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[ConstraintElim] Decompose shl nsw for signed predicates #76961

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Jan 5, 2024
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[ConstraintElim] Decompose shl nsw for signed predicates #76961

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[ConstraintElim] Decompose shl nsw for signed predicates
nikic Jan 4, 2024
74f4d0e
Address review comments
nikic Jan 5, 2024
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12 changes: 12 additions & 0 deletions llvm/lib/Transforms/Scalar/ConstraintElimination.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -517,6 +517,18 @@ static Decomposition decompose(Value *V,
return Result;
}

// (shl nsw x, shift) is (mul nsw x, (1<<shift)), with the exception of
// shift == bw-1.
if (match(V, m_NSWShl(m_Value(Op0), m_ConstantInt(CI)))) {
uint64_t Shift = CI->getValue().getLimitedValue();
if (Shift < Ty->getIntegerBitWidth() - 1) {
assert(Shift < 64 && "Would overflow");
auto Result = decompose(Op0, Preconditions, IsSigned, DL);
Result.mul(int64_t(1) << Shift);
return Result;
}
}

return V;
}

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43 changes: 37 additions & 6 deletions llvm/test/Transforms/ConstraintElimination/shl.ll
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Expand Up @@ -1283,8 +1283,7 @@ define i1 @shl_nsw_x8_slt_x7(i8 %start, i8 %high) {
; CHECK-NEXT: [[C_1:%.*]] = icmp slt i8 [[START_SHL_3]], [[HIGH]]
; CHECK-NEXT: call void @llvm.assume(i1 [[C_1]])
; CHECK-NEXT: [[START_MUL_7:%.*]] = mul nsw i8 [[START]], 7
; CHECK-NEXT: [[T_1:%.*]] = icmp slt i8 [[START_MUL_7]], [[HIGH]]
; CHECK-NEXT: ret i1 [[T_1]]
; CHECK-NEXT: ret i1 true
;
%c.0 = icmp sge i8 %high, 0
call void @llvm.assume(i1 %c.0)
Expand Down Expand Up @@ -1327,11 +1326,9 @@ define i1 @shl_nsw_sign_implication(i8 %x) {
; CHECK-NEXT: [[CMP1:%.*]] = icmp slt i8 [[X]], 0
; CHECK-NEXT: br i1 [[CMP1]], label [[IF:%.*]], label [[ELSE:%.*]]
; CHECK: if:
; CHECK-NEXT: [[CMP2:%.*]] = icmp slt i8 [[SHL]], 0
; CHECK-NEXT: ret i1 [[CMP2]]
; CHECK-NEXT: ret i1 true
; CHECK: else:
; CHECK-NEXT: [[CMP3:%.*]] = icmp sge i8 [[SHL]], 0
; CHECK-NEXT: ret i1 [[CMP3]]
; CHECK-NEXT: ret i1 true
;
%shl = shl nsw i8 %x, 2
%cmp1 = icmp slt i8 %x, 0
Expand Down Expand Up @@ -1361,3 +1358,37 @@ define i1 @shl_nsw_by_bw_minus_1(i64 %x) {
%t.1 = icmp slt i64 %x, 0
ret i1 %t.1
}

; Shift returns poison in this case, just make sure we don't crash.
define i1 @shl_nsw_by_bw(i64 %x) {
; CHECK-LABEL: @shl_nsw_by_bw(
; CHECK-NEXT: [[X_SHL:%.*]] = shl nsw i64 [[X:%.*]], 64
; CHECK-NEXT: [[C_1:%.*]] = icmp slt i64 [[X_SHL]], 0
; CHECK-NEXT: call void @llvm.assume(i1 [[C_1]])
; CHECK-NEXT: [[T_1:%.*]] = icmp slt i64 [[X]], 0
; CHECK-NEXT: ret i1 [[T_1]]
;
%x.shl = shl nsw i64 %x, 64
%c.1 = icmp slt i64 %x.shl, 0
call void @llvm.assume(i1 %c.1)

%t.1 = icmp slt i64 %x, 0
ret i1 %t.1
}

; Shift returns poison in this case, just make sure we don't crash.
define i1 @shl_nsw_by_bw_plus_1(i64 %x) {
; CHECK-LABEL: @shl_nsw_by_bw_plus_1(
; CHECK-NEXT: [[X_SHL:%.*]] = shl nsw i64 [[X:%.*]], 65
; CHECK-NEXT: [[C_1:%.*]] = icmp slt i64 [[X_SHL]], 0
; CHECK-NEXT: call void @llvm.assume(i1 [[C_1]])
; CHECK-NEXT: [[T_1:%.*]] = icmp slt i64 [[X]], 0
; CHECK-NEXT: ret i1 [[T_1]]
;
%x.shl = shl nsw i64 %x, 65
%c.1 = icmp slt i64 %x.shl, 0
call void @llvm.assume(i1 %c.1)

%t.1 = icmp slt i64 %x, 0
ret i1 %t.1
}