On Sun, Nov 22, 2020 at 5:38 PM J.W. Jagersma <jwjager...@gmail.com> wrote:
>
> On 2020-11-21 12:27, J.W. Jagersma wrote:
> > ...
> > Another idea I had is to introduce a new operand modifier, eg. '-', which
> > would signify that the output *must* be considered clobbered on exception,
> > and it would be an error if a copy is not possible. Then the meaning of '+'
> > can be extended to say that the output must be valid regardless of whether
> > an
> > exception occured or not. Modifier '=' would mean the same as '+' with the
> > additional implication that it is okay to eliminate earlier assignments, so
> > that its value on the EH edge would be undefined.
>
> I've been working on implementing this, but I ran into an issue:
>
> First of all, in the first version of this patch I had to make sure that
> debug stmts were inserted across the fallthrough edge, so that they don't
> appear at the end a basic block. I was surprised to find that this is no
> longer necessary.
>
> Next I discovered that the following test case fails (returns -1):
>
> int f ()
> {
> int i = 0;
> try { asm ("mov %0, 1; ud2" : "+r" (i)); }
> catch (...) { }
> return i - 1;
> }
>
> And this does succeed with a memory operand.
>
> It seems that something changed in the past few months, and now asm register
> outputs are already being assigned via a temporary variable, somewhere. Does
> anyone know where that might be?
It's likely out-of SSA / RTL expansion inserting a bogus copy or doing
coalescing on the EH edge. Semantics of throwing asm stmts need to be
honored there.
Richard.
