> Well, I'm not sure I agree that they are wrong. Consider:
>
> S0: r1 = ...
> S1: r2 = r1 + 10
> S2: r1 = r2 + 10 { REG_EQUAL r1 + 20 }
> S3: r3 = r1 + 10
>
> Clearly it would be wrong to use the REG_EQUAL note on S2 to optimize S3 to:
>
> S0: r1 = ...
> S1: r2 = r1 + 10
> S2: r1 = r1 + 20
> S3: r3 = r1 + 30
But the note is wrong by itself. The semantics is clear: the note means that
r1 is equal to r1 + 20 right after S2, which doesn't make any sense.
> However, It seems to me that this would be valid if e.g. the webizer
> turns the above into:
>
> S0: r1 = ...
> S1: r2 = r1 + 10
> S2: r4 = r2 + 10 { REG_EQUAL r1 + 20 }
> S3: r3 = r4 + 10
>
> because now the optimization would be valid:
>
> S0: r1 = ...
> S1: r2 = r1 + 10
> S2: r4 = r1 + 20
> S3: r3 = r1 + 30
Sure, but we have no guarantee that the RTL stream will be massaged that way.
> So self-referencing REG_EQUAL notes should IMHO be considered valid,
> and transformations using the notes should just be careful to
> invalidate the equivalence before processing the insn that the note
> appears on. cse.c doesn't appear to do this, however.
IMO that's a recipe for bugs.
> On a completely different note:
>
> df-problems.c has this comment:
>
> /* Remove the notes that refer to dead registers. As we
> have at most
> one REG_EQUAL/EQUIV note, all of EQ_USES will refer to this
> note so we need to purge the complete EQ_USES vector when removing the note
> using df_notes_rescan. */
>
> But ira.c:update_equiv_regs creates insns with a REG_EQUAL *and* a
> REG_EQUIV note:
> (gdb)
> update_equiv_regs () at ../../trunk/gcc/ira.c:3177
> 3177 = gen_rtx_INSN_LIST (VOIDmode, insn, NULL_RTX);
> 2: debug_rtx((rtx)0x7ffff4df5e58) = (insn 638 637 639 85 (parallel [
> (set (reg:SI 891)
> (minus:SI (reg:SI 890)
> (reg:SI 546 [ D.45472 ])))
> (clobber (reg:CC 17 flags))
> ]) ../../trunk/gcc/value-prof.c:928 283 {*subsi_1}
> (expr_list:REG_EQUIV (mem:SI (plus:DI (reg/v/f:DI 204 [ e12 ])
> (const_int 52 [0x34])) [4 e12_223->probability+0 S4 A32])
> (expr_list:REG_UNUSED (reg:CC 17 flags)
> (expr_list:REG_EQUAL (minus:SI (const_int 10000 [0x2710])
> (reg:SI 546 [ D.45472 ]))
> (nil)))))
> void
> (gdb)
>
> Is that valid?
Yes, the comment is wrong and should have been removed in r183719:
http://gcc.gnu.org/ml/gcc-patches/2012-01/msg01547.html
--
Eric Botcazou
