09/05/2012 11:16, Eric Botcazou: >> I have another issue in DCE pass after changing word_mode from SImode to >> HImode. >> Indeed, in subreg1 pass, SI moves such as >> ... >> (insn 42 41 43 (set (reg:SI 85) (reg/f:SI 83)) >> (insn 46 45 47 (set (reg:SI 8 a1) (reg:SI 85)) >> >> are split into HImode word moves: >> ... >> (insn 98 97 99 2 (set (subreg:HI (reg:SI 106) 0) (reg:HI 104)) >> (insn 99 98 100 2 (set (subreg:HI (reg:SI 106) 2) (reg:HI 105 [+2 ])) >> (insn 100 99 47 2 (set (reg:SI 8 a1) (reg:SI 106)) >> >> Looking at DCE pass in (dce_process_block), it seems that when reverse >> parsing the block insns for dead set eliminations, the liveness of reg >> 106 which is set when parsing insn 100, is removed when parsing insn 99. >> So the insn 98 is deleted because the reg 106 is not seen live anymore... >> >> Is it a bug, or is it because it is not possible to have hardware >> registers which are not splittable in word_mode? Any thoughts? > > Yes, this looks like a bug, since the SET in insn 99 doesn't modify the high > part of the multi-word (reg:SI 106), as per the documented rules for SUBREG. >
It was a backend bug... When trying to workaround the first reload bug, I defined REGMODE_NATURAL_SIZE to return a mode rather than a mode size, which makes DF_REF_PARTIAL not being set for (subreg:HI (reg:SI 106) 0). Sorry about that, and thank you for pointing the DCE bug out. Aurélien
