[PATCH] Teach evrp that main's argc argument is always non-negative for C family (PR tree-optimization/89350)

Jakub Jelinek jakub@redhat.com
Tue Apr 30 15:11:00 GMT 2019


On Tue, Apr 30, 2019 at 09:03:07AM -0600, Jeff Law wrote:
> On 2/16/19 12:12 AM, Jakub Jelinek wrote:
> > Both the C and C++ standard guarantee that the argc argument to main is
> > non-negative, the following patch sets (or adjusts) the corresponding
> > SSA_NAME_RANGE_INFO.  While main is just one, with IPA VRP it can also
> > propagate etc.  I had to change one testcase because it started optimizing
> > it better (the test has been folded away), so no sinking was done.
> > 
> > Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
> > 
> > 2019-02-16  Jakub Jelinek  <jakub@redhat.com>
> > 
> > 	PR tree-optimization/89350
> > 	* gimple-ssa-evrp.c: Include tree-dfa.h and langhooks.h.
> > 	(maybe_set_main_argc_range): New function.
> > 	(execute_early_vrp): Call it.
> > 
> > 	* gcc.dg/tree-ssa/vrp122.c: New test.
> > 	* gcc.dg/tree-ssa/ssa-sink-3.c (main): Rename to ...
> > 	(bar): ... this.
> My recollection is this was somewhat controversial WRT standards
> interpretation.  Additionally, it's just not clear how often this will
> help in the real world and Martin adjusted compute_objsize and the
> restrict pass to avoid the false positive.
> 
> Do we want to continue with this patch independently or not?  My
> inclination would be no.

I wanted to follow-up on this and restrict it to for now to C++ where it is
non-controversial.  Would be of course nice if the C WG discusses it too and
clarifies if needed, I agree the wording isn't 100% clear and the
requirement shouldn't be on whatever use does with that parameter inside of
main, but should be a contract on when main is called and if user decides to
call main multiple times in C, that contract should not be violated even
during the subsequent calls.

	Jakub



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