[llvm-dev] SSE instructions and alignment of the return value of 'new'

[Kevin Choi via llvm-dev]

I think as you alluded to, movaps xmm, m128 requires m128 to be 16 byte
aligned to load 4 single precision fp into xmm.
Glibc had a bug open for not supporting variable alignment on malloc/new as
standard mandates it, but they decided not to fix according to this bug
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=15795
The issue is 'resolved wontfix' but I'm not exactly sure what they decided
to do -- it seems they tried to put in workaround in gcc as glibc won't
budge. If workaround is in gcc, clang for sure would also need that.

-Kevin

On Mon, Oct 2, 2017 at 6:44 PM, Craig Topper via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Does the crash happen if you compile with -fnew-alignment=8? That's
> supposed to change what clang assumes the alignment of memory allocated
> with new will be.
>
> ~Craig
>
> On Mon, Oct 2, 2017 at 3:11 PM, Riyaz Puthiyapurayil via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> I have some programs crashing when I upgraded from clang 3.9.1 to clang
>> 4.0.1.
>>
>>
>>
>> Debugging this I found the reason for the crash. This is happening in the
>> following assembly fragment for a piece of code allocating a class object
>> (size: 24 bytes) using operator new and then initializing it:
>>
>>
>>
>>    0x00002aaaafc145f3 <+35>:    callq  0x2aaaafdf5f90 <operator
>> new(unsigned long)>
>>
>>    0x00002aaaafc145f8 <+40>:    mov    %rax,%r13
>>
>>    0x00002aaaafc145fb <+43>:    xorps  %xmm0,%xmm0
>>
>> => 0x00002aaaafc145fe <+46>:    movaps %xmm0,0x0(%r13)
>>
>>
>>
>> The value in %r13 (from the return value of operator new) is not
>> appropriately aligned causing the crash. The memory allocation is done by a
>> custom memory allocator that is returning 8-byte aligned blocks. The memory
>> allocator has not changed between the two versions of the program (the one
>> using clang 3.9.1 versus the one using clang 4.0.1). The version of
>> libstdc++ is also the same. The command line options to clang are unchanged
>> (-msse2 is specified in both cases). But I found that clang 3.9.1 is not
>> generating SSE instructions but clang 4.0.1 is generating them in the above
>> case.
>>
>>
>>
>> The fix in our code is to make an API call to configure the custom
>> allocator to always return appropriately aligned memory. But I would like
>> to know if there is a known change in LLVM or clang to assume that malloc
>> will return > 8 byte aligned memory based on the allocation size  or if
>> this has always been the case. I want to know if my program compiled with
>> 3.9.1 also has a problem that was just not exposed in testing.
>>
>>
>>
>> Thanks in advance.
>>
>>
>>
>> /Riyaz
>>
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>> llvm-dev at lists.llvm.org
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>>
>>
>
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