[PATCH] Don't localize globals that may serve as leak checker roots.

[Alexey Samsonov]

On Thu, Sep 26, 2013 at 1:02 PM, Duncan Sands <duncan.sands at gmail.com>wrote:

> Hi Alexey,
>
>
> On 25/09/13 13:32, Alexey Samsonov wrote:
>
>> Hi nicholas,
>>
>> This change prevents GlobalOpt from turning globals into
>> stack-allocated locals, if these variables may serve as a leak checker
>> root. For instance, transforming
>>    Foo *x;
>>    int main() { x = new Foo(); }
>> into
>>    int main() { Foo *x = new Foo(); }
>> will make LeakSanitizer treat "x" as definitely leaked, which is
>> undesirable.
>>
>
> having magic semantics for globals with a certain type seems like a bad
> idea
> to me.  Here the global in effect has a secret external use, but that isn't
> clear from the IR.  In my opinion it's only asking for trouble and it
> would be
> better to make it explicit by requiring all GC roots to be used by (eg) a
> new
> llvm.gc_root global, analogous to llvm.used.
>

Just to clarify this - who should create llvm.gc_root array - do you
suggest to add a new __attribute__
the user should specify, or to emit llvm.gc_root in frontend, or in the
backend before running
optimization passes?


>
> Ciao, Duncan.
>
>
>> http://llvm-reviews.chandlerc.**com/D1754<http://llvm-reviews.chandlerc.com/D1754>
>>
>> Files:
>>    test/Transforms/GlobalOpt/**metadata.ll
>>    test/Transforms/GlobalOpt/**dont-localize-pointer.ll
>>    lib/Transforms/IPO/GlobalOpt.**cpp
>>
>> Index: test/Transforms/GlobalOpt/**metadata.ll
>> ==============================**==============================**=======
>> --- test/Transforms/GlobalOpt/**metadata.ll
>> +++ test/Transforms/GlobalOpt/**metadata.ll
>> @@ -3,24 +3,24 @@
>>   ; PR6112 - When globalopt does RAUW(@G, %G), the metadata reference
>> should drop
>>   ; to null.  Function local metadata that references @G from a different
>> function
>>   ; to that containing %G should likewise drop to null.
>> - at G = internal global i8** null
>> + at G = internal global i32 0
>>
>>   define i32 @main(i32 %argc, i8** %argv) {
>>   ; CHECK-LABEL: @main(
>>   ; CHECK: %G = alloca
>> -  store i8** %argv, i8*** @G
>> +  store i32 %argc, i32* @G
>>     ret i32 0
>>   }
>>
>>   define void @foo(i32 %x) {
>> -  call void @llvm.foo(metadata !{i8*** @G, i32 %x})
>> +  call void @llvm.foo(metadata !{i32* @G, i32 %x})
>>   ; CHECK: call void @llvm.foo(metadata !{null, i32 %x})
>>     ret void
>>   }
>>
>>   declare void @llvm.foo(metadata) nounwind readnone
>>
>>   !named = !{!0}
>>
>> -!0 = metadata !{i8*** @G}
>> +!0 = metadata !{i32* @G}
>>   ; CHECK: !0 = metadata !{null}
>> Index: test/Transforms/GlobalOpt/**dont-localize-pointer.ll
>> ==============================**==============================**=======
>> --- /dev/null
>> +++ test/Transforms/GlobalOpt/**dont-localize-pointer.ll
>> @@ -0,0 +1,10 @@
>> +; RUN: opt -S -globalopt < %s | FileCheck %s
>> +
>> + at G = internal global i8** null
>> +
>> +define i32 @main(i32 %argc, i8** %argv) {
>> +; CHECK-LABEL: @main(
>> +; CHECK-NOT: alloca
>> +  store i8** %argv, i8*** @G
>> +  ret i32 0
>> +}
>> Index: lib/Transforms/IPO/GlobalOpt.**cpp
>> ==============================**==============================**=======
>> --- lib/Transforms/IPO/GlobalOpt.**cpp
>> +++ lib/Transforms/IPO/GlobalOpt.**cpp
>> @@ -1951,7 +1951,8 @@
>>         GV->getType()->getElementType(**)->isSingleValueType() &&
>>         GS.AccessingFunction->getName(**) == "main" &&
>>         GS.AccessingFunction->**hasExternalLinkage() &&
>> -      GV->getType()->**getAddressSpace() == 0) {
>> +      GV->getType()->**getAddressSpace() == 0 &&
>> +      !isLeakCheckerRoot(GV)) {
>>       DEBUG(dbgs() << "LOCALIZING GLOBAL: " << *GV);
>>       Instruction &FirstI = const_cast<Instruction&>(*GS.**
>> AccessingFunction
>>
>>  ->getEntryBlock().begin());
>>
>>
>>
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>>
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-- 
Alexey Samsonov, MSK
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