vla.hh

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#ifndef VLA_HH
#define VLA_HH
 
#include <span>
 
// VLAs (Variable Length Array's) are part of C99, but not of C++98.
// Though gcc does support VLAs as an extension. For VC++ we have to emulate
// them via alloca.
 
#ifndef _MSC_VER
 
#define VLA(TYPE, NAME, LENGTH) \
        TYPE NAME##_storage[(LENGTH)]; \
        std::span NAME{NAME##_storage, size_t(LENGTH)}
 
#if defined __i386 || defined __x86_64
#define VLA_ALIGNED(TYPE, NAME, LENGTH, ALIGNMENT) \
        TYPE NAME##_storage[(LENGTH)] __attribute__((__aligned__((ALIGNMENT)))); \
        std::span NAME{NAME##_storage, size_t(LENGTH)}
#else
// Except on x86/x86-64, GCC can align VLAs within a stack frame, but it makes
// no guarantees if the requested alignment is larger than the alignment of the
// stack frame itself.
//   http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16660
#define VLA_ALIGNED(TYPE, NAME, LENGTH, ALIGNMENT) \
        UNABLE_TO_GUARANTEE_VLA_ALIGNMENT_ON_THIS_ARCHITECTURE
#endif
 
#else
 
#define VLA(TYPE, NAME, LENGTH) \
        auto NAME##_storage = static_cast<TYPE*>(_alloca(sizeof(TYPE) * (LENGTH))); \
        std::span NAME{NAME##_storage, size_t(LENGTH)}
 
// mfeingol: evil hack alert
#define VLA_ALIGNED(TYPE, NAME, LENGTH, ALIGNMENT) \
        size_t cbAlign##NAME = (ALIGNMENT); \
        void* palloc##NAME = _alloca(sizeof(TYPE) * (LENGTH) + cbAlign##NAME); \
        palloc##NAME = (void*)((size_t(palloc##NAME) + cbAlign##NAME - 1UL) & ~(cbAlign##NAME - 1UL)); \
        auto NAME##_storage = static_cast<TYPE*>(palloc##NAME); \
        std::span NAME{NAME##_storage, size_t(LENGTH)}
 
#endif
 
// Macro to align a buffer that might be used by SSE instructions.
// On platforms without SSE no (extra) alignment is performed.
#ifdef __SSE2__
#define VLA_SSE_ALIGNED(TYPE, NAME, LENGTH) VLA_ALIGNED(TYPE, NAME, LENGTH, 16)
#else
#define VLA_SSE_ALIGNED(TYPE, NAME, LENGTH) VLA(TYPE, NAME, LENGTH)
#endif
 
#endif // VLA_HH