-malign-double

[Jed Brown]

Barry Smith wrote:
> 
>    Agreed this would be a good option to have. The question is how to do
> it without having a morass of nasty nested if-defs.  Note that portably
> getting alignment out of malloc()  alone is already ugly and not as
> simple a code as I would like.
> 
>     To simplify things we could always require 16 byte alignment
> everywhere? But is that a desirable?

I doubt it's harmful, I don't think I understand where the nested ifdefs
come in.  The required alignment only needs to be stated in one place, I
have something like the following in one of my projects.

  /* current */
#define PetscMalloc3(m1,t1,r1,m2,t2,r2,m3,t3,r3)                        \
    (PetscMalloc((m1)*sizeof(t1)+(m2)*sizeof(t2)+(m3)*sizeof(t3),r1)    \
     || (*(r2) = (t2*)(*(r1)+m1),                                       \
         *(r3) = (t3*)(*(r2)+m2),0))

  /* aligned */
#define PetscMalloc3(m1,t1,r1,m2,t2,r2,m3,t3,r3)                        \
  (PetscMalloc((m1)*sizeof(t1)+(m2)*sizeof(t2)+(m3)*sizeof(t3)+2*(PETSC_MEMALIGN-1),r1) \
   || (*(r2) = (t2*)PETSC_ALIGN(*(r1)+m1),                              \
       *(r3) = (t3*)PETSC_ALIGN(*(r2)+m2),0))

#define PETSC_ALIGN(p) PetscNextAligned((uintptr_t)(p),PETSC_MEMALIGN-1)

static inline void *PetscNextAligned(uintptr_t base,uintptr_t mask)
{return (void*)((base + mask) & ~mask);}


Note that this is a no-op if PETSC_MEMALIGN=1 and thus compiles to
exactly what we have now.

Jed

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