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[InstCombine] Optimize (select %x, op(%x), 0) to op(%x) for operations where op(0) == 0 #147605

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Merged
merged 9 commits into from
Jul 17, 2025
Merged
39 changes: 31 additions & 8 deletions llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
Original file line number Diff line number Diff line change
Expand Up @@ -878,7 +878,11 @@ static Instruction *foldSetClearBits(SelectInst &Sel,
// is a vector consisting of 0 and undefs. If a constant compared with x
// is a scalar undefined value or undefined vector then an expression
// should be already folded into a constant.
static Instruction *foldSelectZeroOrMul(SelectInst &SI, InstCombinerImpl &IC) {
//
// This also holds all operations such that Op(0) == 0
// e.g. Shl, Umin, etc
static Instruction *foldSelectZeroOrFixedOp(SelectInst &SI,
InstCombinerImpl &IC) {
auto *CondVal = SI.getCondition();
auto *TrueVal = SI.getTrueValue();
auto *FalseVal = SI.getFalseValue();
Expand All @@ -900,10 +904,23 @@ static Instruction *foldSelectZeroOrMul(SelectInst &SI, InstCombinerImpl &IC) {
// non-zero elements that are masked by undef elements in the compare
// constant.
auto *TrueValC = dyn_cast<Constant>(TrueVal);
if (TrueValC == nullptr ||
!match(FalseVal, m_c_Mul(m_Specific(X), m_Value(Y))) ||
!isa<Instruction>(FalseVal))
if (TrueValC == nullptr || !isa<Instruction>(FalseVal))
return nullptr;

bool FreezeY;
if (match(FalseVal, m_c_Mul(m_Specific(X), m_Value(Y))) ||
match(FalseVal, m_c_And(m_Specific(X), m_Value(Y))) ||
match(FalseVal, m_FShl(m_Specific(X), m_Specific(X), m_Value(Y))) ||
match(FalseVal, m_FShr(m_Specific(X), m_Specific(X), m_Value(Y))) ||
match(FalseVal,
m_c_Intrinsic<Intrinsic::umin>(m_Specific(X), m_Value(Y)))) {
FreezeY = true;
} else if (match(FalseVal, m_IDiv(m_Specific(X), m_Value(Y))) ||
match(FalseVal, m_IRem(m_Specific(X), m_Value(Y)))) {
FreezeY = false;
} else {
return nullptr;
}

auto *ZeroC = cast<Constant>(cast<Instruction>(CondVal)->getOperand(1));
auto *MergedC = Constant::mergeUndefsWith(TrueValC, ZeroC);
Expand All @@ -914,9 +931,15 @@ static Instruction *foldSelectZeroOrMul(SelectInst &SI, InstCombinerImpl &IC) {
return nullptr;

auto *FalseValI = cast<Instruction>(FalseVal);
auto *FrY = IC.InsertNewInstBefore(new FreezeInst(Y, Y->getName() + ".fr"),
FalseValI->getIterator());
IC.replaceOperand(*FalseValI, FalseValI->getOperand(0) == Y ? 0 : 1, FrY);
if (FreezeY) {
auto *FrY = IC.InsertNewInstBefore(new FreezeInst(Y, Y->getName() + ".fr"),
FalseValI->getIterator());
IC.replaceOperand(*FalseValI,
FalseValI->getOperand(0) == Y
? 0
: (FalseValI->getOperand(1) == Y ? 1 : 2),
FrY);
}
return IC.replaceInstUsesWith(SI, FalseValI);
}

Expand Down Expand Up @@ -4104,7 +4127,7 @@ Instruction *InstCombinerImpl::visitSelectInst(SelectInst &SI) {
return Add;
if (Instruction *Or = foldSetClearBits(SI, Builder))
return Or;
if (Instruction *Mul = foldSelectZeroOrMul(SI, *this))
if (Instruction *Mul = foldSelectZeroOrFixedOp(SI, *this))
return Mul;

// Turn (select C, (op X, Y), (op X, Z)) -> (op X, (select C, Y, Z))
Expand Down
7 changes: 3 additions & 4 deletions llvm/test/Transforms/InstCombine/icmp-select.ll
Original file line number Diff line number Diff line change
Expand Up @@ -248,10 +248,9 @@ define i1 @icmp_select_implied_cond_relational_off_by_one(i8 %x, i8 %y) {

define i1 @umin_seq_comparison(i8 %x, i8 %y) {
; CHECK-LABEL: @umin_seq_comparison(
; CHECK-NEXT: [[CMP1:%.*]] = icmp eq i8 [[X:%.*]], 0
; CHECK-NEXT: [[CMP21:%.*]] = icmp ule i8 [[X]], [[Y:%.*]]
; CHECK-NEXT: [[CMP2:%.*]] = select i1 [[CMP1]], i1 true, i1 [[CMP21]]
; CHECK-NEXT: ret i1 [[CMP2]]
; CHECK-NEXT: [[Y:%.*]] = freeze i8 [[Y1:%.*]]
; CHECK-NEXT: [[CMP21:%.*]] = icmp ule i8 [[X:%.*]], [[Y]]
; CHECK-NEXT: ret i1 [[CMP21]]
;
%min = call i8 @llvm.umin.i8(i8 %x, i8 %y)
%cmp1 = icmp eq i8 %x, 0
Expand Down
221 changes: 221 additions & 0 deletions llvm/test/Transforms/InstCombine/select-fixed-zero.ll
Original file line number Diff line number Diff line change
@@ -0,0 +1,221 @@
; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
; RUN: opt -S -passes=instcombine < %s | FileCheck %s

; (select (icmp x, 0, eq), 0, (umin x, y)) -> (umin x, y)
define i64 @umin_select(i64 %a, i64 %b) {
; CHECK-LABEL: @umin_select(
; CHECK-NEXT: [[B_FR:%.*]] = freeze i64 [[B:%.*]]
; CHECK-NEXT: [[UMIN:%.*]] = call i64 @llvm.umin.i64(i64 [[A:%.*]], i64 [[B_FR]])
; CHECK-NEXT: ret i64 [[UMIN]]
;
%cond = icmp eq i64 %a, 0
%umin = call i64 @llvm.umin.i64(i64 %a, i64 %b)
%select = select i1 %cond, i64 0, i64 %umin
ret i64 %select
}

; (select (icmp x, 0, eq), 0, (mul x, y)) -> (mul x, y)
define i64 @mul_select(i64 %a, i64 %b) {
; CHECK-LABEL: @mul_select(
; CHECK-NEXT: [[B_FR:%.*]] = freeze i64 [[B:%.*]]
; CHECK-NEXT: [[MUL:%.*]] = mul i64 [[A:%.*]], [[B_FR]]
; CHECK-NEXT: ret i64 [[MUL]]
;
%cond = icmp eq i64 %a, 0
%mul = mul i64 %a, %b
%select = select i1 %cond, i64 0, i64 %mul
ret i64 %select
}

; (select (icmp x, 0, eq), 0, (mul x, y)) -> (mul x, y)
define i64 @mul_select_comm(i64 %a, i64 %b) {
; CHECK-LABEL: @mul_select_comm(
; CHECK-NEXT: [[B_FR:%.*]] = freeze i64 [[B:%.*]]
; CHECK-NEXT: [[MUL:%.*]] = mul i64 [[B_FR]], [[A:%.*]]
; CHECK-NEXT: ret i64 [[MUL]]
;
%cond = icmp eq i64 %a, 0
%mul = mul i64 %b, %a
%select = select i1 %cond, i64 0, i64 %mul
ret i64 %select
}

; (select (icmp x, 0, eq), 0, (shl x, y)) -> (shl x, y)
define i64 @shl_select(i64 %a, i64 %b) {
; CHECK-LABEL: @shl_select(
; CHECK-NEXT: [[COND:%.*]] = icmp eq i64 [[A:%.*]], 0
; CHECK-NEXT: [[SHL:%.*]] = shl i64 [[A]], [[B_FR:%.*]]
; CHECK-NEXT: [[SELECT:%.*]] = select i1 [[COND]], i64 0, i64 [[SHL]]
; CHECK-NEXT: ret i64 [[SELECT]]
;
%cond = icmp eq i64 %a, 0
%shl = shl i64 %a, %b
%select = select i1 %cond, i64 0, i64 %shl
ret i64 %select
}

; (select (icmp x, 0, eq), 0, (and x, y)) -> (and x, y)
define i64 @and_select(i64 %a, i64 %b) {
; CHECK-LABEL: @and_select(
; CHECK-NEXT: [[B_FR:%.*]] = freeze i64 [[B:%.*]]
; CHECK-NEXT: [[AND:%.*]] = and i64 [[A:%.*]], [[B_FR]]
; CHECK-NEXT: ret i64 [[AND]]
;
%cond = icmp eq i64 %a, 0
%and = and i64 %a, %b
%select = select i1 %cond, i64 0, i64 %and
ret i64 %select
}

; (select (icmp x, 0, eq), 0, (and x, y)) -> (and x, y)
define i64 @and_select_comm(i64 %a, i64 %b) {
; CHECK-LABEL: @and_select_comm(
; CHECK-NEXT: [[B_FR:%.*]] = freeze i64 [[B:%.*]]
; CHECK-NEXT: [[AND:%.*]] = and i64 [[B_FR]], [[A:%.*]]
; CHECK-NEXT: ret i64 [[AND]]
;
%cond = icmp eq i64 %a, 0
%and = and i64 %b, %a
%select = select i1 %cond, i64 0, i64 %and
ret i64 %select
}

; (select (icmp x, 0, ne), (ashr x, y), 0) -> (ashr x, y)
define i64 @ashr_select(i64 %a, i64 %b) {
; CHECK-LABEL: @ashr_select(
; CHECK-NEXT: [[COND_NOT:%.*]] = icmp eq i64 [[A:%.*]], 0
; CHECK-NEXT: [[ASHR:%.*]] = ashr i64 [[A]], [[B_FR:%.*]]
; CHECK-NEXT: [[SELECT:%.*]] = select i1 [[COND_NOT]], i64 0, i64 [[ASHR]]
; CHECK-NEXT: ret i64 [[SELECT]]
;
%cond = icmp ne i64 0, %a
%ashr = ashr i64 %a, %b
%select = select i1 %cond, i64 %ashr, i64 0
ret i64 %select
}

; (select (icmp x, 0, ne), (lshr x, y), 0) -> (lshr x, y)
define i64 @lshr_select(i64 %a, i64 %b) {
; CHECK-LABEL: @lshr_select(
; CHECK-NEXT: [[COND_NOT:%.*]] = icmp eq i64 [[A:%.*]], 0
; CHECK-NEXT: [[LSHR:%.*]] = lshr i64 [[A]], [[B_FR:%.*]]
; CHECK-NEXT: [[SELECT:%.*]] = select i1 [[COND_NOT]], i64 0, i64 [[LSHR]]
; CHECK-NEXT: ret i64 [[SELECT]]
;
%cond = icmp ne i64 0, %a
%lshr = lshr i64 %a, %b
%select = select i1 %cond, i64 %lshr, i64 0
ret i64 %select
}

; (select (icmp x, 0, eq), 0, fshr(x, x, y)) -> fshr(x, x, y)
define i64 @fshr_select(i64 %a, i64 %b) {
; CHECK-LABEL: @fshr_select(
; CHECK-NEXT: [[B_FR:%.*]] = freeze i64 [[B:%.*]]
; CHECK-NEXT: [[FSHR:%.*]] = call i64 @llvm.fshr.i64(i64 [[A:%.*]], i64 [[A]], i64 [[B_FR]])
; CHECK-NEXT: ret i64 [[FSHR]]
;
%cond = icmp eq i64 %a, 0
%fshr = call i64 @llvm.fshr.i64(i64 %a, i64 %a, i64 %b)
%select = select i1 %cond, i64 0, i64 %fshr
ret i64 %select
}

; (select (icmp x, 0, eq), 0, (fshl x, x, y)) -> (fshl x, x, y)
define i64 @fshl_select(i64 %a, i64 %b) {
; CHECK-LABEL: @fshl_select(
; CHECK-NEXT: [[B_FR:%.*]] = freeze i64 [[B:%.*]]
; CHECK-NEXT: [[FSHL:%.*]] = call i64 @llvm.fshl.i64(i64 [[A:%.*]], i64 [[A]], i64 [[B_FR]])
; CHECK-NEXT: ret i64 [[FSHL]]
;
%cond = icmp eq i64 %a, 0
%fshl = call i64 @llvm.fshl.i64(i64 %a, i64 %a, i64 %b)
%select = select i1 %cond, i64 0, i64 %fshl
ret i64 %select
}

; (select (icmp x, 0, eq), 0, (fshr x, z, y)) -> leave as is
define i64 @fshr_select_no_combine(i64 %a, i64 %b, i64 %c) {
; CHECK-LABEL: @fshr_select_no_combine(
; CHECK-NEXT: [[COND:%.*]] = icmp eq i64 [[A:%.*]], 0
; CHECK-NEXT: [[FSHR:%.*]] = call i64 @llvm.fshr.i64(i64 [[A]], i64 [[B:%.*]], i64 [[C:%.*]])
; CHECK-NEXT: [[SELECT:%.*]] = select i1 [[COND]], i64 0, i64 [[FSHR]]
; CHECK-NEXT: ret i64 [[SELECT]]
;
%cond = icmp eq i64 %a, 0
%fshr = call i64 @llvm.fshr.i64(i64 %a, i64 %b, i64 %c)
%select = select i1 %cond, i64 0, i64 %fshr
ret i64 %select
}

; (select (icmp x, 0, eq), 0, (sdiv x, y)) -> (sdiv x, y)
define i64 @sdiv_select(i64 %a, i64 %b) {
; CHECK-LABEL: @sdiv_select(
; CHECK-NEXT: [[DIV:%.*]] = sdiv i64 [[A:%.*]], [[B_FR:%.*]]
; CHECK-NEXT: ret i64 [[DIV]]
;
%cond = icmp eq i64 %a, 0
%div = sdiv i64 %a, %b
%select = select i1 %cond, i64 0, i64 %div
ret i64 %select
}

; (select (icmp x, 0, eq), 0, (udiv x, y)) -> (udiv x, y)
define i64 @udiv_select(i64 %a, i64 %b) {
; CHECK-LABEL: @udiv_select(
; CHECK-NEXT: [[DIV:%.*]] = udiv i64 [[A:%.*]], [[B_FR:%.*]]
; CHECK-NEXT: ret i64 [[DIV]]
;
%cond = icmp eq i64 %a, 0
%div = udiv i64 %a, %b
%select = select i1 %cond, i64 0, i64 %div
ret i64 %select
}

; (select (icmp x, 0, eq), 0, (srem x, y)) -> (srem x, y)
define i64 @srem_select(i64 %a, i64 %b) {
; CHECK-LABEL: @srem_select(
; CHECK-NEXT: [[REM:%.*]] = srem i64 [[A:%.*]], [[B:%.*]]
; CHECK-NEXT: ret i64 [[REM]]
;
%cond = icmp eq i64 %a, 0
%rem = srem i64 %a, %b
%select = select i1 %cond, i64 0, i64 %rem
ret i64 %select
}

; (select (icmp x, 0, eq), 0, (urem x, y)) -> (urem x, y)
define i64 @urem_select(i64 %a, i64 %b) {
; CHECK-LABEL: @urem_select(
; CHECK-NEXT: [[REM:%.*]] = urem i64 [[A:%.*]], [[B:%.*]]
; CHECK-NEXT: ret i64 [[REM]]
;
%cond = icmp eq i64 %a, 0
%rem = urem i64 %a, %b
%select = select i1 %cond, i64 0, i64 %rem
ret i64 %select
}

; (select (icmp x, 0, eq), 0, (icmp x, 0, slt)) -> (icmp x, 0, slt)
define i1 @icmp_slt_select(i64 %a) {
; CHECK-LABEL: @icmp_slt_select(
; CHECK-NEXT: [[ICMP:%.*]] = icmp slt i64 [[A:%.*]], 0
; CHECK-NEXT: ret i1 [[ICMP]]
;
%cond = icmp eq i64 %a, 0
%icmp = icmp slt i64 %a, 0
%select = select i1 %cond, i1 0, i1 %icmp
ret i1 %select
}

; (select (icmp x, 0, eq), 0, (sub 0, x)) -> (sub 0, x)
define i64 @sub_select(i64 %a) {
; CHECK-LABEL: @sub_select(
; CHECK-NEXT: [[SUB:%.*]] = sub i64 0, [[A:%.*]]
; CHECK-NEXT: ret i64 [[SUB]]
;
%cond = icmp eq i64 %a, 0
%sub = sub i64 0, %a
%select = select i1 %cond, i64 0, i64 %sub
ret i64 %select
}
7 changes: 3 additions & 4 deletions llvm/test/Transforms/InstCombine/select.ll
Original file line number Diff line number Diff line change
Expand Up @@ -893,10 +893,9 @@ define i32 @test56(i16 %x) {

define i32 @test57(i32 %x, i32 %y) {
; CHECK-LABEL: @test57(
; CHECK-NEXT: [[AND:%.*]] = and i32 [[X:%.*]], [[Y:%.*]]
; CHECK-NEXT: [[TOBOOL:%.*]] = icmp eq i32 [[X]], 0
; CHECK-NEXT: [[DOTAND:%.*]] = select i1 [[TOBOOL]], i32 0, i32 [[AND]]
; CHECK-NEXT: ret i32 [[DOTAND]]
; CHECK-NEXT: [[Y:%.*]] = freeze i32 [[Y1:%.*]]
; CHECK-NEXT: [[AND:%.*]] = and i32 [[X:%.*]], [[Y]]
; CHECK-NEXT: ret i32 [[AND]]
;
%and = and i32 %x, %y
%tobool = icmp eq i32 %x, 0
Expand Down
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