Why does GCC 6 assume data is 16-byte aligned?
c++, gcc, gcc6, glibc, memory-alignment
Solution
It's possible that the compiler has a reason to think the object has an alignment ≥ 16 bytes. It's possible to find out what the compiler thinks the alignment is by using the `alignof()` operator in C++11. GCC has an extension `__alignof__` that is available in C and earlier C++ versions.
A structure's alignment is the highest alignment of anything in it, recursively. There could be something in there with higher alignment than expected.
While the C++11 standard guarantees that memory returned by `new` is aligned to the value needed by the "fundamental alignment requirement" of any object, this only applies to standard types and objects made of them. Using C++11 `alignas()` or the `__attribute__((aligned(x)))` GCC extension to request higher alignment might exceed what `new` provides.
A solution to this would be to use `std::aligned_alloc()` (C++11 or later) or `posix_memalign()` (POSIX-only but < C++11) to get aligned memory. This could be coupled with the placement form of the `new` operator to construct the object in that memory or class specific operator overloads of `new` and `delete`.
Problem
(Sorry in advance for not having managed to reduce my problem to a simple failing test case...) I have faced issues with upgrading to GCC 6.3.0 to build our codebase (relevant flags: `-O3 -m32`). Specifically, my application segfaults within a struct ctor call because of GCC optimizations. In this ctor, GCC used `movaps` : ``` movaps %xmm0,0x30a0(%ebx) ``` `movaps` requires the operand to be 16-byte aligned. But at this point in time, `%ebx` points to my object, which is not necessarily 16-byte aligned. From glibc : “The address of a block returned by malloc or realloc in GNU systems is always a multiple of eight (or sixteen on 64-bit systems).“ Hence the segfault (when built with `-O3 -m32`). Why does it seem like GCC assumed the allocated object would be 16-byte aligned ? Am I misunderstanding something ? Notes: - No alignment hints or attributes on this struct - Object has been initialized via default `new` operator - Depends on the level of optimization: - PASS: `-m32 -O2` - FAIL: `-m32 -O2 -ftree-slp-vectorize` - PASS: `-m32 -O3 -fno-tree-slp-vectorize` - FAIL: `-m32 -O3` This other project, seem to have hit similar issues : https://github.com/godotengine/godot/issues/4623 Their investigation points to `-fvect-cost-model=dynamic`. Investigation on my codebase rather points to `-ftree-slp-vectorize`.