c# - Initialization of memory allocated with stackalloc - Stack ...
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From the spec: 18.8 Stack allocation. The content of the newly allocated memory is undefined.
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Initializationofmemoryallocatedwithstackalloc
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IfI'mallocatingmemorywithstackallocinC#,isthatmemoryinitialized(with0)?
Thedocumentationdoesn'tspeakofthatandonlytellsthatthecorrectamountisreserved.
Inmytestssuchmemorydefaultedto0,butthatdoesn'tmeanit'sguaranteedthough.
c#unsafestackalloc
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askedDec30,2011at11:37
frumbaelafrumbaela
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Yes,thespecsaysitisundefined,butthecompileremitslocallocCILintructionforstackalloc.AndthisiswhatECMASpecssaysaboutlocalloc:
Thelocallocinstructionallocatessize(typenativeunsignedint)
bytesfromthelocaldynamicmemorypoolandreturnstheaddress(a
managedpointer,type&)ofthefirstallocatedbyte.Theblockof
memoryreturnedisinitializedto0onlyiftheinitializeflagonthe
methodistrue(seePartitionI).Theareaofmemoryisnewly
allocated.Whenthecurrentmethodreturnsthelocalmemorypoolis
availableforreuse.
Theinitializeflag,alsoknownaslocalsinitflag,isemittedforeverymethodbythecompilerbecauseitisrequiredforverifiablecode.
Pleaselookatthisissueoncoreclraskingtostopzeroingmemoryonstackalloc.Attheendoftheissuejkotassays:
Thecurrentplanis:
C#willkeepzeroinitializingbydefault.Changingthedefaultwould
betoobreaking.Wehaveasetofissuesopenedtomakethezero
initializationdonebytheJITmoreefficientorreduceneedforit
(#13827,#13823,#13825)Folkswhoreallywanttogetthelastbitof
performancebyavoidingzeroinitializationcanusecustomILLinker
step(mono/linker#159)whentheyknowwhattheyaredoing.Wedothis
forCoreLibtoday(viaVMhack,butweshouldswitchtotheILLinker),
andweplantoexperimentwiththisinCoreFX(dotnet/corefx#25956).
Basedontheresultsoftheseexperiments,wemayconsiderintroducing
amorestreamlinedwaytodothisinfuture.@ahsonkhanYoushould
considerexperimentingwithitinCoreFXLabaswellifyoubelieve
thatitwouldhelp.
Andseethiscsharplangproposal
Sotheconclusionis:thememoryisinitializedtozeroinpractice
Howeverthereisabug/featureinthecompilerthatpreventsfromemitinglocalsinitflag.Unsafemethodsthatdon'tdeclareothervariablesandusethestackallocatedvariableonlytopassingittoothermethod,don'tgetmarkedwiththelocalsinitflag.
Hereisanexampleofsuchbug/feature:
usingSystem;
usingSystem.Diagnostics;
usingSystem.Runtime.CompilerServices;
namespaceInformalTests
{
classProgram
{
constintn=100_000_000;
staticunsafevoidMain(string[]args)
{
varwatch=Stopwatch.StartNew();
for(inti=0;i
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