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

[Riyaz Puthiyapurayil via llvm-dev]

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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