[llvm-commits] [llvm] r56885 - in /llvm/trunk: include/llvm/Target/TargetLowering.h lib/CodeGen/SelectionDAG/SelectionDAG.cpp lib/Target/X86/X86ISelLowering.cpp lib/Target/X86/X86ISelLowering.h lib/Target/X86/X86Subtarget.cpp lib/Target/X86/X86Su

[Evan Cheng]

On Sep 30, 2008, at 2:52 PM, Bill Wendling wrote:

> On Tue, Sep 30, 2008 at 2:47 PM, Dan Gohman <gohman at apple.com> wrote:
>>
>> On Sep 30, 2008, at 2:22 PM, Bill Wendling wrote:
>>
>>> Author: void
>>> Date: Tue Sep 30 16:22:07 2008
>>> New Revision: 56885
>>>
>>> URL: http://llvm.org/viewvc/llvm-project?rev=56885&view=rev
>>> Log:
>>> Add the new `-no-builtin' flag. This flag is meant to mimic the GCC
>>> `-fno-builtin' flag. Currently, it's used to replace "memset" with
>>> "_bzero"
>>> instead of "__bzero" on Darwin10+. This arguably violates the
>>> meaning of this
>>> flag, but is currently sufficient. The meaning of this flag should
>>> become more
>>> specific over time.
>>
>> First, -fno-builtin is only intended to apply to user calls. It is  
>> not
>> intended to apply to libcalls that the compiler generates. For  
>> example,
>> specifying -fno-builtin-memcpy does not tell the compiler not to
>> generate calls to memcpy.
>>
>> Second, it looks like there is confusion about the leading  
>> underscores.
>> The previous code used "__bzero" on Darwin 10, which is actually
>> printed as "___bzero" because of Darwin ABI rules. By introducing new
>> code that uses "_bzero", it will be printed as "__bzero" on Darwin,
>> which doesn't seem to be what was intended here.
>>
> That's what was intended.
>
>> And furthermore, if "bzero" was intended, meaning printing in the
>> output as "_bzero" on Darwin, that's an even more flagrant abuse of
>> the -fno-builtin flag.  -fno-builtin doesn't mean that the compiler  
>> can
>> call functions with names in the user namespace not reserved by the C
>> standard.
>>
> I would have loved to do that, but people had problems with it.

I am confused by the meaning of -fno-builtin:
        -fno-builtin
        -fno-builtin-function
            Don't recognize built-in functions that do not begin with
            __builtin_ as prefix.

Does this actually disable the optimization of memset to bzero?

Evan


>
>
> -bw
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