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

[Craig Topper via llvm-dev]

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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>
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