http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29560
--- Comment #7 from Georg-Johann Lay <gjl at gcc dot gnu.org> 2011-07-23 14:50:58 UTC --- Created attachment 24816 --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=24816 Test case for 16-bit shifts that use low part only Reopened this optimization issue. For the attached test case, there are sometimes REG_UNUSED notes for the high part like with avr-gcc-4.6.1 -Os -dP shift1: ; (insn 8 4 17 (set (reg:HI 24 r24 [50]) ; (ashift:HI (reg:HI 24 r24 [ x ]) ; (reg/v:QI 22 r22 [orig:47 s ] [47]))) foo.c:3 68 {ashlhi3} ; (expr_list:REG_DEAD (reg/v:QI 22 r22 [orig:47 s ] [47]) ; (expr_list:REG_UNUSED (reg:QI 25 r25) ; (nil)))) rjmp 2f ; 8 ashlhi3/1 [length = 6] 1: lsl r24 rol r25 2: dec r22 brpl 1b So that there is a way to map ashlhi3 to ashlqi3, i.e. 16-bit shift to a 8-bit shift. Unfortunately, these notes are not always present like in shift2: shift2: ; (insn 15 4 8 (set (reg:HI 18 r18) ; (reg:HI 24 r24 [ x ])) foo.c:10 10 {*movhi} ; (nil)) movw r18,r24 ; 15 *movhi/1 [length = 1] ; (insn 8 15 9 (set (reg:HI 18 r18) ; (ashift:HI (reg:HI 18 r18) ; (reg/v:QI 22 r22 [orig:46 s ] [46]))) foo.c:10 68 {ashlhi3} ; (expr_list:REG_DEAD (reg/v:QI 22 r22 [orig:46 s ] [46]) ; (nil))) rjmp 2f ; 8 ashlhi3/1 [length = 6] 1: lsl r18 rol r19 2: dec r22 brpl 1b The notes are not present in pass .split2 so a split won't help. The notes appear to be available in .peephole2 so that could be a fix. Moreover, the notes won't be back-propagated so that an optimization will cover at most one insn: the shift insn.
