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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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11 changes: 11 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 +517,17 @@ static Decomposition decompose(Value *V,
return Result;
}

// shl nsw x, shift is mul nsw x, (1<<shift),
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nit: might be good to wrap the expressions in () or something else to separate them from the interleave text

// with the exception of shift == bw-1.
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nit: reflow comment?

if (match(V, m_NSWShl(m_Value(Op0), m_ConstantInt(CI)))) {
uint64_t Shift = CI->getValue().getLimitedValue();
if (Shift < Ty->getIntegerBitWidth() - 1 && Shift < 64) {
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Looks like there's no test case for Shift >= 64 at the moment. IIUC this is to avoid overflows when doing 1 << Shift? We won't reach this code if the types are wider than 64 bit and if the type is <= i64 then shifts >= 64 will produce poison. Might be helpful for future readers to document the condition

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Yeah, this check is actually redundant. I'll replace it with an assert.

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Sounds good!

auto Result = decompose(Op0, Preconditions, IsSigned, DL);
Result.mul(int64_t(1) << Shift);
return Result;
}
}

return V;
}

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9 changes: 3 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
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