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The current demand propagator for addition will mark all input bits at and right of the alive output bit as alive. But carry won't propagate beyond a bit for which both operands are zero (or one/zero in the case of subtraction) so a more accurate answer is possible given known bits. I derived a propagator by working through truth tables and using a bit-reversed addition to make demand ripple to the right, but I'm not sure how to make a convincing argument for its correctness in the comments yet. Nevertheless, here's a minimal implementation and test to get feedback. This would help in a situation where, for example, four bytes (<128) packed into an int are added with four others SIMD-style but only one of the four results is actually read. Known A: 0_______0_______0_______0_______ Known B: 0_______0_______0_______0_______ AOut: 00000000001000000000000000000000 AB, current: 00000000001111111111111111111111 AB, patch: 00000000001111111000000000000000 Committed on behalf of: @rrika (Erika) Differential Revision: https://reviews.llvm.org/D72423
53 lines
1.4 KiB
C++
53 lines
1.4 KiB
C++
//===- llvm/unittest/Support/KnownBitsTest.h - KnownBits tests ------------===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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//
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// This file implements helpers for KnownBits and DemandedBits unit tests.
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//
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//===----------------------------------------------------------------------===//
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#ifndef LLVM_UNITTESTS_SUPPORT_KNOWNBITSTEST_H
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#define LLVM_UNITTESTS_SUPPORT_KNOWNBITSTEST_H
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#include "llvm/Support/KnownBits.h"
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namespace {
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using namespace llvm;
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template <typename FnTy> void ForeachKnownBits(unsigned Bits, FnTy Fn) {
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unsigned Max = 1 << Bits;
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KnownBits Known(Bits);
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for (unsigned Zero = 0; Zero < Max; ++Zero) {
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for (unsigned One = 0; One < Max; ++One) {
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Known.Zero = Zero;
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Known.One = One;
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if (Known.hasConflict())
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continue;
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Fn(Known);
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}
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}
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}
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template <typename FnTy>
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void ForeachNumInKnownBits(const KnownBits &Known, FnTy Fn) {
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unsigned Bits = Known.getBitWidth();
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unsigned Max = 1 << Bits;
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for (unsigned N = 0; N < Max; ++N) {
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APInt Num(Bits, N);
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if ((Num & Known.Zero) != 0 || (~Num & Known.One) != 0)
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continue;
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Fn(Num);
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}
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}
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} // end anonymous namespace
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#endif
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