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[InstCombine] canShiftBinOpWithConstantRHS(): drop bogus signbit check

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Summary:
In D61918 i was looking at dropping it in DAGCombiner `visitShiftByConstant()`,
but as @craig.topper pointed out, it was copied from here.

That check claims that the transform is illegal otherwise.
That isn't true:
1. For `ISD::ADD`, we only process `ISD::SHL` outer shift => sign bit does not matter
   https://rise4fun.com/Alive/K4A
2. For `ISD::AND`, there is no restriction on constants:
   https://rise4fun.com/Alive/Wy3
3. For `ISD::OR`, there is no restriction on constants:
   https://rise4fun.com/Alive/GOH
3. For `ISD::XOR`, there is no restriction on constants:
   https://rise4fun.com/Alive/ml6

So, why is it there then?
As far as i can tell, it dates all the way back to original check-in rL7793.
I think we should just drop it.

Reviewers: spatel, craig.topper, efriedma, majnemer

Reviewed By: spatel

Subscribers: llvm-commits, craig.topper

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D61938

llvm-svn: 361043
This commit is contained in:
Roman Lebedev 2019-05-17 15:52:49 +00:00
parent 4c3058cdb8
commit 6efaf72ba7
4 changed files with 25 additions and 43 deletions
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34
lib/Transforms/InstCombine/InstCombineShifts.cpp
View File

@ -312,35 +312,17 @@ static Value *getShiftedValue(Value *V, unsigned NumBits, bool isLeftShift,
// If this is a bitwise operator or add with a constant RHS we might be able
// to pull it through a shift.
static bool canShiftBinOpWithConstantRHS(BinaryOperator &Shift,
BinaryOperator *BO,
const APInt &C) {
bool IsValid = true; // Valid only for And, Or Xor,
bool HighBitSet = false; // Transform ifhigh bit of constant set?
BinaryOperator *BO) {
switch (BO->getOpcode()) {
default: IsValid = false; break; // Do not perform transform!
default:
return false; // Do not perform transform!
case Instruction::Add:
IsValid = Shift.getOpcode() == Instruction::Shl;
break;
return Shift.getOpcode() == Instruction::Shl;
case Instruction::Or:
case Instruction::Xor:
HighBitSet = false;
break;
case Instruction::And:
HighBitSet = true;
break;
return true;
}
// If this is a signed shift right, and the high bit is modified
// by the logical operation, do not perform the transformation.
// The HighBitSet boolean indicates the value of the high bit of
// the constant which would cause it to be modified for this
// operation.
//
if (IsValid && Shift.getOpcode() == Instruction::AShr)
IsValid = C.isNegative() == HighBitSet;
return IsValid;
}
Instruction *InstCombiner::FoldShiftByConstant(Value *Op0, Constant *Op1,
@ -507,7 +489,7 @@ Instruction *InstCombiner::FoldShiftByConstant(Value *Op0, Constant *Op1,
// shift is the only use, we can pull it out of the shift.
const APInt *Op0C;
if (match(Op0BO->getOperand(1), m_APInt(Op0C))) {
if (canShiftBinOpWithConstantRHS(I, Op0BO, *Op0C)) {
if (canShiftBinOpWithConstantRHS(I, Op0BO)) {
Constant *NewRHS = ConstantExpr::get(I.getOpcode(),
cast<Constant>(Op0BO->getOperand(1)), Op1);
@ -551,7 +533,7 @@ Instruction *InstCombiner::FoldShiftByConstant(Value *Op0, Constant *Op1,
const APInt *C;
if (!isa<Constant>(FalseVal) && TBO->getOperand(0) == FalseVal &&
match(TBO->getOperand(1), m_APInt(C)) &&
canShiftBinOpWithConstantRHS(I, TBO, *C)) {
canShiftBinOpWithConstantRHS(I, TBO)) {
Constant *NewRHS = ConstantExpr::get(I.getOpcode(),
cast<Constant>(TBO->getOperand(1)), Op1);
@ -570,7 +552,7 @@ Instruction *InstCombiner::FoldShiftByConstant(Value *Op0, Constant *Op1,
const APInt *C;
if (!isa<Constant>(TrueVal) && FBO->getOperand(0) == TrueVal &&
match(FBO->getOperand(1), m_APInt(C)) &&
canShiftBinOpWithConstantRHS(I, FBO, *C)) {
canShiftBinOpWithConstantRHS(I, FBO)) {
Constant *NewRHS = ConstantExpr::get(I.getOpcode(),
cast<Constant>(FBO->getOperand(1)), Op1);

8
test/Transforms/InstCombine/pull-binop-through-shift.ll
View File

@ -198,8 +198,8 @@ define i32 @and_nosignbit_ashr(i32 %x) {
define i32 @or_signbit_ashr(i32 %x) {
; CHECK-LABEL: @or_signbit_ashr(
; CHECK-NEXT: [[T0:%.*]] = or i32 [[X:%.*]], -65536
; CHECK-NEXT: [[R:%.*]] = ashr i32 [[T0]], 8
; CHECK-NEXT: [[TMP1:%.*]] = lshr i32 [[X:%.*]], 8
; CHECK-NEXT: [[R:%.*]] = or i32 [[TMP1]], -256
; CHECK-NEXT: ret i32 [[R]]
;
%t0 = or i32 %x, 4294901760 ; 0xFFFF0000
@ -219,8 +219,8 @@ define i32 @or_nosignbit_ashr(i32 %x) {
define i32 @xor_signbit_ashr(i32 %x) {
; CHECK-LABEL: @xor_signbit_ashr(
; CHECK-NEXT: [[T0:%.*]] = xor i32 [[X:%.*]], -65536
; CHECK-NEXT: [[R:%.*]] = ashr i32 [[T0]], 8
; CHECK-NEXT: [[T0:%.*]] = ashr i32 [[X:%.*]], 8
; CHECK-NEXT: [[R:%.*]] = xor i32 [[T0]], -256
; CHECK-NEXT: ret i32 [[R]]
;
%t0 = xor i32 %x, 4294901760 ; 0xFFFF0000

18
test/Transforms/InstCombine/pull-conditional-binop-through-shift.ll
View File

@ -221,9 +221,9 @@ define i32 @and_signbit_select_ashr(i32 %x, i1 %cond) {
}
define i32 @and_nosignbit_select_ashr(i32 %x, i1 %cond) {
; CHECK-LABEL: @and_nosignbit_select_ashr(
; CHECK-NEXT: [[T0:%.*]] = and i32 [[X:%.*]], 2147418112
; CHECK-NEXT: [[T1:%.*]] = select i1 [[COND:%.*]], i32 [[T0]], i32 [[X]]
; CHECK-NEXT: [[R:%.*]] = ashr i32 [[T1]], 8
; CHECK-NEXT: [[TMP1:%.*]] = ashr i32 [[X:%.*]], 8
; CHECK-NEXT: [[TMP2:%.*]] = and i32 [[TMP1]], 8388352
; CHECK-NEXT: [[R:%.*]] = select i1 [[COND:%.*]], i32 [[TMP2]], i32 [[TMP1]]
; CHECK-NEXT: ret i32 [[R]]
;
%t0 = and i32 %x, 2147418112 ; 0x7FFF0000
@ -234,9 +234,9 @@ define i32 @and_nosignbit_select_ashr(i32 %x, i1 %cond) {
define i32 @or_signbit_select_ashr(i32 %x, i1 %cond) {
; CHECK-LABEL: @or_signbit_select_ashr(
; CHECK-NEXT: [[T0:%.*]] = or i32 [[X:%.*]], -65536
; CHECK-NEXT: [[T1:%.*]] = select i1 [[COND:%.*]], i32 [[T0]], i32 [[X]]
; CHECK-NEXT: [[R:%.*]] = ashr i32 [[T1]], 8
; CHECK-NEXT: [[TMP1:%.*]] = ashr i32 [[X:%.*]], 8
; CHECK-NEXT: [[TMP2:%.*]] = or i32 [[TMP1]], -256
; CHECK-NEXT: [[R:%.*]] = select i1 [[COND:%.*]], i32 [[TMP2]], i32 [[TMP1]]
; CHECK-NEXT: ret i32 [[R]]
;
%t0 = or i32 %x, 4294901760 ; 0xFFFF0000
@ -259,9 +259,9 @@ define i32 @or_nosignbit_select_ashr(i32 %x, i1 %cond) {
define i32 @xor_signbit_select_ashr(i32 %x, i1 %cond) {
; CHECK-LABEL: @xor_signbit_select_ashr(
; CHECK-NEXT: [[T0:%.*]] = xor i32 [[X:%.*]], -65536
; CHECK-NEXT: [[T1:%.*]] = select i1 [[COND:%.*]], i32 [[T0]], i32 [[X]]
; CHECK-NEXT: [[R:%.*]] = ashr i32 [[T1]], 8
; CHECK-NEXT: [[TMP1:%.*]] = ashr i32 [[X:%.*]], 8
; CHECK-NEXT: [[TMP2:%.*]] = xor i32 [[TMP1]], -256
; CHECK-NEXT: [[R:%.*]] = select i1 [[COND:%.*]], i32 [[TMP2]], i32 [[TMP1]]
; CHECK-NEXT: ret i32 [[R]]
;
%t0 = xor i32 %x, 4294901760 ; 0xFFFF0000

8
test/Transforms/InstCombine/trunc.ll
View File

@ -125,8 +125,8 @@ define i16 @ashr_mul(i8 %X, i8 %Y) {
define i32 @trunc_ashr(i32 %X) {
; CHECK-LABEL: @trunc_ashr(
; CHECK-NEXT: [[B:%.*]] = or i32 [[X:%.*]], -2147483648
; CHECK-NEXT: [[C:%.*]] = ashr i32 [[B]], 8
; CHECK-NEXT: [[TMP1:%.*]] = lshr i32 [[X:%.*]], 8
; CHECK-NEXT: [[C:%.*]] = or i32 [[TMP1]], -8388608
; CHECK-NEXT: ret i32 [[C]]
;
%A = zext i32 %X to i36
@ -138,8 +138,8 @@ define i32 @trunc_ashr(i32 %X) {
define <2 x i32> @trunc_ashr_vec(<2 x i32> %X) {
; CHECK-LABEL: @trunc_ashr_vec(
; CHECK-NEXT: [[B:%.*]] = or <2 x i32> [[X:%.*]], <i32 -2147483648, i32 -2147483648>
; CHECK-NEXT: [[C:%.*]] = ashr <2 x i32> [[B]], <i32 8, i32 8>
; CHECK-NEXT: [[TMP1:%.*]] = lshr <2 x i32> [[X:%.*]], <i32 8, i32 8>
; CHECK-NEXT: [[C:%.*]] = or <2 x i32> [[TMP1]], <i32 -8388608, i32 -8388608>
; CHECK-NEXT: ret <2 x i32> [[C]]
;
%A = zext <2 x i32> %X to <2 x i36>