[llvm] r282433 - Allow StringRef to be constructed from a null pointer.

[Evgenii Stepanov via llvm-commits]

Stack trace:
#0  0xb74b3850 in _GLOBAL__sub_I_DiagnosticInfo.cpp () at
/code/llvm/include/llvm/Support/CommandLine.h:291
#1  0xb747acd7 in __dl__ZN6soinfo16CallConstructorsEv.part.10 () from
z/system/bin/linker
#2  0xb747df8f in
__dl__ZL29__linker_init_post_relocationR19KernelArgumentBlockj () from
z/system/bin/linker
#3  0xb74808c8 in __dl___linker_init () from z/system/bin/linker
#4  0xb7481704 in __dl__start () from z/system/bin/linker
#5  0x00000001 in ?? ()

Source:
291  void setDescription(StringRef S) { HelpStr = S; }

Asm:
=> 0xb74b3850 <+480>: movdqa %xmm2,0x40(%esp)

This is an unaligned load. Your change must have triggered a bug in
some other code.


On Wed, Sep 28, 2016 at 2:49 PM, Zachary Turner <zturner at google.com> wrote:
> Sorry, I take that back.  If this change had any effect on existing code,
> then llvm-symbolizer should have been asserting before my patch
>
> On Wed, Sep 28, 2016 at 2:47 PM Zachary Turner <zturner at google.com> wrote:
>>
>> That is quite surprising, I admit I haven't seen the stack trace, but I'm
>> inclined to say that something is wrong with the bisection method.  The only
>> thing this change could possibly do is allow an object to be constructed
>> where previously a construction would have failed to compile.  So if this
>> change had any effect on existing code, then llvm-symbolizer shouldn't have
>> even been compiling before.
>>
>> On Wed, Sep 28, 2016 at 2:36 PM Evgenii Stepanov
>> <eugeni.stepanov at gmail.com> wrote:
>>>
>>> This change broke llvm-symbolizer on android/x86 (and only x86), SEGV
>>> at startup. Trying to get a stack trace...
>>>
>>> On Mon, Sep 26, 2016 at 1:08 PM, Zachary Turner via llvm-commits
>>> <llvm-commits at lists.llvm.org> wrote:
>>> > Author: zturner
>>> > Date: Mon Sep 26 15:08:05 2016
>>> > New Revision: 282433
>>> >
>>> > URL: http://llvm.org/viewvc/llvm-project?rev=282433&view=rev
>>> > Log:
>>> > Allow StringRef to be constructed from a null pointer.
>>> >
>>> > Differential Revision: https://reviews.llvm.org/D24904
>>> >
>>> > Modified:
>>> >     llvm/trunk/include/llvm/ADT/StringRef.h
>>> >
>>> > Modified: llvm/trunk/include/llvm/ADT/StringRef.h
>>> > URL:
>>> > http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/ADT/StringRef.h?rev=282433&r1=282432&r2=282433&view=diff
>>> >
>>> > ==============================================================================
>>> > --- llvm/trunk/include/llvm/ADT/StringRef.h (original)
>>> > +++ llvm/trunk/include/llvm/ADT/StringRef.h Mon Sep 26 15:08:05 2016
>>> > @@ -73,14 +73,14 @@ namespace llvm {
>>> >      /// Construct an empty string ref.
>>> >      /*implicit*/ StringRef() : Data(nullptr), Length(0) {}
>>> >
>>> > +    /// Disable conversion from nullptr.  This prevents things like
>>> > +    /// if (S == nullptr)
>>> >      StringRef(std::nullptr_t) = delete;
>>> >
>>> >      /// Construct a string ref from a cstring.
>>> > +    LLVM_ATTRIBUTE_ALWAYS_INLINE
>>> >      /*implicit*/ StringRef(const char *Str)
>>> > -      : Data(Str) {
>>> > -        assert(Str && "StringRef cannot be built from a NULL
>>> > argument");
>>> > -        Length = ::strlen(Str); // invoking strlen(NULL) is undefined
>>> > behavior
>>> > -      }
>>> > +        : Data(Str), Length(Str ? ::strlen(Str) : 0) {}
>>> >
>>> >      /// Construct a string ref from a pointer and length.
>>> >      LLVM_ATTRIBUTE_ALWAYS_INLINE
>>> >
>>> >
>>> > _______________________________________________
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>>> > llvm-commits at lists.llvm.org
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