How do I modify SSA and copy basic blocks?

[Steve Ellcey]

I decided to take a crack at the coremark optimization (PR 54742) for switch
statements.  Since I couldn't figure out the existing jump threading
optimization enough to extend it properly, I decided to try a plugin
optimization pass just for this case and see if I could get that to work.

The basic idea is to find a path from where a variable is assigned a constant
value to where that variable is used in a switch statement.  Then I want to
duplicate the blocks in that path (thus removing any alternative entry points
into the path that may give the variable a different value) and plug it into
the cfg.

I think I have code that finds the path that I am interested in, but when
I try to use copy_bbs to copy the basic blocks in order to create my new path,
I get segfaults.  I was wondering if anyone could help me understand what I
need to do, in addition to calling copy_bbs, to create my new path and keep
the various ssa and cfg information up to date, either by modifying it or by
blowing it away and regenerating it, I am not worried about compilation speed
at this point so if regenerating all the SSA/cfg data is easiest, I am happy
to do that.

Attached is my plugin as it exists right now and a simple test case using a
switch statement.  I am not including the actual coremark code because it is
copyrighted.  If you run my example you will see output showing 4 places where
I think I can do my optimization.  For example we assign t the value of 0 in
block 2 and from there we go to block 8 and (possibly) to block 3 where we have
our switch statement.

So what I try to do is copy blocks 8 and 3 and change the edge from block
2 to block 8 to go from block 2 to my new copy of block 8 which should
then go to the new copy of block 3.  After this we should be able to
optimize the new copy of block 3 to finish with a goto to the 'case 0'
label instead of with a switch/goto.

If you remove the '#if 0' in my code where I comment out the call to
copy_bbs, you will get a seg fault, this is the code that I need help
with.  For example, If I copy block 8 and block 3, and there is an edge
from 8 to 3, does copy_bbs automatically create a new edge pointing from the
copy of block 8 to block 3 and replace that in the copied blocks?  I think it
does, but I am not sure. 

Steve Ellcey
sellcey@imgtec.com



Output from the test program using my new optimization phase.

In plugin, registering new pass
Block 2 assigns variable t a constant value of 0
Basic blocks (leading from basic block 2 to switch) are:
  8
  3
Block 4 assigns variable t a constant value of 1
Basic blocks (leading from basic block 4 to switch) are:
  7
  8
  3
Block 5 assigns variable t a constant value of 2
Basic blocks (leading from basic block 5 to switch) are:
  7
  8
  3
Block 6 assigns variable t a constant value of 1
Basic blocks (leading from basic block 6 to switch) are:
  7
  8
  3

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