Re: Compile time increases on Diego's SPEC box

[Richard Henderson <rth at redhat dot com>]

On Sun, Jul 17, 2005 at 01:48:44PM -0400, Daniel Berlin wrote:
> As i expected, the sixtrack slowdown is entirely in global-alloc, in
> daten.f
> 
> with -fno-tree-promote-statics
>  global alloc          :   0.23 ( 4%) usr   0.00 ( 0%) sys   0.24 ( 3%)
> wall     448 kB ( 3%) ggc
> 
> without:
> 
>  global alloc          : 375.96 (83%) usr   5.00 (62%) sys 453.83 (82%)
> wall   10714 kB ( 5%) ggc
> 
> 
> Cute, no?
> 
> It just has a lot more variables to deal with now that they aren't
> statics that live in memory (a lot of things now get SRA'd that didn't
> before, etc).

Would you care to characterize the number of extra variables created
at the tree level and the number of extra pseudos created at the rtl
level?  I could see that there's room to be lazy and create hundreds
of extra unnecessary variables that will exacerbate this problem.

For instance, if there's a global variable X that gets promoted to a
local, and a function FOO that kills X, one could do

	x1_1 = X;
	// use x1_1
	foo  ();
	x2_2 = X;
	// use x2_2

or 

	x_1 = X;
	// use x_1
	foo ();
	x_2 = X;
	// x_2

wherein the difference is that the first case uses two variables for
the different live ranges, and the second uses only one.


r~