maybe_infinite_loop?

[Mike Stump <mrs at apple dot com>]

We still have some lno bits in our tree.  We tried to remove them and  
found:

gzip +0.5%
vpr -0.4%
gcc -3.2%
mcf -0.3%
crafty +0.2%
parser +0.2%
perlbmk -2.2%
gap +0.2%
vortex -0.1%
bzip2 +1.9%
twolf -0.7%

on x86 (probably a core2 duo) in our 4.2 tree (with the rest of our  
local patches).  -3.2% means a 3.2% better codegen (roughly) with the  
lno bits.  I didn't rerun the numbers for mainline to see if they are  
still applicable.

Of all the LNO bits, the last major bits seems to be the below bit.   
I don't even know if it is responsible for the benefit we see.  I  
thought I'd mention it, as a 2-3% win on two of the spec tests seems  
worthwhile.

I'd be interested in finding someone that might be interested in  
tracking down where the benefit comes from in the patch and pushing  
into mainline what goodness there is to be had from the patch.  Any  
takers?  If I can find someone, I'd be happy to send out the version  
of the patch for mainline.  [ hum just 567 lines] On second though,  
I'll just include at the end for reference.  Note, there is one soft  
conflict resolution to resolve in going from the 4.2 context to  
mainline, which I've not yet resolved.

2004-07-13  Zdenek Dvorak  <rakdver@atrey.karlin.mff.cuni.cz>

        * Makefile.in (tree-ssa-loop.o, tree-ssa-dce.o): Add function.h
        dependency.
        * builtins.c (expand_builtin): Handle  
BUILT_IN_MAYBE_INFINITE_LOOP.
        * builtins.def (BUILT_IN_MAYBE_INFINITE_LOOP): New builtin.
        * function.h (struct function): Add marked_maybe_inf_loops  
field.
        * timevar.def (TV_MARK_MILOOPS): New timevar.
        * tree-flow.h (mark_maybe_infinite_loops): Declare.
        * tree-optimize.c (init_tree_optimization_passes): Add
        pass_mark_maybe_inf_loops.
        * tree-pass.h (pass_mark_maybe_inf_loops): Declare.
        * tree-ssa-dce.c: Include function.h.
        (find_obviously_necessary_stmts): Mark back edges only if  
they were
        not marked already.
        (perform_tree_ssa_dce): Do not call mark_dfs_back_edges here.
        * tree-ssa-loop-niter.c (unmark_surely_finite_loop,
        mark_maybe_infinite_loops): New functions.
        * tree-ssa-loop.c: Include function.h.
        (tree_mark_maybe_inf_loops, gate_tree_mark_maybe_inf_loops,
        pass_mark_maybe_inf_loops): New pass.
        * tree-ssa-operands.c (function_ignores_memory_p): Add
        BUILT_IN_MAYBE_INFINITE_LOOP.

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