Re: [02/23] rtlanal: Remove noop_move_p REG_EQUAL condition

[Jeff Law via Gcc-patches]

On 11/13/20 1:12 AM, Richard Sandiford via Gcc-patches wrote:
> noop_move_p currently keeps any instruction that has a REG_EQUAL
> note, on the basis that the equality might be useful in future.
> But this creates a perverse incentive not to add potentially-useful
> REG_EQUAL notes, in case they prevent an instruction from later being
> removed as dead.
>
> The condition originates from flow.c:life_analysis_1 and predates
> the changes tracked by the current repository (1992).  It probably
> made sense when most optimisations were done on RTL rather than FE
> trees, but it seems counterproductive now.
>
> gcc/
>       * rtlanal.c (noop_move_p): Don't check for REG_EQUAL notes.
I would  guess this was primarily for the old libcall mechanism where
we'd have a self-copy at the end of the sequence with a REG_EQUAL note
for the expression's natural form.  All that's been broken for a long
time.  So I'm not going to lose any sleep if we want to remove this
little chunk of code.

OK

jeff