better code

8/9/2019 Better Code

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Writing better code withWriting better code withhelp from the compilerhelp from the compiler

ThiagThiagQt Developer Days & LinuxCon urope ! "Qt Developer Days & LinuxCon urope ! "


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Who am I?


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Example scenario

Interview queson

You have 2 MB of data and you want to calculate how many bits are set, how wMemory usage is not a constraint (within reason).


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staticunsignedchardata[2*1024*1024];

intbitcount(){

intresult=0;

for(inti=0;i


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Approach 2: use a lookup table

staticunsignedchardata[2*1024*1024];e!ternconstushortbitcounttable[%&%];

intbitcount(){ intresult=0; for(inti=0;i


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My answer

# Use the POPCNT instrucon

$ %dded with the &rst 'ntel orei* genera+on, ehalem (--.2, but se/arate


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How o you use the !"!#$% instruction?

# Write assembly

# Use the GCC intrinsic: builtin+o+count()

# Use the Intel intrinsic: ,,+o+cntu&2()


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When can I use the instruction?

# Use uncondionally

# Chec! CPUI"

# #s! the lin!er $or hel%

# Chec! i$ surroundin& code already requires a CPU that su%%orts the anyway


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#hoosin& the solution

# What a'ects the choice:

$ 01s it will run on

$ om/ilers 5 toolchains it will be com/iled with

$ 6ibraries you7re using


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"ther architectures

# Intrinsics e(ist $or #)* and PowerPC too +Neon and #lvec,

# Not all com%iler $eatures wor! on those architectures yet

# -ut not discussed on this %resentaon


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(sing intrinsics(sing intrinsics


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'inin& out which intrinsic to use

# Use the ."*/ 0u!e


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Examples usin& intrinsics

# The %o%ulaon count # Calculan& C)C12

staticunsignedchardata[2*1024*1024];

intbitcount(){ intresult=0; for(inti=0;i


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Where are intrinsics allowe?

3or all com%ilers: recent enou&h +e4&4/ GCC 546 $or #782/ 549 $or #78;2

Compiler Permitted usage

Microsoft )isual *tudio+nywhere, no special build options re-uired

.ntel C// Compiler

Clang +nywhere, as long as code generation is enabled

0e1g1, $%a&'# $%a&'2# $%arch=core$a&'$i# etc1 acti3CC '14 or earlier

3CC '15

Code generation enabled6 or

functions decorated with attribute((target(

0etc12


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How I sol(e this for )t *+,

# *acro $or tesn& with 4 u"2 u1> u? goes !else

staticuintcrc"2(@@@){ +693:-7:A3(); return0;!endif


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7untime dispatching7untime dispatching


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-untime ispatchin& basics

;,"etect CPU

2,"etermine best im%lementaon1,)un it

With 3CC '148

0doesn9t wor: withClang, .CC or M*)C2

&oidfunctionsse2();&oidfunctionlain();

&oidfunction(){

if(builtincusuor functionsse2(); else functionlain();!

&oidfunctionsse2();&oidfunctionlain();&oidfunction()

{ if(*-6suorts553 functionsse2(); else functionlain();!


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Ientifyin& the #!.

# )unnin& CPUI" le= as an e(ercise to the reader

# >ust remember: cachethe result

e'ternintBtcufeatures;e'tern&oidBCetect-u8eatures(&oid);

staticinlineintB-u8eatures(){ intfeatures=Btcufeatures;

if(+69D3E(features==0)){ BCetect-u8eatures(); features=Btcufeatures; ! returnfeatures;!


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#heckin& surrounin& coe


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!uttin& it to&ether

# )esult on ?5@bit: uncondional call to the ..A2 version

&oidfunctionsse2();

&oidfunctionlain();&oidfunction()

{

if(B-u7as8eature(5532))

functionsse2();

else

functionlain();

!


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&oid*%e%cF(&oid*#const&oid*#sizet)

attribute((ifunc(resol&e%e%cF)));

decltFe(%e%cF)%e%cFa&'#%e%cFsse2;

autoresol&e%e%cF()

{

returnB-u7as8eature(:GH)I%e%cFa&'J%e%cFsse2;

!

&oid*%e%cF(&oid*#const&oid*#sizet)

attribute((ifunc(resol&e%e%cF)));

&oid*%e%cFa&'(&oid*#const&oid*#sizet);

&oid*%e%cFsse2(&oid*#const&oid*#sizet);static &oid*(*resol&e%e%cF(&oid))(&oid*#const&oid*#sizet)

{

returnB-u7as8eature(:GH)I%e%cFa&'J%e%cFsse2;

!

Askin& the linker an ynamic linker for help

# )equires:

$ :libc 2.88.8, Binu+ls 2.29.8, : . 5 ' 8.9

$ ot su//orted with lang or on %ndroid (due to Bionic)


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/## ,+0 autoispatcher a+k+a+ 3'unction Multi 4ersionin&56

# CBB only

attribute((target(ocnt)))

intbitcount()

{

intresult=0; for(inti=0;i


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'inin& better answers to inter(iew 7uestions

# Dow would you write a $uncon that returns a 12@bit random num

# Dow would you ero@e(tend a bloc! o$ data $rom H@ to ;?@bitEF

# Dow do you calculate the ne(t %ower o$ 2 $or a &iven non@ero inte

uint"2ne'toKer.f,Ko(uint"2&)

{

&$$;

&L=&MM1;

&L=&MM2;

&L=&MM4; &L=&MM?;

&L=&MM1>;

&;

return&;

!


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8etter answer

uint"2ne'toKer.f,Ko'?>(uint"2&)

{

intid'=bitscanre&erse(&);

return26


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9ummary

# 0earn $rom the ."*: use intrinsics

# Chec! the CPU at com%ile me/ run me and dis%atch

# Use library/ com%iler and lin!er tools


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eroextenin& from ; to 1


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=eft to the whims of the compiler ">6

-.. 4/$ 2d$c ,odu (3rsi3ra!1)3!,,1 2d51 add 60!13r10

2d5 ,oda 3!,,13!,,& 2d55 +un+c7hb8 3!,,03!,,1 2d5d +un+c7lb8 3!,,03!,,& 2da1 ,odu 3!,,10!10(3rdi3ra!2) 2da9 ,odu 3!,,&(3rdi3ra!2) 2dac add 60!103ra! 2db0 c,+ 3r53r10 2db& :b 2d$c

.lang &/4 210 ,o (3rsi 21 +un+c7lb8 3!,,0

215 ,o 0!$(3r 21f +un+c7lb8 3!,,0 21%& +and 3!,,0 21%9 +and 3!,,0 21%b ,odu 3!,,1 2190 ,odu 3!,,2 219% add 60!10 219a c,+ 3rc!3 219d :ne 210

.. 14 9d& ,o (3r$3rsi1)3!,,1 9d5 +un+c7lb8 3!,,03!,,1 9dd ,oda 3!,,1(3rdi3r$2) 9e& add 60!$3r$ 9e9 c,+ 3ra!3r$ 9ea :b 9d&


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=eft to the whims of the compiler "> ma(x26

-.. 4/5 2bb% ,odu (3rsi3rdi1)3,,0 2bbb add 60!13r11

2bbf +,oz!b8 3!,,03,,1 2bc4 e!tracti12$ 60!13,,03!,,0 2bca +,oz!b8 3!,,03,,0 2bcf ,oda 3,,1(3rb!3rdi2) 2bd4 ,oda 3,,00!20(3rb!3rdi2) 2bda add 60!203rdi 2bde c,+ 3r113ra! 2be1 :a 2bb%

.lang &/4 21a0 ,odu 0!2 21a ,odu 0!1

21aa ,odu (3rs 21ae +,oz!b8 3!,, 21b& +,oz!b8 3!,, 21b$ +,oz!b8 3!,, 21bd ,odu 3,, 21c2 ,odu 3,, 21c9 ,odu 3,, 21cb add 60!% 21cf add 60!&

21d& add 60!ffff 21d9 c,+ 3rc! 21da :ne 21a0

.. 14 9dc +,oz!b8 (3r$3rsi1)3,,0 9e2 ,odu 3,,0(3rdi3r$2) 9e$ add 60!103r$ 9ec c,+ 3ra!3r$ 9ef :b 9dc


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Helpin& out the compiler

# GCCs im%lementaon was the best with ..A2

$ ' /roduces be;er code for %


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#oe &enerate with the intrinsics

efore 2d$c ,odu (3rsi3ra!1)3!,,1 2d51 add 60!13r10

2d5 ,oda 3!,,13!,,& 2d55 +un+c7hb8 3!,,03!,,1 2d5d +un+c7lb8 3!,,03!,,& 2da1 ,odu 3!,,10!10(3rdi3ra!2) 2da9 ,odu 3!,,&(3rdi3ra!2) 2dac add 60!103ra! 2db0 c,+ 3r53r10 2db& :b 2d$c

>fter 2d90 ,odu (3rsi 2d9 add 60!10

2d95 add 3rc! 2d9c c,+ 3r$3 2d9f ,oda 3!,,0 2d$& +un+c7hb8 3!,,1 2d$9 +un+c7lb8 3!,,1 2d$b ,odu 3!,,0 2d51 ,odu 3!,,2 2d5% ,o 3ra! 2d55 :ne 2d90

-eJer or worseE


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Extenin& to A42 support

constchar*e=strsize;

Btrdiffoffset=0; KeOregoingtoreadstr[offset@@offset1P](1>bFtes) for(;stroffset1P){ const%12?ichunN=,,loadusi12$((%12?i*)(stroffset));load 1> bFtifdef:GH2 zeroe'tendtoanE//register

const%2P>ie'tended=,,2%cte+u$e+i1%(chunN); store "2 bFtes

,,2%storeusi2%((%2P>i*)(dstoffset)#e'tended);else const %12?i null/asN = %%set1ei"2(0);

unacN the first ? bFtes# adding Kith zeros

const %12?i first7alf = %%unacNloei?(chunN# null/asN); %%storeusi12?((%12?i*)(dst offset)# first7alf); store 1> bFtes

unacN the last ? bFtes# adding Kith zeros const %12?i second7alf = %%unacNhiei? (chunN# null/asN); %%storeusi12?((%12?i*)(dst offset ?)# second7alf); store ne't 1> bF

endif !


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Thiago Macieirathiago1macieira

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