| Message ID | 58CA9BAB0200007800143CE4@prv-mh.provo.novell.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On 16/03/17 13:05, Jan Beulich wrote: > Stop creating an excessively large array on the stack, by properly > taking into account the array element size when establishing its > element count (and of course also when calculating the pointer to > be actually used to access the memory). What in practice does this do? It looks like it reduces the size of the array by 4 or 8 times? ~Andrew > > Signed-off-by: Jan Beulich <jbeulich@suse.com> > > --- a/xen/arch/x86/x86_emulate/x86_emulate.c > +++ b/xen/arch/x86/x86_emulate/x86_emulate.c > @@ -553,10 +553,10 @@ typedef union { > * the compiler for automatic variables. Use this helper to instantiate a > * suitably aligned variable, producing a pointer to access it. > */ > -#define DECLARE_ALIGNED(type, var) \ > - long __##var[sizeof(type) + __alignof(type) - __alignof(long)]; \ > - type *const var##p = \ > - (void *)((long)(__##var + __alignof(type) - __alignof(long)) \ > +#define DECLARE_ALIGNED(type, var) \ > + long __##var[(sizeof(type) + __alignof(type)) / __alignof(long) - 1]; \ > + type *const var##p = \ > + (void *)(((long)__##var + __alignof(type) - __alignof(__##var)) \ > & -__alignof(type)) > > #ifdef __GCC_ASM_FLAG_OUTPUTS__ > > >
>>> On 16.03.17 at 14:09, <andrew.cooper3@citrix.com> wrote: > On 16/03/17 13:05, Jan Beulich wrote: >> Stop creating an excessively large array on the stack, by properly >> taking into account the array element size when establishing its >> element count (and of course also when calculating the pointer to >> be actually used to access the memory). > > What in practice does this do? It looks like it reduces the size of the > array by 4 or 8 times? Correct - it was this much oversized, bloating the stack frame of the function quite a bit. Jan
On 16/03/17 13:31, Jan Beulich wrote: >>>> On 16.03.17 at 14:09, <andrew.cooper3@citrix.com> wrote: >> On 16/03/17 13:05, Jan Beulich wrote: >>> Stop creating an excessively large array on the stack, by properly >>> taking into account the array element size when establishing its >>> element count (and of course also when calculating the pointer to >>> be actually used to access the memory). >> What in practice does this do? It looks like it reduces the size of the >> array by 4 or 8 times? > Correct - it was this much oversized, bloating the stack frame of the > function quite a bit. Ok, although it would be helpful if this was called out more clearly in the commit message. Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
--- a/xen/arch/x86/x86_emulate/x86_emulate.c +++ b/xen/arch/x86/x86_emulate/x86_emulate.c @@ -553,10 +553,10 @@ typedef union { * the compiler for automatic variables. Use this helper to instantiate a * suitably aligned variable, producing a pointer to access it. */ -#define DECLARE_ALIGNED(type, var) \ - long __##var[sizeof(type) + __alignof(type) - __alignof(long)]; \ - type *const var##p = \ - (void *)((long)(__##var + __alignof(type) - __alignof(long)) \ +#define DECLARE_ALIGNED(type, var) \ + long __##var[(sizeof(type) + __alignof(type)) / __alignof(long) - 1]; \ + type *const var##p = \ + (void *)(((long)__##var + __alignof(type) - __alignof(__##var)) \ & -__alignof(type)) #ifdef __GCC_ASM_FLAG_OUTPUTS__