[PATCH] auto-inc-dec: Don't add incs/decs to bare CLOBBERs [PR93124]

[Jeff Law]

On Wed, 2020-01-22 at 12:11 +0000, Richard Sandiford wrote:
> In this PR, auto-inc-dec was trying to turn:
> 
>     (set (reg X) (plus (reg X) (const_int N)))
>     (clobber (mem (reg X)))
> 
> into:
> 
>     (clobber (mem (pre_modify (reg X) ...)))
> 
> But bare clobber insns are just there to describe dataflow.  They're
> not supposed to generate any code.
> 
> This is part 2 of the fix for PR93124.  The reason we have the
> clobber above is that cprop replaced (reg sp) with (reg X) in:
> 
>     (clobber (mem (reg sp)))
> 
> Part 1 stops that from happening, but I still think we should
> prevent auto-inc-dec from "optimising" bare USEs and CLOBBERs
> as a belt-and-braces fix.
> 
> Tested on aarch64-linux-gnu and x86_64-linux-gnu.  OK to install?
> 
> Richard
> 
> 
> 2020-01-22  Richard Sandiford  <richard.sandiford@arm.com>
> 
> gcc/
> 	PR rtl-optimization/93124
> 	* auto-inc-dec.c (merge_in_block): Don't add auto inc/decs to
> 	bare USE and CLOBBER insns.
> 
> gcc/testsuite/
> 	* gcc.dg/torture/pr93124.c: New test.
OK. 

Jeff
>