[PATCH] R600/SI: Use source modifiers for f64 fabs / fneg

[Matt Arsenault]

On Aug 4, 2014, at 1:09 AM, Michel Dänzer <michel at daenzer.net> wrote:

> On 02.08.2014 11:33, Matt Arsenault wrote:
>> f32 has a comment and patterns about the behavior with 0 and negate
>> modifier which I'm unsure about. I think the source modifier should
>> work if the source is 0, since the negate modifier is supposed to be
>> applied before abs. Does this pattern need to be skipped for f64
>> too?
> 
> The original problem was that fneg was implemented as V_ADD_F32_e64 of 0
> and the operand with the negate source modifier set. This resulted in
> +0.0 instead of -0.0 (i.e. the sign bit was not set) for an argument of
> +0.0. There's a (graphics) piglit test which failed because of that.
> 
> Given that FNEG_SI and FABS_SI directly manipulate the sign bit, I
> wouldn't expect that to require any special treatment anymore. As you
> say, the source modifiers should work fine when folding these into other
> operations.
> 


> So, I suspect the purpose of the (fneg (fabs)) pattern isn't to avoid
> that problem but to avoid expanding both FABS_SI and FABS_SI.


Yes, that seems to be what it accomplishes. f64 fneg is still implemented that way because there is no v_sub_f64. Here is an updated set that implements that for f64. These also switch to preferring using the literal constant as the first operand, rather than loading the immediate into a register.

-------------- next part --------------
A non-text attachment was scrubbed...
Name: 0001-R600-SI-Refactor-fneg-fabs-patterns.patch
Type: application/octet-stream
Size: 2036 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20140805/ab886f8f/attachment.obj>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: 0002-R600-SI-Use-source-modifier-for-f64-fabs.patch
Type: application/octet-stream
Size: 6397 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20140805/ab886f8f/attachment-0001.obj>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: 0003-R600-SI-Use-source-modifiers-for-f64-fneg.patch
Type: application/octet-stream
Size: 10622 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20140805/ab886f8f/attachment-0002.obj>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: 0004-R600-SI-Improve-handling-of-fabs-fneg.patch
Type: application/octet-stream
Size: 11481 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20140805/ab886f8f/attachment-0003.obj>