Re: [trunk] patch to remove REG_NO_CONFLICT_BLOCKS

[Vladimir Makarov <vmakarov at redhat dot com>]

Kenneth Zadeck wrote:
> The third patch, has a modification to ra-conflict.c to compute the 
> information that is/was contained in REG_NO_CONFLICT blocks.   Because 
> it is computed from first principals, it actually finds cases of two 
> multiword regs that could be allocated on top of each other but could 
> not otherwise overlap.  It also contains enhancements to global to use 
> this information to do this kind of allocation.   By doing this in 
> global, there is now no reason to perform the allocation in 
> local-alloc.   Local alloc is still called but returns after 
> update-equiv-regs. We have also changed things so that global is 
> called for -O0.   This adds .5% to the compile time, but cuts the size 
> of the .o files by 5%.  After peter's and my changes last year to the 
> conflict builder, there really is very little compile time benefit to 
> having local-alloc around.
It would be nice to remove local-alloc.  But it would be not so easy to 
do.  Please, check c99-int-const-1.c.  Even after Peter's conflict 
matrix triangulation, I guess the test compilation will need 1GB memory 
machine for -O0.  Such big memory is not necessary for -O2.  So removing 
local-alloc will need to switch some optimizations on too (although I 
did not analyze which ones).

Also a few years ago I did some experiments on removing local-alloc too 
and I found a visible performance degradation (as I remember about 1%).

So removing local-alloc would be a bad idea without performance testing.

To be honest I don't care about local-alloc removing because I stopped 
my work on improving the current register allocator long ago and put my 
efforts into a new register allocator which I hope will be a default RA 
soon for some architectures.
> All four patches need to be tested together, but this division makes 
> it possible for different people to review the parts under their 
> control.   However, if some backend maintainer wanted to just review 
> everything, that would great.   I do want to ask that someone test 
> these patches on other architectures for both correctness and 
> performance.
> Comments, suggestions, attacks ....
Please, don't look at this as an attack but it would be nice to see some 
performance numbers what removing REG_NO_CONFLICT_BLOCKS and byte 
accurate  conflict detection (without removing local-alloc) gives.  I 
guess there should be some improvement.  I like general direction to 
remove REG_NO_CONFLICT_BLOCKS and byte accurate  conflict detection.  I 
think it is ok even it gives no visible improvement and makes  compiler 
a bit slower.