Write better* C++ - Amazon Simple Storage Service · Write better* C++ with help from your friend...

Write better* C++with help from your friend Clang

René Milk 23rd October 2013living knowledgeWWU Münster

WESTFÄLISCHEWILHELMS-UNIVERSITÄTMÜNSTER

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WESTFÄLISCHEWILHELMS-UNIVERSITÄTMÜNSTER Write better* C++ 2 /31

Contents

What is Clang?

Static analysis

Dynamic analysis (sanitizers)

Tools

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LLVM1

LLVM (formerly Low Level Virtual Machine) is a compiler infrastructure written in C++; it isdesigned for compile-time, link-time, run-time, and "idle-time" optimization of programswritten in arbitrary programming languages.

Clang2

Clang is a compiler front end for the C, C++, Objective-C and Objective-C++ programminglanguages. It uses LLVM as its back end and has been part of its releases since LLVM 2.6.

1https://en.wikipedia.org/wiki/LLVM2https://en.wikipedia.org/wiki/Clang

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https://en.wikipedia.org/wiki/LLVM

https://en.wikipedia.org/wiki/Clang

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Clang features

I fast compiles with low memory usage 3

I detailed “fix-it” notes for warnings and errorsI build on top of libclang, API allows for extensive toolingI fully4 supports c++11 since version 3.3

3http://clang.llvm.org/features.html#performance4http://clang.llvm.org/cxx_status.html

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http://clang.llvm.org/features.html#performance

http://clang.llvm.org/cxx_status.html

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for this talk we’ll take “better” as referring to:

I code with fewer bugsI shorter compile timesI consistent formattingI not faster executionI not code quality in terms of style, complexity, etc.

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static code (program) analysis5

Static program analysis is the analysis of computer software that is performed withoutactually executing programs (analysis performed on executing programs is known asdynamic analysis).

5https://en.wikipedia.org/wiki/Static_program_analysis

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https://en.wikipedia.org/wiki/Static_program_analysis

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Why you should compile at high warning levels

Catch code segments that are valid C++, but (probably) not what you meant to write.I typosI unintentional operator usageI potential data over/underflowsI dead code

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error free build 6= error free code

/// \file warnings_00.cc

#include <cstddef>#include <string>

5int main(int argn, char** argv){

auto name = argv[0];int* i = 0;

10 unsigned long long bigstuff = 8 << 32;if ( name == "foo" )

char* name = "bar";else if ( name = NULL )

bigstuff = -1;15 for ( ; bigstuff < 0; i++);

bigstuff--;return bigstuff**i;

}

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how many warnings did that generate?

default “most"

6 13

Table : clang version 3.4-1 exp1

”most“ warnings settingsclang++ -g -std=c++11 -Wsign-conversion -O0 -g -Wall -Wextra -Wc++0x-compat-Winline -pedantic -Wredundant-decls -Wloop-analysis -Wstring-compare -Wshadow-Wunused-variable -Wundef -Wnon-virtual-dtor -Wdocumentation -Wshorten-64-to-32-Wused-but-marked-unused -Wdisabled-macro-expansion -Wparentheses-Wsometimes-uninitialized -Wconditional-uninitialized -Wfloat-equal -Wswitch-enum-Warray-bounds -Wcovered-switch-default -Wunreachable-code-Wnon-literal-null-conversion -Wtautological-compare

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sign-conversion / tautological-compare

/// \file warnings_01.cc#include <cstddef>

int main(int, char**)5 {

std::size_t bigstuff = -1;return bigstuff < 0;

}

warnings_01.cc:5:28: warning: implicit conversion changes signedness: ’int’ to’std::size_t’ (aka ’unsigned long’) [-Wsign-conversion]

std::size_t bigstuff = -1;~~~~~~~~ ^~

warnings_01.cc:6:21: warning: comparison of unsigned expression < 0 is alwaysfalse [-Wtautological-compare]

return bigstuff < 0;~~~~~~~~ ^ ~

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parentheses

/// \file warnings_02.ccint main(int argn, char**){

if (argn = 1)5 return -1;

}

warnings_02.cc:4:14: warning: using the result of an assignment asa condition without parentheses [-Wparentheses]

if (argn = 1)~~~~~^~~

warnings_02.cc:4:14: note: place parentheses around the assignment to silence this warningif (argn = 1)

^( )

warnings_02.cc:4:14: note: use ’==’ to turn this assignment into an equality comparisonif (argn = 1)

^==

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loop-analysis / empty-body


#include <iostream>int main(int argn, char** argv)

5 {for(;argn > 0;);

std::cout << argv[--argn] << std::endl;}

warnings_03.cc:6:10: warning: variable ’argn’ used in loop conditionnot modified in loop body [-Wloop-analysis]

for(;argn > 0;);^~~~

warnings_03.cc:6:20: warning: for loop has empty body [-Wempty-body]for(;argn > 0;);

^warnings_03.cc:6:20: note: put the semicolon on a separate line to silence this warning

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switch / return-type


enum Things { Dog, Frog };

5 int foo(Things thing) {switch (thing) {

case Dog: return 9001;}

}10

int main(int, char**) { return 0;}

warnings_04.cc:6:13: warning: enumeration value ’Frog’ not handled in switch [-Wswitch]switch (thing) {

^warnings_04.cc:9:1: warning: control may reach end of non-void function [-Wreturn-type]}^

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covered-switch-default


enum Things { Dog, Frog };

5 int bar(Things thing) {int ret;switch (thing) {

case Dog: case Frog: ret = 0;default: ret = -1;

10 }return ret;

}

int main(int, char**) { return 0;}

warnings_05.cc:9:9: warning: default label in switch which coversall enumeration values [-Wcovered-switch-default]

default: ret = -1;^

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array-bounds


const int ones[] = {1};

5 int main() {const auto bar = ones[1];return bar;

}

warnings_06.cc:4:22: warning: array index 1 is past the end of the array(which contains 1 element) [-Warray-bounds]

const auto bar = ones[1];^ ~

warnings_06.cc:1:1: note: array ’ones’ declared hereconst int ones[] = {1};^

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the analyzer plugin

#include <cstddef>#include <string>

inline int the_meaning_of_life_and_everything() { return 42; }5 int main(int, char**)

{const int* i = nullptr;auto j = the_meaning_of_life_and_everything();j = sizeof(int);

10 return j+*i;}

static_00.cc:8:10: warning: Value stored to ’j’ during its initialization is never readauto j = the_meaning_of_life_and_everything();

^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~static_00.cc:10:14: warning: Dereference of null pointer (loaded from variable ’i’)

return j+*i;^~

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available sanitizers

I Address Sanitizer - out-of-bounds checks, use-after-free,...I Thread Sanitizer - finds data racesI Memory Sanitizer - detects uninitialized readsI (soon?) Undefined Behavior Sanitizer - ???

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What is a sanitizer?

I inserts instrumentation code into the binaryI “replays” memory access in shadow memoryI runs algorithms to find patterns of unwanted behaviourI incurs runtime and memory penalty when activated

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memory sanitizer - unitialized read

int main(int argc, char**){

int* i = new int[666];if (i[0])

5 return -1;}

==14624== WARNING: MemorySanitizer: use-of-uninitialized-value#0 0x7fdb3ee1b9e6 in main instrumentation_01.cc:6#1 0x7fdb3d9a3de4 in __libc_start_main /build/buildd/eglibc-2.17/csu/libc-start.c:260#2 0x7fdb3ee1b89c in _start (01+0x6589c)

SUMMARY: MemorySanitizer: use-of-uninitialized-value instrumentation_01.cc:6 mainExiting

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address sanitizer - heap-use-after-free I

#include <memory>#include <iostream>using namespace std;struct YYZ {

5 int length () const { return (4-int(radio_)*60)+25; }bool radio_;YYZ(bool radio = false) : radio_(radio){}

};

10 struct Player {const YYZ& song_;Player(const YYZ& song) : song_(song) {}int length() const { return song_.length(); }

};15 int main(int, char**)

{unique_ptr<Player> player(nullptr);{

YYZ* yyz = new YYZ;20 player = unique_ptr<Player>(new Player(*yyz));

delete yyz;}auto le = player->length();std::cout << le;

25 return le;}

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heap-use-after-free II

===================================================================12704==ERROR: A6ddressSanitizer: heap-use-after-free on address 0x60200000eff0

at pc 0x473841 bp 0x7fff778bdcc0 sp 0x7fff778bdcb8READ of size 1 at 0x60200000eff0 thread T0

#0 0x473840 in YYZ::length() const instrumentation_02.cc:5#1 0x472f78 in main instrumentation_02.cc:23#2 0x7f73aac68de4 in __libc_start_main /build/buildd/eglibc-2.17/csu/libc-start.c:260#3 0x472dac in _start (02+0x472dac)

0x60200000eff0 is located 0 bytes inside of 1-byte region [0x60200000eff0,0x60200000eff1)freed by thread T0 here:

#0 0x45eb29 in operator delete(void*) (02+0x45eb29)#1 0x472f68 in main instrumentation_02.cc:21#2 0x7f73aac68de4 in __libc_start_main /build/buildd/eglibc-2.17/csu/libc-start.c:260

previously allocated by thread T0 here:#0 0x45e829 in operator new(unsigned long) (02+0x45e829)#1 0x472f12 in main instrumentation_02.cc:19#2 0x7f73aac68de4 in __libc_start_main /build/buildd/eglibc-2.17/csu/libc-start.c:260

SUMMARY: AddressSanitizer: heap-use-after-free instrumentation_02.cc:5 YYZ::length() const

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heap-use-after-free III

Shadow bytes around the buggy address:0x0c047fff9da0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa0x0c047fff9db0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa0x0c047fff9dc0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa0x0c047fff9dd0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa0x0c047fff9de0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa

=>0x0c047fff9df0: fa fa fa fa fa fa fa fa fa fa 00 fa fa fa[fd]fa0x0c047fff9e00: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa0x0c047fff9e10: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa0x0c047fff9e20: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa0x0c047fff9e30: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa0x0c047fff9e40: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa

Shadow byte legend (one shadow byte represents 8 application bytes):Addressable: 00Partially addressable: 01 02 03 04 05 06 07Heap left redzone: faHeap right redzone: fbFreed heap region: fdStack left redzone: f1Stack mid redzone: f2Stack right redzone: f3Stack partial redzone: f4Stack after return: f5Stack use after scope: f8Global redzone: f9Global init order: f6Poisoned by user: f7ASan internal: fe

==12704==ABORTING

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Tools built with (lib)Clang

I clang-formatI clang-modernizeI include-what-you-use (iwyu)I cldoc

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Example clang-format (before)

/// \file format.cc#include <cstddef>#include <string>int main(int argn,char** argv){auto name=argv[0];int* i=0;unsigned long

5 long bigstuff=8<<32;if(name=="foo")char* name="bar";else if(name=NULL)bigstuff=-1;for(;bigstuff<0;i++);bigstuff--;return bigstuff**i;}

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Example clang-format (after)

clang-format-3.4 -style=LLVM format.cc > format_after.cc/// \file format.cc#include <cstddef>#include <string>int main(int argn, char **argv) {

5 auto name = argv[0];int *i = 0;unsigned long long bigstuff = 8 << 32;if (name == "foo")

char *name = "bar";10 else if (name = NULL)

bigstuff = -1;for (; bigstuff < 0; i++)

;bigstuff--;

15 return bigstuff * *i;}

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Example clang-modernize (before)

/// \file modernize.cc#include <vector>

template <class T>5 void increment(std::vector<T>& vec) {

for (typename std::vector<T>::iterator it = vec.begin();it != vec.end(); ++it)

{++(*it);

10 }}

int main() {std::vector<int> a(2,4); increment(a);}

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Example clang-modernize (after)

clang-modernize -format-style=LLVM modernize.cc/// \file modernize.cc#include <vector>

template <class T>5 void increment(std::vector<T>& vec) {

for (auto &elem : vec) {++(elem);}

}10

int main() {std::vector<int> a(2,4); increment(a);}

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Example iwyu (input)

/// \file iwyu.cc

#include <vector>#include <assert.h>

5 #include <iterator>#include <memory>#include <sstream>#include <iostream>

10 int main(){

using namespace std;cout << abs(long(-1));

}

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Example iwyu (output)

iwyu -std=c++11 iwyu.cciwyu.cc should add these lines:#include <cstdlib> // for abs

iwyu.cc should remove these lines:5 - #include <assert.h> // lines 2-2

- #include <iterator> // lines 3-3- #include <memory> // lines 4-4- #include <sstream> // lines 5-5- #include <vector> // lines 1-1

10The full include-list for iwyu.cc:#include <cstdlib> // for abs#include <iostream> // for cout, ostream---

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Example cldoc

http://jessevdk.github.io/cldoc/example/

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Thank you foryour attention.

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