[tree-ssa] Add flag to disable dominator optimizations [patch]

[Diego Novillo <dnovillo at redhat dot com>]

Adds -ftree-dominator-opts to experiment with enabling/disabling the
dominator optimizations we do while we convert the program into SSA.

The flag is enabled by default at -O1 and higher.  If -ftree-pre is
given and the optimization level is >= 2, then it's disabled
automatically.  So, for now the behaviour is exactly the same, except
that we can disable to test various combinations.

The patch renames the existing rewrite_stmt into
rewrite_and_optimize_stmt and adds a new simple minded rewrite_stmt
function that merely renames the statement's operands.  I could've added
code to the existing function instead, but the function is hairy enough
and the flag would have to be checked for every invocation.

Bootstrapped and tested on x86 with and without -ftree-dominator-opts.


Diego.

Attachment: 20030610-flag-for-dominator-opts.diff
Description: Text document