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Re: [PATCH 1/7] Add __builtin_speculation_safe_value

On 24/07/18 18:26, Richard Biener wrote:
> So, please make resolve_overloaded_builtin return a no-op on such targets
> which means you can remove the above warning.  Maybe such targets
> shouldn't advertise / initialize the builtins at all?

So I tried to make resolve_overloaded_builtin collapse the builtin
entirely if it's not needed by the machine, transforming

  x = __b_s_s_v (y);


  x = y;

but I can't see how to make any side-effects on the optional second
argument hang around.  It's somewhat obscure, but if the user does write

 x = __b_s_s_v (y, z++);

then z++ does still need to be performed.

The problem seems to be that the callers of resolve_overloaded_builtin
expect just a simple value result - they can't, for example, deal with a
statement list and just calling save_expr on the argument isn't enough;
so I can't see an obvious way to force the z++ expression back into the
token stream at this point.

Any ideas?  The alternative seems to be that we must keep the call until
such time as the builtins are lowered during expansion, which pretty
much loses all the benefits you were looking for.


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