[PATCH] fwprop: Prevent infinite looping (PR79405)

[Jeff Law]

On 03/23/2017 08:58 AM, Segher Boessenkool wrote:
> The algorithm fwprop uses never reconsiders a possible propagation,
> although it could succeed if the def (in the def-use to propagate)
> has changed.  This causes fwprop to do infinite propagations, like
> in the scenario in the PR, where we end up with effectively
>   B = A
>   A = B
>   D = A
> where only propagations into the last statement are still tried, and
> that loops (it becomes D = B, then back to D = A, etc.)
>
> Fixing this properly isn't easy; this patch instead limits the number
> of propagations performed to the number of uses we originally had,
> which is the maximum number of propagations that can be done if there
> are no such infinite loops.
>
> Bootstrapped and regression checked on powerpc64-linux {-m64,-m32};
> is this okay for trunk?
>
>
> Segher
>
>
> 2017-03-23  Segher Boessenkool  <segher@kernel.crashing.org>
>
> 	PR rtl-optimization/79405
> 	* fwprop.c (propagations_left): New variable.
> 	(forward_propagate_into): Decrement it.
> 	(fwprop_init): Initialize it.
> 	(fw_prop): If the variable has reached zero, stop propagating.
> 	(fwprop_addr): Ditto.
>
> gcc/testsuite/
> 	PR rtl-optimization/79405
> 	gcc.dg/pr79405.c: New testcase.
I installed this as a stopgap for gcc-7.  The bug is still open 
targetting gcc-8  to force us to come back and look at Richi's suggestion.

jeff