On 9 February 2011 22:28, Richard Guenther <richard.guent...@gmail.com> wrote: > 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). >
OK. We are using the same branch pattern for INT as well as all modes. For int, generating "brle" should be fine I think where as float this is wrong. So that leaves us to define a separate branch pattern for floating mode. It would be very interesting to know why a direct-conditional branch is turned to reverse-conditional branch at all... I have been skimming through jump.c and cfglayout.c for some clues. Anitha
