Jakub Jelinek <ja...@redhat.com> writes:
> On Sun, Oct 15, 2023 at 12:43:10PM +0100, Richard Sandiford wrote:
>> It seemed like there was considerable support for bumping the minimum
>> to beyond 4.8. I think we should wait until a decision has been made
>> before adding more 4.8 workarounds.
>
> I think adding a workaround until that decision is made and perhaps
> removing it afterwards will make life easier for people still using gcc 4.8.
>
>> Having a conditional explicit constructor is dangerous because it changes
>> semantics. E.g. consider:
>>
>> #include <new>
>>
>> union u { int x; };
>> void f(u *ptr) { new(ptr) u; }
>> void g(u *ptr) { new(ptr) u(); }
>>
>> g(ptr) zeros ptr->x whereas f(ptr) doesn't. If we add "u() {}" then g()
>> does not zero ptr->x.
>>
>> So if we did add the workaround, it would need to be unconditional,
>> like you say.
>
> What about using more directed workaround then?
>
> Like (just stage1 build tested, perhaps with comment why we do that)
> below? Seems at least in stage1 it is the only problematic spot.
>
> --- a/gcc/cse.cc
> +++ b/gcc/cse.cc
> @@ -4951,8 +4951,14 @@ cse_insn (rtx_insn *insn)
> && is_a <scalar_int_mode> (mode, &int_mode)
> && (extend_op = load_extend_op (int_mode)) != UNKNOWN)
> {
> +#if GCC_VERSION >= 5000
> struct rtx_def memory_extend_buf;
> rtx memory_extend_rtx = &memory_extend_buf;
> +#else
> + alignas (alignof (rtx_def)) unsigned char
> + memory_extended_buf[sizeof (rtx_def)];
Looks like the simpler "alignas (rtx_def)" should work.
LGTM otherwise FWIW.
Richard
> + rtx memory_extend_rtx = (rtx) &memory_extended_buf[0];
> +#endif
>
> /* Set what we are trying to extend and the operation it might
> have been extended with. */
>
>
> Jakub
