEXT: i32 21, label %[[END]]
+; OPTNOLUT-NEXT: i32 48, label %[[END]]
+; OPTNOLUT-NEXT: i32 16, label %[[END]]
+; OPTNOLUT-NEXT: ]
; OPTNOLUT: [[DEFAULT]]:
; OPTNOLUT-NEXT: [[CMP:%.*]] = icmp eq i32 [[X]], 80
; OPTNOLUT-NEXT: [[SEL:%.*]] = select i1 [[CMP]], i1 false, i1 true
-; OPTNOLUT-NEXT: br label %[[END:.*]]
-; OPTNOLUT: [[SWITCH_HOLE_CHECK]]:
-; OPTNOLUT-NEXT: [[SWITCH_MASKINDEX:%.*]] = zext i32 [[SWITCH_TABLEIDX]] to i64
-; OPTNOLUT-NEXT: [[SWITCH_SHIFTED:%.*]] = lshr i64 4294967333, [[SWITCH_MASKINDEX]]
-; OPTNOLUT-NEXT: [[SWITCH_LOBIT:%.*]] = trunc i64 [[SWITCH_SHIFTED]] to i1
-; OPTNOLUT-NEXT: br i1 [[SWITCH_LOBIT]], label %[[END]], label %[[DEFAULT]]
+; OPTNOLUT-NEXT: br label %[[END]]
; OPTNOLUT: [[END]]:
-; OPTNOLUT-NEXT: [[RES:%.*]] = phi i1 [ [[SEL]], %[[DEFAULT]] ], [ false, %[[SWITCH_HOLE_CHECK]] ]
+; OPTNOLUT-NEXT: [[RES:%.*]] = phi i1 [ false, %[[ENTRY]] ], [ false, %[[ENTRY]] ], [ false, %[[ENTRY]] ], [ false, %[[ENTRY]] ], [ [[SEL]], %[[DEFAULT]] ]
; OPTNOLUT-NEXT: ret i1 [[RES]]
;
; TTINOLUT-LABEL: define i1 @single_value_with_mask(
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