PATCH: [gcc3.5 improvement branch] Very Simple constant propagation

[Caroline Tice]

Using "time" and measuring it on the C++ code from PR8361 (which I have 
been informed is a
pretty good compile time benchmark), I get

95.750u 7.430s 1:45.27 98.0%    0+0k 56+320io 0pf+0w   using gcc 3.4, 
-O3 -ffast-math,

  and I get

96.000u 7.340s 1:43.75 99.6%    0+0k 0+320io 0pf+0w   using my patch, 
adding the -fss-const-prop flag

So it appears to have a very small impact on compile time.  The impact 
on performance will depend a lot
on the code you use it on (although performance should only get better, 
never worse).  In the case that
prompted this patch, it caused the elimination of a floating point 
multiplication from a loop that was
executed many times, which had a major positive impact on performance.

The tree-ssa branch currently exhibits the problem behavior that 
prompted this patch.  However Jan
Hubicka has suggested a possible alternative approach for tree-ssa to 
take.

-- Caroline Tice


On Jan 15, 2004, at 3:14 PM, Steven Bosscher wrote:

> On Friday 16 January 2004 00:00, Caroline Tice wrote:
>> This patch has been tested on an Apple G4 running apple-darwin.  It
>> bootstraps and passes DejaGnu tests.  Is this okay to commit to the 
>> gcc
>> 3.5 improvements branch? Or gcc 3.4?
>
> What are the compile time impacts of this pass, and what is its impact
> on the performance of the resulting code?  Also, is this still relevant
> after tree-ssa hits the branch for 3.5 (assuming it does)?.
>
> Gr.
> Steven
>