On 11/12/20 7:02 PM, Xionghu Luo via Gcc wrote: > Hi all, > > In PR51505(https://gcc.gnu.org/bugzilla/show_bug.cgi?id=51505), Paolo Bonzini > added the code to delete REG_EQUAL notes in df_remove_dead_eq_notes: > > gcc/df-problems.c: > df_remove_dead_eq_notes (rtx_insn *insn, bitmap live) > { > ... > case REG_EQUAL: > case REG_EQUIV: > { > /* 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. */ > df_ref use; > bool deleted = false; > > FOR_EACH_INSN_EQ_USE (use, insn) > if (DF_REF_REGNO (use) >= FIRST_PSEUDO_REGISTER > && DF_REF_LOC (use) > && (DF_REF_FLAGS (use) & DF_REF_IN_NOTE) > && !bitmap_bit_p (live, DF_REF_REGNO (use)) > && loc_mentioned_in_p (DF_REF_LOC (use), XEXP (link, 0))) > { > deleted = true; > break; > } > if (deleted) > { > rtx next; > if (REG_DEAD_DEBUGGING) > df_print_note ("deleting: ", insn, link); > next = XEXP (link, 1); > free_EXPR_LIST_node (link); > *pprev = link = next; > df_notes_rescan (insn); > } > ... > } > > > while I have a test case as below: > > > typedef long myint_t; > __attribute__ ((noinline)) myint_t > hash_loop (myint_t nblocks, myint_t hash) > { > int i; > for (i = 0; i < nblocks; i++) > hash = ((hash + 13) | hash) + 0x66546b64; > return hash; > } > > before cse1: > > 22: L22: > 16: NOTE_INSN_BASIC_BLOCK 4 > 17: r125:DI=r120:DI+0xd > 18: r118:DI=r125:DI|r120:DI > 19: r126:DI=r118:DI+0x66540000 > 20: r120:DI=r126:DI+0x6b64 > REG_EQUAL r118:DI+0x66546b64 > 21: r119:DI=r119:DI-0x1 > 23: r127:CC=cmp(r119:DI,0) > 24: pc={(r127:CC!=0)?L22:pc} > REG_BR_PROB 955630228 > > The dump in cse1: > > 16: NOTE_INSN_BASIC_BLOCK 4 > 17: r125:DI=r120:DI+0xd > 18: r118:DI=r125:DI|r120:DI > REG_DEAD r125:DI > REG_DEAD r120:DI > 19: r126:DI=r118:DI+0x66540000 > REG_DEAD r118:DI > 20: r120:DI=r126:DI+0x6b64 > REG_DEAD r126:DI > 21: r119:DI=r119:DI-0x1 > 23: r127:CC=cmp(r119:DI,0) > 24: pc={(r127:CC!=0)?L22:pc} > REG_DEAD r127:CC > REG_BR_PROB 955630228 > ; pc falls through to BB 6 > > > 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 Jeff ps. Note that a REG_EQUAL note is valid at a particular point in the IL -- it is not a function-wide equivalence. So you have to be careful using such values as they can be invalidated by other statements. Contrast to a REG_EQUIV note where the equivalence is global and you don't have to worry about invalidation.
