http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57503
--- Comment #6 from Georg-Johann Lay <gjl at gcc dot gnu.org> --- Created attachment 30252 --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=30252&action=edit .165r.expand: dump file with -Os -mmcu=atmega168 -fno-expensive-optimizations The tree-ssa part for widening_mul can be disabled by -fno-expensive-optimizations, but the code is still wrong. Cf. the attached .expand dump of long func1 (unsigned char a, unsigned char b, int c) { unsigned ab = a * b; return (long) ab * c; } ab is computed in insn 10 with zero-extended inputs: (insn 8 5 9 2 (set (reg:HI 53 [ D.1447 ]) (zero_extend:HI (reg/v:QI 49 [ a ]))) cecky.c:4 -1 (nil)) (insn 9 8 10 2 (set (reg:HI 54 [ D.1447 ]) (zero_extend:HI (reg/v:QI 50 [ b ]))) cecky.c:4 -1 (nil)) (insn 10 9 11 2 (set (reg:HI 55 [ D.1447 ]) (mult:HI (reg:HI 53 [ D.1447 ]) (reg:HI 54 [ D.1447 ]))) cecky.c:4 -1 This is correct, but then in insn 11 ab gets sign-extended even though it is unsigned: (insn 11 10 12 2 (set (reg:SI 56 [ D.1448 ]) (sign_extend:SI (reg:HI 55 [ D.1447 ]))) cecky.c:5 -1 (nil)) Insn combine combines this wrong extension into a widening multiplication.
