Herb Derby | 35ae65d | 2017-08-11 14:00:50 -0400 | [diff] [blame^] | 1 | /* |
| 2 | * Copyright 2017 Google Inc. |
| 3 | * |
| 4 | * Use of this source code is governed by a BSD-style license that can be |
| 5 | * found in the LICENSE file. |
| 6 | */ |
| 7 | |
| 8 | #ifndef SkSafeMath_DEFINED |
| 9 | #define SkSafeMath_DEFINED |
| 10 | |
| 11 | // SkSafeMath always check that a series of operations do not overflow. |
| 12 | // This must be correct for all platforms, because this is a check for safety at runtime. |
| 13 | |
| 14 | class SkSafeMath { |
| 15 | public: |
| 16 | SkSafeMath() = default; |
| 17 | |
| 18 | bool ok() const { return fOK; } |
| 19 | explicit operator bool() const { return fOK; } |
| 20 | |
| 21 | size_t mul(size_t x, size_t y) { |
| 22 | return sizeof(size_t) == sizeof(uint64_t) ? mul64(x, y) : mul32(x, y); |
| 23 | } |
| 24 | |
| 25 | size_t add(size_t x, size_t y) { |
| 26 | size_t result = x + y; |
| 27 | fOK &= result >= x; |
| 28 | return result; |
| 29 | } |
| 30 | |
| 31 | private: |
| 32 | uint32_t mul32(uint32_t x, uint32_t y) { |
| 33 | uint64_t bx = x; |
| 34 | uint64_t by = y; |
| 35 | uint64_t result = bx * by; |
| 36 | fOK &= result >> 32 == 0; |
| 37 | return result; |
| 38 | } |
| 39 | |
| 40 | uint64_t mul64(uint64_t x, uint64_t y) { |
| 41 | if (x <= std::numeric_limits<uint64_t>::max() >> 32 |
| 42 | && y <= std::numeric_limits<uint64_t>::max() >> 32) { |
| 43 | return x * y; |
| 44 | } else { |
| 45 | auto hi = [](uint64_t x) { return x >> 32; }; |
| 46 | auto lo = [](uint64_t x) { return x & 0xFFFFFFFF; }; |
| 47 | |
| 48 | uint64_t lx_ly = lo(x) * lo(y); |
| 49 | uint64_t hx_ly = hi(x) * lo(y); |
| 50 | uint64_t lx_hy = lo(x) * hi(y); |
| 51 | uint64_t hx_hy = hi(x) * hi(y); |
| 52 | uint64_t result = 0; |
| 53 | result = this->add(lx_ly, (hx_ly << 32)); |
| 54 | result = this->add(result, (lx_hy << 32)); |
| 55 | fOK &= (hx_hy + (hx_ly >> 32) + (lx_hy >> 32)) == 0; |
| 56 | |
| 57 | #if defined(SK_DEBUG) && defined(__clang__) && defined(__x86_64__) |
| 58 | auto double_check = (unsigned __int128)x * y; |
| 59 | SkASSERT(result == (double_check & 0xFFFFFFFFFFFFFFFF)); |
| 60 | SkASSERT(!fOK || (double_check >> 64 == 0)); |
| 61 | #endif |
| 62 | |
| 63 | return result; |
| 64 | } |
| 65 | } |
| 66 | bool fOK = true; |
| 67 | }; |
| 68 | |
| 69 | #endif//SkSafeMath_DEFINED |