[PATCH 23/23] fwprop: Rewrite to use RTL SSA

[Jeff Law]

On 11/13/20 1:24 AM, Richard Sandiford via Gcc-patches wrote:
> This patch rewrites fwprop.c to use the RTL SSA framework.  It tries
> as far as possible to mimic the old behaviour, even in caes where
> that doesn't fit naturally with the new framework.  I've added ???
> comments to mark those places, but I think “fixing” them should
> be done separately to make bisection easier.
>
> In particular:
>
> * The old implementation iterated over uses, and after a successful
>   substitution, the new insn's uses were added to the end of the list.
>   The pass still processed those uses, but because it processed them at
>   the end, it didn't fully optimise one instruction before propagating
>   it into the next.
>
>   The new version follows the same approach for comparison purposes,
>   but I'd like to drop that as a follow-on patch.
>
> * The old implementation operated on single use sites (DF_REF_LOCs).
>   This doesn't work well for instructions with match_dups, where it's
>   necessary to update both an operand and its dups at the same time.
>   For example, attempting to substitute into a divmod instruction would
>   fail because only the div or the mod side would be updated.
>
>   The new version again follows this to some extent for comparison
>   purposes (although not exactly).  Again I'd like to drop it as a
>   follow-on patch.
>
>   One difference is that if a register occurs in multiple MEM addresses
>   in a set, the new version will try to update them all at once.  This is
>   what causes the SVE ACLE st4* output to improve.
>
> Also, the old version didn't naturally guarantee termination (PR79405),
> whereas the new one does.
>
> gcc/
> 	* fwprop.c: Rewrite to use the RTL SSA framework.
>
> gcc/testsuite/
> 	* gcc.dg/rtl/x86_64/test-return-const.c.before-fwprop.c: Don't
> 	expect insn updates to be deferred.
> 	* gcc.target/aarch64/sve/acle/asm/st4_s8.c: Expect the addition
> 	to be folded into the address.
> 	* gcc.target/aarch64/sve/acle/asm/st4_s8.c: Likewise.
Consider killing the ADD_NOTES bits.

s/eqaul/equal/ to fix a typo.

Naturally I'm happy at how much by-hand RTL analysis code just
disappears with this change :-)

Ideally you'll drop this in tomorrow and we can get a fresh run of all
the targets in my tester before the weekend.  I won't be stressed if we
see some fallout, but I don't expect much.  I'll help track them down if
they occur.

Thanks for your patience.

Jeff