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

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.

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/testsuite/
	PR rtl-optimization/115881
	* gcc.c-torture/compile/pr115881.c: New test.

0 files changed, 0 insertions, 0 deletions