Re: Minor codegen silliness in ExecInterpExpr()

On 2017-09-28 18:39:03 -0400, Tom Lane wrote:
> Andres Freund <andres(at)anarazel(dot)de> writes:
> > On 2017-09-28 16:21:34 -0400, Tom Lane wrote:
> >> We could save a pointless register spill
> >> and reload if there were a temporary variable in there,
>
> > Makes sense. Do you want to make it so, or shall I?
>
> I just finished testing a patch, as attached. On my machine (again,
> not latest gcc: 4.4.7 on RHEL6 x86_64), it reduces the code size of
> execExprInterp.o by a fraction of a percent, and it seems to offer
> a slight benefit in "pgbench -S" performance although I'd not put
> much stock in that being reproducible.

Cool.

> + * Note: the reason for using a temporary variable "d", here and in
> + * other places, is that some compilers think "*op->resvalue = f();"
> + * requires them to evaluate op->resvalue into a register before
> + * calling f(), just in case f() is able to modify op->resvalue
> + * somehow. The extra line of code can save a useless register spill
> + * and reload, on architectures without many registers.

I'd remove the "without many registers" bit - that's really more an
functioncall ABI question (#caller vs #callee saved registers) than
about the actual architecture.

Greetings,

Andres Freund