Joakim Tjernlund <joakim.tjernl...@transmode.se> writes:
> Ian Lance Taylor <i...@google.com> wrote on 2010/04/25 20:07:03:
>>
>> Joakim Tjernlund <joakim.tjernl...@transmode.se> writes:
>>
>> > Noticed that gcc 4.3.4 doesn't optimize "add with carry" properly:
>
> BTW, I can see in gcc src:
> (define_insn ""
> [(set (match_operand:CC 0 "cc_reg_operand" "=x,?y")
> (compare:CC
> (plus:SI (gt:SI (match_operand:SI 1 "gpc_reg_operand" "r,r")
> (const_int 0))
> (match_operand:SI 2 "gpc_reg_operand" "r,r"))
> (const_int 0)))
> (clobber (match_scratch:SI 3 "=&r,&r"))]
> "TARGET_32BIT"
> "@
> {a|addc} %3,%1,%1\;{sfe|subfe} %3,%1,%3\;{aze.|addze.} %3,%2
> #"
> [(set_attr "type" "compare")
> (set_attr "length" "12,16")])
> so there seems to be possible for gcc to emit them but how do
> I make gcc do that?
That unfortunately does not do what you want. That instruction
sequence uses a scratch register to set a condition register to
whether (a + b) > 0.
Ian
