On Tue, Oct 29, 2013 at 6:18 PM, Cong Hou <co...@google.com> wrote: >>> For the define_expand I added as below, the else body is there to >>> avoid fall-through transformations to ABS operation in optabs.c. >>> Otherwise ABS will be converted to other operations even that we have >>> corresponding instructions from SSSE3. >> >> No, it wont be. >> >> Fallthrough will generate the pattern that will be matched by the insn >> pattern above, just like you are doing by hand below. > > > I think the case is special for abs(). In optabs.c, there is a > function expand_abs() in which the function expand_abs_nojump() is > called. This function first tries the expand function defined for the > target and if it fails it will try max(v, -v) then shift-xor-sub > method. If I don't generate any instruction for SSSE3, the > fall-through will be max(v, -v). I have tested it on my machine.
Huh, strange. Then you can rename previous pattern to abs<mode>2_1 and call it from the new expander instead of expanding it manually. Please also add a small comment, describing the situation to prevent future "optimizations" in this place. Thanks, Uros.
