Re: RTL Expand pass - Difference in float and int datatypes

[Richard Guenther <richard dot guenther at gmail dot com>]

On Wed, Feb 9, 2011 at 5:55 PM, Anitha Boyapati
<anitha.boyapati@gmail.com> wrote:
> On 9 February 2011 20:34, Ian Lance Taylor <iant@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.