[llvm-dev] [PPC, APFloat] Add full PPCDoubleDouble to APFloat

[Hal Finkel via llvm-dev]

----- Original Message -----

> From: "Tim Shen" <timshen at google.com>
> To: "Hal Finkel" <hfinkel at anl.gov>
> Cc: resistor at mac.com, "Fiona Glaser" <escha at apple.com>, "Stephen
> Canon" <scanon at apple.com>, "Eric Christopher" <echristo at gmail.com>,
> llvm-dev at lists.llvm.org
> Sent: Monday, October 3, 2016 3:52:12 PM
> Subject: Re: [PPC, APFloat] Add full PPCDoubleDouble to APFloat

> On Mon, Oct 3, 2016 at 1:02 PM Hal Finkel < hfinkel at anl.gov > wrote:

> > There might be some tradeoff here between performance and
> > maintainability.
> 

> I don't think that it's a tradeoff. We can have both. For example:

> struct IEEEPayload {
> union Significand { integerPart part; integerPart *parts; }
> significand;
> int exponent;
> int category : 3;
> int sign : 1;
> };

> struct PPCDoubleDoublePayload {
> struct {
> int exponent1, exponent2;
> integerPart significands[]; // flexible array member, or
> llvm::TrailingObjects instead.
> } *heapdata;

> int category : 3;

> int sign : 2;

> };

> class APFloat {
> fltSemantics *semantics;
> union {
> IEEEPayload ieee;
> PPCDoubleDoublePayload ppc;
> } payload;
> };

> It's just an example of how we can use the same shallow space.

> My previous point is that, I want to build an internal abstraction in
> the hope to reduce technical debt and maintenance burden, but I\we
> need to do it correctly.
That looks good to me (using llvm::TrailingObjects seems a bit cleaner than the trailing array). 

-Hal 

> > APFloat already isn't *fast*, so I'm not super worried about
> > introducing some extra checks inside the functions, but I'm more
> > worried about increasing the size of the APFloat object itself,
> > because we might have *a lot* of these in memory at once. As a
> > result, I'd prefer a solution that did not increase the size of the
> > base APFloat object (because then everyone pays a price for this
> > and
> > most people won't use it).
> 

> > Thanks again,
> 
> > Hal
> 

> > > > Thanks again,
> > > 
> > 
> 
> > > > Hal
> > > 
> > 
> 

> > > > > From: "Tim Shen" < timshen at google.com >
> > > 
> > 
> 
> > > > > To: llvm-dev at lists.llvm.org
> > > 
> > 
> 
> > > > > Cc: resistor at mac.com , "Hal Finkel" < hfinkel at anl.gov >,
> > > > > "Fiona
> > > > > Glaser" < escha at apple.com >, "Stephen Canon"
> > > 
> > 
> 
> > > > > < scanon at apple.com >, "Eric Christopher" < echristo at gmail.com
> > > > > >
> > > 
> > 
> 
> > > > > Sent: Friday, September 30, 2016 4:43:12 PM
> > > 
> > 
> 
> > > > > Subject: [PPC, APFloat] Add full PPCDoubleDouble to APFloat
> > > 
> > 
> 
> > > > >
> > > 
> > 
> 
> > > > >
> > > 
> > 
> 
> > > > >
> > > 
> > 
> 
> > > > >
> > > 
> > 
> 
> > > > > I have found some internal test failures due to the wrong
> > > > > constant
> > > 
> > 
> 
> > > > > folding on ppc_fp128.
> > > 
> > 
> 
> > > > >
> > > 
> > 
> 
> > > > >
> > > 
> > 
> 
> > > > > As documented in APFloat::PPCDoubleDouble, APFloat doesn't
> > > > > support
> > > 
> > 
> 
> > > > > PowerPC double-double correctly <
> > > 
> > 
> 
> > > > > https://github.com/llvm-mirror/llvm/blob/492acdd450bcdf9837494d6da029ed064f14fc33/lib/Support/APFloat.cpp#L74
> > > 
> > 
> 
> > > > > >.
> > > 
> > 
> 
> > > > >
> > > 
> > 
> 
> > > > >
> > > 
> > 
> 
> > > > > To support this, we need to add a second tuple of (sign,
> > > > > exponent,
> > > 
> > 
> 
> > > > > significand) to APFloat. I wonder where should I start to
> > > > > change,
> > > > > so
> > > 
> > 
> 
> > > > > that it's less hacky? I certainly expect refactoring to come.
> > > > > :)
> > > 
> > 
> 
> > > > >
> > > 
> > 
> 
> > > > >
> > > 
> > 
> 
> > > > > Thanks!
> > > 
> > 
> 

> > > > --
> > > 
> > 
> 
> > > > Hal Finkel
> > > 
> > 
> 
> > > > Lead, Compiler Technology and Programming Languages
> > > 
> > 
> 
> > > > Leadership Computing Facility
> > > 
> > 
> 
> > > > Argonne National Laboratory
> > > 
> > 
> 

> > --
> 

> > Hal Finkel
> 
> > Lead, Compiler Technology and Programming Languages
> 
> > Leadership Computing Facility
> 
> > Argonne National Laboratory
> 

-- 

Hal Finkel 
Lead, Compiler Technology and Programming Languages 
Leadership Computing Facility 
Argonne National Laboratory 
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