Hello,
Is it necessary for an insn to have REG_UNUSED notes for a CLOBBER?
There are lots such notes on x86 for the flags register:
11: {r68:DI=r61:DI<<0x8;clobber flags:CC;}
REG_DEAD r61:DI
REG_UNUSED flags:CC
In most places where the compiler looks at REG_UNUSED notes, it is for
an unused result of a SET in a multiple_sets insn. There are a few
exceptions, e.g. worker functions for note_stores such as in
mode-switching.c:reg_becomes_live(), but that looks more like a
mistake than intentional: reg_becomes_live makes non-REG_UNUSED
CLOBBERs live but it doesn't calculate REG_UNUSED notes before running
so it may be looking at invalid notes, or looking for missing notes.
It seems to me that a REG_UNUSED note on a CLOBBER is unnecessary, and
it also goes against the documented purpose of a REG_UNUSED note in
reg-notes.def:
/* Identifies a register set in this insn and never used. */
REG_NOTE (UNUSED)
After all, a CLOBBER is not a register set, so I think a patch like
the one below is necessary.
Thoughts?
Ciao!
Steven
* df-problems.c (df_note_bb_compute): Do not set REG_UNUSED notes
on CLOBBERs.
Index: df-problems.c
===================================================================
--- df-problems.c (revision 196182)
+++ df-problems.c (working copy)
@@ -3276,11 +3276,12 @@ df_note_bb_compute (unsigned int bb_inde
{
df_ref def = *def_rec;
unsigned int dregno = DF_REF_REGNO (def);
- df_create_unused_note (insn,
- def, live, artificial_uses, &debug);
-
if (!DF_REF_FLAGS_IS_SET (def, DF_REF_MUST_CLOBBER |
DF_REF_MAY_CLOBBER))
- bitmap_set_bit (do_not_gen, dregno);
+ {
+ df_create_unused_note (insn,
+ def, live, artificial_uses, &debug);
+ bitmap_set_bit (do_not_gen, dregno);
+ }
if (!DF_REF_FLAGS_IS_SET (def, DF_REF_PARTIAL |
DF_REF_CONDITIONAL))
bitmap_clear_bit (live, dregno);
