CSE deletes valid REG_EQUAL?

Segher Boessenkool segher@kernel.crashing.org
Fri Nov 13 13:55:16 GMT 2020


Hi guys,

On Thu, Nov 12, 2020 at 09:10:14PM -0700, Jeff Law wrote:
> On 11/12/20 7:02 PM, Xionghu Luo via Gcc wrote:
> > The output shows "REQ_EQUAL r118:DI+0x66546b64" is deleted by df_remove_dead_eq_notes,
> > but r120:DI is not REG_DEAD here, so is it correct here to check insn use and find that
> > r118:DI is dead then do the delete?
> 
> It doesn't matter where the death occurs, any REG_DEAD note will cause
> the REG_EQUAL note to be removed.  So given the death note for r118,
> then any REG_EQUAL note that references r118 will be removed.  This is
> overly pessimistic as the note may still be valid/useful at some
> points.  See
> 
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92291

The note even *always* is valid where it is!  As you say, the REG_EQUAL
is only (necessarily) valid at *that* insn.  And this insn *sets* the
reg (that is the only case when a REG_EQ* is allowed at all), so if the
eq reg is dead (i.e. unused) the whole insn is, and the insn can be
removed.

Removing all REG_EQUAL notes for regs that become dead anywhere seems
to just be a thinko?  All pseudos are dead somewhere!  (Yeah okay,
infinite loops, but other than that.)


Segher


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