[llvm] r222121 - Optimize switch lookup tables with linear mapping.
Erik Eckstein
eeckstein at apple.com
Mon Nov 17 01:13:58 PST 2014
Author: eeckstein
Date: Mon Nov 17 03:13:57 2014
New Revision: 222121
URL: http://llvm.org/viewvc/llvm-project?rev=222121&view=rev
Log:
Optimize switch lookup tables with linear mapping.
This is a simple optimization for switch table lookup:
It computes the output value directly with an (optional) mul and add if there is a linear mapping between index and output.
Example:
int f1(int x) {
switch (x) {
case 0: return 10;
case 1: return 11;
case 2: return 12;
case 3: return 13;
}
return 0;
}
generates:
define i32 @f1(i32 %x) #0 {
entry:
%0 = icmp ult i32 %x, 4
br i1 %0, label %switch.lookup, label %return
switch.lookup:
%switch.offset = add i32 %x, 10
ret i32 %switch.offset
return:
ret i32 0
}
Modified:
llvm/trunk/lib/Transforms/Utils/SimplifyCFG.cpp
llvm/trunk/test/Transforms/SimplifyCFG/X86/switch_to_lookup_table.ll
Modified: llvm/trunk/lib/Transforms/Utils/SimplifyCFG.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Utils/SimplifyCFG.cpp?rev=222121&r1=222120&r2=222121&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Utils/SimplifyCFG.cpp (original)
+++ llvm/trunk/lib/Transforms/Utils/SimplifyCFG.cpp Mon Nov 17 03:13:57 2014
@@ -70,6 +70,7 @@ static cl::opt<bool> HoistCondStores(
cl::desc("Hoist conditional stores if an unconditional store precedes"));
STATISTIC(NumBitMaps, "Number of switch instructions turned into bitmaps");
+STATISTIC(NumLinearMaps, "Number of switch instructions turned into linear mapping");
STATISTIC(NumLookupTables, "Number of switch instructions turned into lookup tables");
STATISTIC(NumLookupTablesHoles, "Number of switch instructions turned into lookup tables (holes checked)");
STATISTIC(NumSinkCommons, "Number of common instructions sunk down to the end block");
@@ -3656,6 +3657,11 @@ namespace {
// store that single value and return it for each lookup.
SingleValueKind,
+ // For tables where there is a linear relationship between table index
+ // and values. We calculate the result with a simple multiplication
+ // and addition instead of a table lookup.
+ LinearMapKind,
+
// For small tables with integer elements, we can pack them into a bitmap
// that fits into a target-legal register. Values are retrieved by
// shift and mask operations.
@@ -3673,6 +3679,10 @@ namespace {
ConstantInt *BitMap;
IntegerType *BitMapElementTy;
+ // For LinearMapKind, these are the constants used to derive the value.
+ ConstantInt *LinearOffset;
+ ConstantInt *LinearMultiplier;
+
// For ArrayKind, this is the array.
GlobalVariable *Array;
};
@@ -3685,7 +3695,7 @@ SwitchLookupTable::SwitchLookupTable(Mod
Constant *DefaultValue,
const DataLayout *DL)
: SingleValue(nullptr), BitMap(nullptr), BitMapElementTy(nullptr),
- Array(nullptr) {
+ LinearOffset(nullptr), LinearMultiplier(nullptr), Array(nullptr) {
assert(Values.size() && "Can't build lookup table without values!");
assert(TableSize >= Values.size() && "Can't fit values in table!");
@@ -3730,6 +3740,43 @@ SwitchLookupTable::SwitchLookupTable(Mod
return;
}
+ // Check if we can derive the value with a linear transformation from the
+ // table index.
+ if (isa<IntegerType>(ValueType)) {
+ bool LinearMappingPossible = true;
+ APInt PrevVal;
+ APInt DistToPrev;
+ assert(TableSize >= 2 && "Should be a SingleValue table.");
+ // Check if there is the same distance between two consecutive values.
+ for (uint64_t I = 0; I < TableSize; ++I) {
+ ConstantInt *ConstVal = dyn_cast<ConstantInt>(TableContents[I]);
+ if (!ConstVal) {
+ // This is an undef. We could deal with it, but undefs in lookup tables
+ // are very seldom. It's probably not worth the additional complexity.
+ LinearMappingPossible = false;
+ break;
+ }
+ APInt Val = ConstVal->getValue();
+ if (I != 0) {
+ APInt Dist = Val - PrevVal;
+ if (I == 1) {
+ DistToPrev = Dist;
+ } else if (Dist != DistToPrev) {
+ LinearMappingPossible = false;
+ break;
+ }
+ }
+ PrevVal = Val;
+ }
+ if (LinearMappingPossible) {
+ LinearOffset = cast<ConstantInt>(TableContents[0]);
+ LinearMultiplier = ConstantInt::get(M.getContext(), DistToPrev);
+ Kind = LinearMapKind;
+ ++NumLinearMaps;
+ return;
+ }
+ }
+
// If the type is integer and the table fits in a register, build a bitmap.
if (WouldFitInRegister(DL, TableSize, ValueType)) {
IntegerType *IT = cast<IntegerType>(ValueType);
@@ -3765,6 +3812,16 @@ Value *SwitchLookupTable::BuildLookup(Va
switch (Kind) {
case SingleValueKind:
return SingleValue;
+ case LinearMapKind: {
+ // Derive the result value from the input value.
+ Value *Result = Builder.CreateIntCast(Index, LinearMultiplier->getType(),
+ false, "switch.idx.cast");
+ if (!LinearMultiplier->isOne())
+ Result = Builder.CreateMul(Result, LinearMultiplier, "switch.idx.mult");
+ if (!LinearOffset->isZero())
+ Result = Builder.CreateAdd(Result, LinearOffset, "switch.offset");
+ return Result;
+ }
case BitMapKind: {
// Type of the bitmap (e.g. i59).
IntegerType *MapTy = BitMap->getType();
Modified: llvm/trunk/test/Transforms/SimplifyCFG/X86/switch_to_lookup_table.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/SimplifyCFG/X86/switch_to_lookup_table.ll?rev=222121&r1=222120&r2=222121&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/SimplifyCFG/X86/switch_to_lookup_table.ll (original)
+++ llvm/trunk/test/Transforms/SimplifyCFG/X86/switch_to_lookup_table.ll Mon Nov 17 03:13:57 2014
@@ -895,7 +895,7 @@ sw.bb1: br label %return
sw.bb2: br label %return
sw.default: br label %return
return:
- %x = phi i32 [ 3, %sw.default ], [ 5, %sw.bb2 ], [ 7, %sw.bb1 ], [ 9, %entry ]
+ %x = phi i32 [ 3, %sw.default ], [ 5, %sw.bb2 ], [ 7, %sw.bb1 ], [ 10, %entry ]
ret i32 %x
; CHECK-LABEL: @threecases(
; CHECK-NOT: switch i32
@@ -977,3 +977,104 @@ return:
; CHECK: switch i32
; CHECK-NOT: @switch.table
}
+
+; We can use linear mapping.
+define i8 @linearmap1(i32 %c) {
+entry:
+ switch i32 %c, label %sw.default [
+ i32 10, label %return
+ i32 11, label %sw.bb1
+ i32 12, label %sw.bb2
+ i32 13, label %sw.bb3
+ ]
+sw.bb1: br label %return
+sw.bb2: br label %return
+sw.bb3: br label %return
+sw.default: br label %return
+return:
+ %x = phi i8 [ 3, %sw.default ], [ 3, %sw.bb3 ], [ 8, %sw.bb2 ], [ 13, %sw.bb1 ], [ 18, %entry ]
+ ret i8 %x
+; CHECK-LABEL: @linearmap1(
+; CHECK: entry:
+; CHECK-NEXT: %switch.tableidx = sub i32 %c, 10
+; CHECK: switch.lookup:
+; CHECK-NEXT: %switch.idx.cast = trunc i32 %switch.tableidx to i8
+; CHECK-NEXT: %switch.idx.mult = mul i8 %switch.idx.cast, -5
+; CHECK-NEXT: %switch.offset = add i8 %switch.idx.mult, 18
+; CHECK-NEXT: ret i8 %switch.offset
+}
+
+; Linear mapping in a different configuration.
+define i32 @linearmap2(i8 %c) {
+entry:
+ switch i8 %c, label %sw.default [
+ i8 -10, label %return
+ i8 -11, label %sw.bb1
+ i8 -12, label %sw.bb2
+ i8 -13, label %sw.bb3
+ ]
+sw.bb1: br label %return
+sw.bb2: br label %return
+sw.bb3: br label %return
+sw.default: br label %return
+return:
+ %x = phi i32 [ 3, %sw.default ], [ 18, %sw.bb3 ], [ 19, %sw.bb2 ], [ 20, %sw.bb1 ], [ 21, %entry ]
+ ret i32 %x
+; CHECK-LABEL: @linearmap2(
+; CHECK: entry:
+; CHECK-NEXT: %switch.tableidx = sub i8 %c, -13
+; CHECK: switch.lookup:
+; CHECK-NEXT: %switch.idx.cast = zext i8 %switch.tableidx to i32
+; CHECK-NEXT: %switch.offset = add i32 %switch.idx.cast, 18
+; CHECK-NEXT: ret i32 %switch.offset
+}
+
+; Linear mapping with overflows.
+define i8 @linearmap3(i32 %c) {
+entry:
+ switch i32 %c, label %sw.default [
+ i32 10, label %return
+ i32 11, label %sw.bb1
+ i32 12, label %sw.bb2
+ i32 13, label %sw.bb3
+ ]
+sw.bb1: br label %return
+sw.bb2: br label %return
+sw.bb3: br label %return
+sw.default: br label %return
+return:
+ %x = phi i8 [ 3, %sw.default ], [ 44, %sw.bb3 ], [ -56, %sw.bb2 ], [ 100, %sw.bb1 ], [ 0, %entry ]
+ ret i8 %x
+; CHECK-LABEL: @linearmap3(
+; CHECK: entry:
+; CHECK-NEXT: %switch.tableidx = sub i32 %c, 10
+; CHECK: switch.lookup:
+; CHECK-NEXT: %switch.idx.cast = trunc i32 %switch.tableidx to i8
+; CHECK-NEXT: %switch.idx.mult = mul i8 %switch.idx.cast, 100
+; CHECK-NEXT: ret i8 %switch.idx.mult
+}
+
+; Linear mapping with with multiplier 1 and offset 0.
+define i8 @linearmap4(i32 %c) {
+entry:
+ switch i32 %c, label %sw.default [
+ i32 -2, label %return
+ i32 -1, label %sw.bb1
+ i32 0, label %sw.bb2
+ i32 1, label %sw.bb3
+ ]
+sw.bb1: br label %return
+sw.bb2: br label %return
+sw.bb3: br label %return
+sw.default: br label %return
+return:
+ %x = phi i8 [ 3, %sw.default ], [ 3, %sw.bb3 ], [ 2, %sw.bb2 ], [ 1, %sw.bb1 ], [ 0, %entry ]
+ ret i8 %x
+; CHECK-LABEL: @linearmap4(
+; CHECK: entry:
+; CHECK-NEXT: %switch.tableidx = sub i32 %c, -2
+; CHECK: switch.lookup:
+; CHECK-NEXT: %switch.idx.cast = trunc i32 %switch.tableidx to i8
+; CHECK-NEXT: ret i8 %switch.idx.cast
+}
+
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