Pictures MEMCMP RESULTS, X86 name : mismatch | size : 10000 | type : char | group : avx2+bmi | padding : min 国 sd::mismatch 25oo0 图use_mt64s dmemcmp Advanced SIMD Algorithms in Pictures name : mismatch | size : 10000 | type : char | group : avx2+bmi | padding : min use_int64s std:memcemp everalgozmlsmatch ao0 250 2o0 tme 150 1o0 50 se_int54S Advanced SIMD Algorithms in Pictures name : mismatch | size : 10000 | type : char | group : apple_ml | padding : min 400

*>(src))); dst += 16; src += 16; size -= 16; } } else { size_t padding = (16 - (reinterpret_cast(dst) & 15)) & 15; if (padding > 0) { __m128i head = _mm_loadu_si128(reinterpret_cast(src)); __m128i*>(src)); _mm_storeu_si128(reinterpret_cast<__m128i*>(dst), head); dst += padding; src += padding; size -= padding; } __m128i c0, c1, c2, c3, c4, c5, c6, c7; while (size >= 128) { c0 = _mm_l *>(src))); dst += 16; src += 16; size -= 16; } } else { size_t padding = (16 - (reinterpret_cast(dst) & 15)) & 15; if (padding > 0) { __m128i head = _mm_loadu_si128(reinterpret_cast(src));

Call Site uleb128 Call Site Size 4 … Unwind Instructions … offset( &gcc_personality_v0 ) padding padding paddingEntering bar() 142 SRAM Address Value 0x4000 0x???????? 0x3FFC 0x???????? 0x3FF8 Call Site uleb128 Call Site Size 4 … Unwind Instructions … offset( &gcc_personality_v0 ) padding padding paddingExecuting bar()'s Destructor 143 SRAM Address Value 0x4000 0x???????? 0x3FFC 0x? Call Site uleb128 Call Site Size 4 … Unwind Instructions … offset( &gcc_personality_v0 ) padding padding paddingheader call site table action table(s) personality function personality data type

std::cout << sizeof(Widget) << '\n'; // prints 12 // Needs to be 4-byte aligned on x64 char padding1[3]; char padding2[3];Order of Member Data 36 Task, Step 2: Assuming the x64 architecture, what is the std::cout << sizeof(Widget) << '\n'; // prints 24 // Needs to be 8-byte aligned on x64 char padding1[7]; char padding2[7];Order of Member Data 38 Task, Step 3: Assuming the x64 architecture, what is the struct Widget? struct Widget { double d; // Largest first bool b1; bool b2; char padding[6]; }; std::cout << sizeof(Widget) << '\n'; // prints 16Order of Member Data 40 Task, Step 4:

// ... }; On this compiler, sizeof(string) is 32 and alignof(string) is 8. We are paying for padding and a pointer to the vtbl for the virtual member functions and the virtual base class 121We armor; // ... }; sizeof(Character) is 48. Adding sizeof(int) and sizeof(void*) adds 16 including padding, leading to 64. We get 72 due to the virtual base class overhead (the cost of this overhead depends // ... }; On this compiler, sizeof(string) is 32 and alignof(string) is 8. We are paying for padding and a pointer to the vtbl for the virtual member functions 135We want numbers… // ... int main()

#include "catch.hpp" TEST(left_pad_pads_strings_to_a_minimum_length, an_empty_string_with_zero_padding) { } #include "gtest/gtest.h" std::string left_pad(std::string const& str, size_t min_len, char ""); REQUIRE(left_pad("abc", 0) == "abc"); ASSERT_EQ(left_pad("abc", 0), "abc"); SECTION("When padding is < string length, the existing string is returned") { }60 TEST(left_pad_pads_strings_to_a_minimum_length ""); REQUIRE(left_pad("abc", 0) == "abc"); SECTION("When padding is < string length, the existing string is returned") { } SECTION("When padding is == string length, the existing string is returned") {

(sizeof(uint8_t) * 8) − 1; B large+size == sizeof(uint8_t) 14 uint8_t padding[sizeof(size_t) − sizeof(uint8_t)]; C Padding large+size+padding == sizeof(size_t) 15 char data[sizeof(normal) − sizeof(size_t)];

cmp w9, w8 b.ne .LBB0_1 ret Unconditional branch to .LBB1_2 No-op Tooling Padding: current solution, punting to developer /// Specifies that a type is safely writable to byte slices information is leaked into the /// byte slices. /// /// # Safety /// /// A type must not include padding bytes and must be fully initialised to safely implement /// [`WritableToBytes`] (i.e., it doesn't composition of /// writable types in a structure is not necessarily writable because it may result in padding /// bytes. pub unsafe trait WritableToBytes {} pub trait IoBufferWriter { /// Writes the contents

set by open()). */ unsigned int cache_readdir : 1; /** Padding. Reserved for future use*/ unsigned int padding : 25; unsigned int padding2 : 32; /** File handle id. May be filled in by filesystem

doing already. 32 Copyright © 2020 by Stephen C. Dewhurst and Daniel Saks Layout Guarantees and Padding  Regarding the layout of standard-layout classes, C++ guarantees only that:  The first non-static be quite as sure about any other data member.  Any class, even a standard‐layout class, may have padding bytes (also called “slack bytes”) after any non‐static data member. 33 Copyright © 2020 by Stephen &rhs); };  Unfortunately, we may now get some surprising results from our operator ==. 43 x y z padding Copyright © 2020 by Stephen C. Dewhurst and Daniel Saks 42 43Class Layout Copyright © 2020 by