RFA: Fix other/44566

[Mark Mitchell]

Joern Rennecke wrote:

> And I'd also like to know what that does to our bootstrap speed.

What follows is a minor parenthetical in the broader discussion that you
and Joseph are having...

I know this may be perceived as the words of someone who vary rarely
bootstraps the compiler anymore, but I believe that bootstrap speed per
se is nearly irrelevant.  Compile-time is not irrelevant; we care what
the user experience is like.  But to the extent bootstrap time is
independent of compile-time, I don't think we should care.  (OK, I think
we should care about making it 10x greater, but not about making it 25%
greater.)

The reason is that I think that (a) bootstraps are fast enough that it
doesn't make a whole lot of difference, and (b) building the run-time
libraries and running tests takes much more time than the bootstrap per
se.  And, as for the bootstrap per se, I suspect that configure/link
overhead will dominate soon, if it doesn't already; configure and link
steps are (currently) serial, so cannot take advantage of massively
parallel systems in the way that the rest of the build process can.

So, concretely, carrying LTO/WHOPR overhead during the compiler build
itself to me seems OK if that gets us a better compiler and user's can't
tell the difference.  In fact, we probably want to turn on LTO/WHOPR by
default once we feel it's sufficiently reliable since presumably we'll
get better code that way, no matter how the compiler is structured.

-- 
Mark Mitchell
CodeSourcery
mark@codesourcery.com
(650) 331-3385 x713