6502/llvm-6502
1
0
Fork 0
mirror of https://github.com/c64scene-ar/llvm-6502.git synced 2024-09-30 04:56:49 +00:00
Code Issues Projects Releases Wiki Activity

Optimize switch lookup tables with linear mapping.

Browse Source 
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
}



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222121 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Erik Eckstein 2014-11-17 09:13:57 +00:00
parent 17e95ead36
commit 72a1394991
2 changed files with 160 additions and 2 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

59
lib/Transforms/Utils/SimplifyCFG.cpp
View File

@ -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(Module &M,
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(Module &M,
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(Value *Index, IRBuilder<> &Builder) {
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();

103
test/Transforms/SimplifyCFG/X86/switch_to_lookup_table.ll
View File

@ -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
}