On Wed, Feb 9, 2011 at 5:55 PM, Anitha Boyapati <anitha.boyap...@gmail.com> wrote: > On 9 February 2011 20:34, Ian Lance Taylor <i...@google.com> wrote: > >>> I would like to know what prompts gcc to decide if "le" can be used in >>> the expand pass rather than "gt" operator. Or more precisely why it >>> differs incase of float. >> >> The choice of LE when using int is just a happenstance of the way that >> gcc generates code. When gcc comes to RTL generation it happens to be >> looking at an if statement that falls through on true and branches on >> else. So it flips the condition to branch on true (in >> do_compare_rtx_and_jump). The use of volatile doesn't affect this: it >> will still only load the variables precisely as described in the >> program. >> >> The condition can not be flipped for float because that would be an >> invalid transformation if one of the values is NaN. >> > > ok. I get it. > > I would like to understand some more of reverse-conditional branches > in case of float. For the target I am working on (AVR32), there is a > floating point unit which follows IEEE 754. The port is relatively > stable. We are now incrementally building support for these FPU > instructions (like fadd, fmul, fcmp). > > Now for the same test case with float data type, after expand pass, in > one of the passes (outof_cfglayout) the direct-conditional branches > are turned to reverse-conditional branches. > > > Direct-conditional branch > >> (jump_insn 9 8 34 3 gt.c:4 (set (pc) >> (if_then_else (gt:CC (cc0) >> (const_int 0 [0x0])) >> (label_ref 12) >> (pc))) -1 (nil)) > > Reverse-conditional Branch > >> (jump_insn 9 8 34 3 gt.c:4 (set (pc) >> (if_then_else (gt:CC (cc0) >> (const_int 0 [0x0])) >> (pc))) -1 (nil)) >> (label_ref 14) > > (Sorry that I don't have access to the installation right now, so I > just manually modified the reverse-conditional branch. The idea is to > illustrate that label_ref and pc are interchanged). > > The latter pattern is supposed to emit assembly which tests for the > reverse-condition. For instance if the former pattern emits assembly > instruction like "brgt <label>" then the latter pattern is supposed to > emit instruction like "brle <label>"
That's the misunderstanding. The above should emit a brngt <label> (branch if !gt). Richard.
