[llvm-commits] [llvm] r80710 - in /llvm/trunk/lib: CodeGen/AsmPrinter/DwarfException.cpp Target/ARM/AsmPrinter/ARMAsmPrinter.cpp

[Daniel Dunbar]

On Wed, Sep 2, 2009 at 8:17 PM, Chris Lattner<clattner at apple.com> wrote:
> On Sep 2, 2009, at 8:10 PM, Daniel Dunbar wrote:
>> Hi Jim,
>>
>> Actually 256 is a better choice. Unless we actively care about the
>> stack size in some case, 256 is a nice "idiomatic" choice, because of
>> the interaction with raw_svector_ostream.
>>
>> The particular issue is that raw_ostream expects to have a certain
>> size buffer (64 bytes, currently), and so the vector will get
>> immediately resized (malloc'd) if it is smaller than that. I'm open to
>> suggestions about how to improve this API though (some obvious options
>> are make raw_ostream's required buffer size smaller, or make
>> raw_svector_ostream give a compile time assertion when passed a vector
>> that is provably too small).
>>
>> Given the current implementation, 128 is the smallest "safe" choice
>> which will almost always avoid allocation, but since all uses already
>> use 256 I was hoping we could make the API "reasonable" by idiom.
>
> What am I missing here Daniel?  The string being constructed is
> sizeof(PrivateGlobalPrefix) which is a max of 2 + sizeof("_LSDA_")  +
> utostr(Asm->getFunctionNumber()).size()

Yes.

However, since the raw_svector_ostream coordinates with the
raw_ostream to use its own buffer as the stream buffer, and since
raw_ostream requires a buffer of at least 64 bytes, it gets resized.

I'm happy to work on fixing this if someone has a good plan.
raw_ostream doesn't really seem to need its 64 byte buffer, for
example. When I first implemented raw_svector_ostream it happened that
every user was a SmallString<256>...

 - Daniel

> This isn't very big.
>
> -Chris
>
>>
>> - Daniel
>>
>> On Tue, Sep 1, 2009 at 11:55 AM, Jim Grosbach<grosbach at apple.com> wrote:
>>>
>>> Author: grosbach
>>> Date: Tue Sep  1 13:55:08 2009
>>> New Revision: 80710
>>>
>>> URL: http://llvm.org/viewvc/llvm-project?rev=80710&view=rev
>>> Log:
>>> reduce size of SmallString to something more reasonable
>>>
>>> Modified:
>>>   llvm/trunk/lib/CodeGen/AsmPrinter/DwarfException.cpp
>>>   llvm/trunk/lib/Target/ARM/AsmPrinter/ARMAsmPrinter.cpp
>>>
>>> Modified: llvm/trunk/lib/CodeGen/AsmPrinter/DwarfException.cpp
>>> URL:
>>> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/AsmPrinter/DwarfException.cpp?rev=80710&r1=80709&r2=80710&view=diff
>>>
>>>
>>> ==============================================================================
>>> --- llvm/trunk/lib/CodeGen/AsmPrinter/DwarfException.cpp (original)
>>> +++ llvm/trunk/lib/CodeGen/AsmPrinter/DwarfException.cpp Tue Sep  1
>>> 13:55:08 2009
>>> @@ -601,7 +601,7 @@
>>>
>>>  EmitLabel("exception", SubprogramCount);
>>>  if (MAI->getExceptionHandlingType() == ExceptionHandling::SjLj) {
>>> -    SmallString<256> LSDAName;
>>> +    SmallString<16> LSDAName;
>>>    raw_svector_ostream(LSDAName) << MAI->getPrivateGlobalPrefix() <<
>>>      "_LSDA_" << Asm->getFunctionNumber();
>>>    O << LSDAName.str() << ":\n";
>>>
>>> Modified: llvm/trunk/lib/Target/ARM/AsmPrinter/ARMAsmPrinter.cpp
>>> URL:
>>> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/ARM/AsmPrinter/ARMAsmPrinter.cpp?rev=80710&r1=80709&r2=80710&view=diff
>>>
>>>
>>> ==============================================================================
>>> --- llvm/trunk/lib/Target/ARM/AsmPrinter/ARMAsmPrinter.cpp (original)
>>> +++ llvm/trunk/lib/Target/ARM/AsmPrinter/ARMAsmPrinter.cpp Tue Sep  1
>>> 13:55:08 2009
>>> @@ -160,7 +160,7 @@
>>>      std::string Name;
>>>
>>>      if (ACPV->isLSDA()) {
>>> -        SmallString<256> LSDAName;
>>> +        SmallString<16> LSDAName;
>>>        raw_svector_ostream(LSDAName) << MAI->getPrivateGlobalPrefix() <<
>>>          "_LSDA_" << getFunctionNumber();
>>>        Name = LSDAName.str();
>>>
>>>
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>>
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>
>