[cfe-commits] [PATCH] PR12094: set alignment of memory builtins

[Eli Friedman]

On Fri, Mar 2, 2012 at 1:28 PM, Eli Friedman <eli.friedman at gmail.com> wrote:
> On Fri, Mar 2, 2012 at 1:13 PM, Chris Lattner <clattner at apple.com> wrote:
>>
>> On Mar 2, 2012, at 12:09 PM, Eli Friedman wrote:
>>
>>> On Fri, Mar 2, 2012 at 3:57 AM, Jay Foad <jay.foad at gmail.com> wrote:
>>>> http://llvm.org/bugs/show_bug.cgi?id=12094
>>>>
>>>> The attached patch:
>>>>
>>>> - Makes CGBuiltin.cpp's helper function GetPointeeAlignment into a
>>>> proper member of CodeGenFunction and splits it into two:
>>>> -- GetPointeeAlignment, which returns alignment as an unsigned, and
>>>> -- GetPointeeAlignmentValue, which wraps the result in an llvm::Value
>>>> for the convenience of generating ARM NEON intrinsics.
>>>>
>>>> - Uses GetPointeeAlignment to set a sensible alignment on all memory
>>>> intrinsics (memcpy, memset etc.) instead of the conservative default
>>>> of 1.
>>>
>>> I don't think this is really safe; strictly speaking, yes, an int* is
>>> required to be appropriately aligned, but in practice neither clang
>>> nor gcc has ever tried to enforce that, so making that assumption is
>>> going to cause trouble.
>>
>> Hi Eli,
>>
>> Is this a vague concern or a specific one?  Even GCC-4.2 has been doing this optimization.  I consider this "fixing a regression compared to GCC", not an innovation in optimization.
>
> I can't point to a specific example off the top of my head, but I do
> know that we've suggested switching code that uses unaligned loads to
> use memcpy without specifically noting that the type of the pointer
> matters.

s/unalgned/misaligned/

-Eli