Re: [PATCH] recog: Disallow subregs in mode-punned value [PR115881]

[Jeff Law <jeffreyalaw@gmail.com>]

On 7/19/24 11:37 AM, Richard Sandiford wrote:
> In g:9d20529d94b23275885f380d155fe8671ab5353a, I'd extended
> insn_propagation to handle simple cases of hard-reg mode punning.
> The punned "to" value was created using simplify_subreg rather
> than simplify_gen_subreg, on the basis that hard-coded subregs
> aren't generally useful after RA (where hard-reg propagation is
> expected to happen).
> 
> This PR is about a case where the subreg gets pushed into the
> operands of a plus, but the subreg on one of the operands
> cannot be simplified.  Specifically, we have to generate
> (subreg:SI (reg:DI sp) 0) rather than (reg:SI sp), since all
> references to the stack pointer must be via stack_pointer_rtx.
> 
> However, code in x86 (reasonably) expects no subregs of registers
> to appear after RA, except for special cases like strict_low_part.
> This leads to an awkward situation where we can't ban subregs of sp
> (because of the strict_low_part use), can't allow direct references
> to sp in other modes (because of the stack_pointer_rtx requirement),
> and can't allow rvalue uses of the subreg (because of the "no subregs
> after RA" assumption).  It all seems a bit of a mess...
> 
> I sat on this for a while in the hope that a clean solution might
> become apparent, but in the end, I think we'll just have to check
> manually for nested subregs and punt on them.
> 
> Tested on aarch64-linux-gnu & x86_64-linux-gnu.  OK to install?
> 
> Richard
> 
> 
> gcc/
> 	PR rtl-optimization/115881
> 	* recog.cc: Include rtl-iter.h.
> 	(insn_propagation::apply_to_rvalue_1): Check that the result
> 	of simplify_subreg does not include nested subregs.
> 
> gcc/tetsuite/
> 	PR rtl-optimization/115881
> 	* cc.c-torture/compile/pr115881.c: New test.
OK
jeff