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)];
+ 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
