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.
> 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”. :-)
Thanks,
Richard
