Re: [tree-ssa] Fix bootstrap failure caused by tree-ssa-loop.c

[law at redhat dot com]

In message <1070903665.17667.28.camel@p4>, Andrew MacLeod writes:
 >On Sun, 2003-12-07 at 11:57, Diego Novillo wrote:
 >> On Sun, 2003-12-07 at 11:52, Jan Hubicka wrote:
 >> 
 >> > This looks strange.  Why dumping code should be tied with checking?  I
 >> > definitly would like to see dumps out of checking disabled compiler.
 >> > 
 >> See the code.
 >> 
 >> Zdenek just provided an empty tree_ssa_loop_opt() which merely dumps the
 >> function with checking enabled.
 >> 
 >Actually, the flag for the loop optimizations should probably be off by
 >default. Our normal operating method is to have new passes turned off by
 >default until they are in a reasonable state where they demonstrate
 >something of signdficance, and all the bugs are worked out. Its lets the
 >author of the pass fiddle around a lot more without impacting other
 >people.
Correct.  And for this code I already know it's got some problems.  Or more
correctly it interacts badly with the rest of the SSA path.

The dominator tree right now is fundamentally tied to block indices.  If we
add or remove blocks, then we effectively invalidate the dominator tree
(that's probably not a wise thing, but it's the way things are right now).

Anyway, the loop code creates new basic blocks, then re-computes block
indices.  This totally horks the dominator tree -- and does so in a way
that our code to detect the dominator tree being invalid does not detect.

Then later we come along and run the dominator optimizer using this horked
dominator tree -- which results in a variety of incorrect transformations
which are caught by verify_ssa.

You can see this lossage in action if you have the loop code enabled and
do a checking bootstrap with -O1.

jeff