Re: Example program takes 2000 times as long to compile under C++ as C

[Zack Weinberg <zack at wolery dot cumb dot org>]

On Mon, Sep 04, 2000 at 03:26:46PM -0700, Mark Mitchell wrote:
> 
> Well, it depends what you mean.  SAVE_EXPRs are supposed to appear in
> tree structure several times over, and that's part of why I think they
> are evil.  I'm not quite sure why we'd get there *that* many times,
> and I think we should understand that better.
> 
> In this particular case, it looks like we are creating a lot more
> SAVE_EXPRs than we need to; there's nothing about that function that
> should require any SAVE_EXPRs, really.  (There's nothing about what
> you showed that has any side-effects.)  So, I bet that in this case,
> there is an easy fix.  Send me the complete source code for a pared
> down test-case; I promise to look at it today.

Sorry, I was out all day.  I've been using Kelley's original test
case; below is a preprocessed version.  You don't need any special
arguments.  If you want a smaller tree to look at, just cut out

	(inN ? 0xf00 : 0) |

lines.

N.B. If I use a hash table to remember all the trees visited by
calls_setjmp_p_r and simplify_aggr_init_exprs_r, and I clear
walk_subtrees and return on subsequent visits to each tree, processing
time for this test case drops from 90s to 0.04s - both figures with
stage1 cc1plus, so no optimization.  I also see moderate improvement
in time to run the C++ test suite.  But g++.other/defarg[45].C get
bizarre aborts in function.c.  I don't know why yet.  The code in
function.c that triggers has to do with nested functions and there
aren't any in those test cases.

zw

bool in0 ;
bool in1 ;
bool in2 ;
bool in3 ;
bool in4 ;
bool in5 ;
bool in6 ;
bool in7 ;
bool in8 ;
bool in9 ;
bool in10;
bool in11;
bool in12;
bool in13;
bool in14;
bool in15;
unsigned int output;

void mux(void)
{
  output =
      (in0   ?  0x0001 : 0) |
      (in1   ?  0x0002 : 0) |
      (in2   ?  0x0004 : 0) |
      (in3   ?  0x0008 : 0) |
      (in4   ?  0x0010 : 0) |
      (in5   ?  0x0020 : 0) |
      (in6   ?  0x0040 : 0) |
      (in7   ?  0x0080 : 0) |
      (in8   ?  0x0100 : 0) |
      (in9   ?  0x0200 : 0) |
      (in10  ?  0x0400 : 0) |
      (in11  ?  0x0800 : 0) |
      (in12  ?  0x1000 : 0) |
      (in13  ?  0x2000 : 0) |
      (in14  ?  0x4000 : 0) |
      (in15  ?  0x8000 : 0) ;
}