On Wed, Jun 30, 2021 at 11:00 AM Richard Sandiford
<richard.sandif...@arm.com> wrote:
>
> Richard Biener via Gcc-patches <gcc-patches@gcc.gnu.org> writes:
> > Note there's also array_slice<> which could be used to pass non-const
> > vec<>s that are never resized but modified - the only "valid" case of
> > passing a non-const vec<> by value.
>
> Yeah. We'd need a new constructor for that (the current one only
> takes const vec<>&) but I agree it would be a good thing to do.
>
> I realise you weren't saying otherwise, but: array_slice<> can also be
> used for const vec<>s. E.g. array_slice<const int> can't be resized
> or modified.
>
> I think array_slice<> is going to be more efficient as well. E.g.:
>
> void
> f1 (vec<char> &foo)
> {
> for (unsigned int i = 0; i < foo.length (); ++i)
> foo[i] += 1;
> }
>
> void
> f2 (array_slice<char> foo)
> {
> for (unsigned int i = 0; i < foo.size (); ++i)
> foo[i] += 1;
> }
>
> gives:
>
> 000000000000d150 <f1(vec<char, va_heap, vl_ptr>&)>:
> d150: 48 8b 07 mov (%rdi),%rax
> d153: 31 d2 xor %edx,%edx
> d155: 48 85 c0 test %rax,%rax
> d158: 74 26 je d180 <f1(vec<char, va_heap,
> vl_ptr>&)+0x30>
> d15a: 66 0f 1f 44 00 00 nopw 0x0(%rax,%rax,1)
> d160: 3b 50 04 cmp 0x4(%rax),%edx
> d163: 73 12 jae d177 <f1(vec<char, va_heap,
> vl_ptr>&)+0x27>
> d165: 89 d1 mov %edx,%ecx
> d167: 83 c2 01 add $0x1,%edx
> d16a: 80 44 08 08 01 addb $0x1,0x8(%rax,%rcx,1)
> d16f: 48 8b 07 mov (%rdi),%rax
> d172: 48 85 c0 test %rax,%rax
> d175: 75 e9 jne d160 <f1(vec<char, va_heap,
> vl_ptr>&)+0x10>
> d177: c3 retq
> d178: 0f 1f 84 00 00 00 00 nopl 0x0(%rax,%rax,1)
> d17f: 00
> d180: c3 retq
>
> 000000000000d190 <f2(array_slice<char>)>:
> d190: 85 f6 test %esi,%esi
> d192: 74 18 je d1ac
> <f2(array_slice<char>)+0x1c>
> d194: 8d 46 ff lea -0x1(%rsi),%eax
> d197: 48 8d 44 07 01 lea 0x1(%rdi,%rax,1),%rax
> d19c: 0f 1f 40 00 nopl 0x0(%rax)
> d1a0: 80 07 01 addb $0x1,(%rdi)
> d1a3: 48 83 c7 01 add $0x1,%rdi
> d1a7: 48 39 c7 cmp %rax,%rdi
> d1aa: 75 f4 jne d1a0
> <f2(array_slice<char>)+0x10>
> d1ac: c3 retq
>
> where f1 has to reload the length and base each iteration,
> but f2 doesn't.
Of course but that's unfair - by refrence vec<> vs. by value array_slice<>
plus char * pointer stores which destroy all TBAA ... ;)
>
> > But as noted array_slice<> lacks most of the vec<> API so I'm not sure
> > how awkward that option would be. We of course can amend its API as
> > well.
>
> Yeah, that'd be good. The current class follows the principle
> “don't add stuff that isn't needed yet”. :-)
Yes, and that's good of course.
Richard.
> Thanks,
> Richard
