extra instructions lost from -O0 to -O1

[Paolo Bonzini]

Thomas A.M. Bernard wrote:
> Well I found another way to solve the problem by updating the dce for
> not taking out my instructions.
> 
> I inserted "setallocate" as a native operator in the back-end which
> comes from a GIMPLE node and map to the RTL pattern. Earlier in the
> discussion, it's been discussed that the dce was taking out the
> instruction when flag -O1 was engaged. To solve that, in
> 'tree-ssa-dce.c', I flagged this node with the function,
> "mark_stmt_necessary". And it works fine so far. The instruction is not
> omitted anymore by the dce :-)

Do not add it as a GIMPLE node.  Add it as a builtin function, so that
the tree-level DCE will treat like every other call and not remove it.

IOW, do not add new kinds of node.  Use builtins for trees, and unspecs
for RTL.

Paolo