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<p class="MsoNormal"><span style="color:#1F497D">Hi Hal,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">You mentioned “I'd be in favor of changing the current semantics”. Just checking: do you mean the semantics of __fp16?<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">Because that is exactly what we are trying to avoid with introducing a new true half type; changing semantics of fp16 would break backward compatibility.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">> By "when required", do you mean when the result would<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">> be the same as if the operation had been performed in single
<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">> precision? If so, then no, we need different semantics<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">I think that is indeed the case, but I am double checking that.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">Cheers,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">Sjoerd.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif";color:windowtext;mso-fareast-language:EN-GB">From:</span></b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif";color:windowtext;mso-fareast-language:EN-GB">
Hal Finkel [mailto:hfinkel@anl.gov] <br>
<b>Sent:</b> 10 May 2017 17:40<br>
<b>To:</b> Sjoerd Meijer; Martin J. O'Riordan<br>
<b>Cc:</b> 'clang developer list'; nd<br>
<b>Subject:</b> Re: [cfe-dev] [RFC] implementation of _Float16<o:p></o:p></span></p>
</div>
</div>
<p class="MsoNormal"><o:p> </o:p></p>
<p><o:p> </o:p></p>
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<p class="MsoNormal">On 05/10/2017 11:15 AM, Sjoerd Meijer wrote:<o:p></o:p></p>
</div>
<blockquote style="margin-top:5.0pt;margin-bottom:5.0pt">
<p class="MsoNormal"><span style="color:#1F497D">The thing that confused me again is that for simple expressions/examples like this:</span><o:p></o:p></p>
<p class="MsoNormal"><span style="color:#1F497D"> </span><o:p></o:p></p>
<p class="MsoNormal"><span style="color:#1F497D">__fp16 MyAdd(__fp16 a, __fp16 b) {</span><o:p></o:p></p>
<p class="MsoNormal"><span style="color:#1F497D"> return a + b;</span><o:p></o:p></p>
<p class="MsoNormal"><span style="color:#1F497D">}</span><o:p></o:p></p>
<p class="MsoNormal"><span style="color:#1F497D"> </span><o:p></o:p></p>
<p class="MsoNormal"><span style="color:#1F497D">The IR does not include promotions/truncations which you would expect (because operations are done on floats):</span><o:p></o:p></p>
<p class="MsoNormal"><span style="color:#1F497D"> </span><o:p></o:p></p>
<p class="MsoNormal"><span style="color:#1F497D">define half @MyAdd(half %a, half %b) local_unnamed_addr #0 {</span><o:p></o:p></p>
<p class="MsoNormal"><span style="color:#1F497D">entry:</span><o:p></o:p></p>
<p class="MsoNormal"><span style="color:#1F497D"> %0 = fadd half %a, %b</span><o:p></o:p></p>
<p class="MsoNormal"><span style="color:#1F497D"> ret half %0</span><o:p></o:p></p>
<p class="MsoNormal"><span style="color:#1F497D">}</span><o:p></o:p></p>
<p class="MsoNormal"><span style="color:#1F497D"> </span><o:p></o:p></p>
<p class="MsoNormal"><span style="color:#1F497D">But that is only because there is this optimisation that does not include them if it can prove that the result with/without these converts is the same, so in other cases the promotes/truncates are there as expected.</span><o:p></o:p></p>
<p class="MsoNormal"><span style="color:#1F497D"> </span><o:p></o:p></p>
<p class="MsoNormal"><span style="color:#1F497D">This means that Clang produces the necessary promotions when needed, and that a new _Float16 type can also be mapped onto the LLRM IR half type I think (no changes needed). Yes, then the approach could indeed
be to treat it as a native type, and only promote operands to floats when required.</span><o:p></o:p></p>
</blockquote>
<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Times New Roman","serif";mso-fareast-language:EN-GB"><br>
By "when required", do you mean when the result would be the same as if the operation had been performed in single precision? If so, then no, we need different semantics. That having been said, I'd be in favor of changing the current semantics to require explicit
promotions/truncations, change the existing optimization to elide them when they're provably redundant (as we do with other such things), and then only have a single, true, half-precision type. I suspect that we'd need to figure out how to auto-upgrade, but
that seems doable.<br>
<br>
-Hal<br>
<br>
<br>
<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"> </span><o:p></o:p></p>
<p class="MsoNormal"><span style="color:#1F497D">Cheers,</span><o:p></o:p></p>
<p class="MsoNormal"><span style="color:#1F497D">Sjoerd.</span><o:p></o:p></p>
<p class="MsoNormal"><span style="color:#1F497D"> </span><o:p></o:p></p>
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<p class="MsoNormal"><b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif";color:windowtext;mso-fareast-language:EN-GB">From:</span></b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif";color:windowtext;mso-fareast-language:EN-GB">
Hal Finkel [<a href="mailto:hfinkel@anl.gov">mailto:hfinkel@anl.gov</a>] <br>
<b>Sent:</b> 10 May 2017 16:00<br>
<b>To:</b> Martin J. O'Riordan; Sjoerd Meijer<br>
<b>Cc:</b> 'clang developer list'<br>
<b>Subject:</b> Re: [cfe-dev] [RFC] implementation of _Float16</span><o:p></o:p></p>
</div>
</div>
<p class="MsoNormal"> <o:p></o:p></p>
<p> <o:p></o:p></p>
<div>
<p class="MsoNormal">On 05/10/2017 09:01 AM, Martin J. O'Riordan wrote:<o:p></o:p></p>
</div>
<blockquote style="margin-top:5.0pt;margin-bottom:5.0pt">
<p class="MsoNormal"><span style="font-size:12.0pt">Yes, I see how this would be an issue if it is necessary to keep the storage-only versus native types separate.</span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:12.0pt"> </span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:12.0pt">At the moment I have ‘short float’ internally associated with OpenCL’s ‘half’ but I do not enable ‘half’ as a keyword. Independently I have made ‘__fp16’ when used with our target also be a synonym for ‘short
float/half’ (simply to avoid adding a new keyword). This in turn is bound to the IEEE FP16 using ‘HalfFormat = &llvm::APFloat::IEEEhalf();’.</span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:12.0pt"> </span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:12.0pt">In our case it is always a native type and never a storage only type, so coupling ‘__fp16’ to ‘half’ made sense. Certainly if the native versus storage-only variants were distinct, then this association I
have made would have to be decoupled (not a big-deal).</span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:12.0pt"> </span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:12.0pt">Another approach might be to always work with FP16 as-if native, but to provide only Load/Store instructions in the TableGen descriptions for FP16, and to adapt lowering to always perform the arithmetic using
FP32 if the selected target does not support native FP16 - would that be feasible in your case? In this way it is not really any different to how targets that have no FPU can use an alternative integer based implementation (with the help of ‘compiler-rt’).</span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:12.0pt"> </span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:12.0pt">I can certainly see how something like ‘ADD’ of ‘f16’ could be changed to use ‘Expand’ in lowering rather than ‘Legal’ as a function of the selected target (or some other target specific option) - we just
marked it ‘Legal’ and provided the corresponding instructions in TableGen with very little custom lowering necessary. I have a mild concern that LLVM would have to have an ‘f16’ which is native and another kind-of ‘f16’ restricted to being only storage.</span><o:p></o:p></p>
</blockquote>
<p class="MsoNormal"><span style="font-size:12.0pt"><br>
Why? That should only be true if they have different semantics.<br>
<br>
-Hal<br>
<br>
<br>
<br>
</span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:12.0pt"> </span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:12.0pt">Thanks,</span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:12.0pt"> </span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:12.0pt"> MartinO</span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:12.0pt"> </span><o:p></o:p></p>
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<p class="MsoNormal"><b><span lang="EN-US" style="color:windowtext;mso-fareast-language:EN-IE">From:</span></b><span lang="EN-US" style="color:windowtext;mso-fareast-language:EN-IE"> Sjoerd Meijer [<a href="mailto:Sjoerd.Meijer@arm.com">mailto:Sjoerd.Meijer@arm.com</a>]
<br>
<b>Sent:</b> 10 May 2017 14:19<br>
<b>To:</b> Martin J. O'Riordan <a href="mailto:martin.oriordan@movidius.com"><martin.oriordan@movidius.com></a>; 'Hal Finkel'
<a href="mailto:hfinkel@anl.gov"><hfinkel@anl.gov></a><br>
<b>Cc:</b> 'clang developer list' <a href="mailto:cfe-dev@lists.llvm.org"><cfe-dev@lists.llvm.org></a><br>
<b>Subject:</b> RE: [cfe-dev] [RFC] implementation of _Float16</span><o:p></o:p></p>
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<p class="MsoNormal"> <o:p></o:p></p>
<p class="MsoNormal"><span style="color:#1F497D">Hi Hal, Martin,</span><o:p></o:p></p>
<p class="MsoNormal"><span style="color:#1F497D"> </span><o:p></o:p></p>
<p class="MsoNormal"><span style="color:#1F497D">Thanks for the feedback.</span><o:p></o:p></p>
<p class="MsoNormal"><span style="color:#1F497D">Yes, the issue indeed is that ‘__fp16’ is already used to implement a storage-only type. And earlier I wrote that I don’t expect LLVM IR changes, but now I am not so sure anymore if both types map onto the same
half LLVM IR type. With two half precision types, __fp16 and _Float16, where one is a storage only type and the other a native type, somehow the distinction between these two must be made I think.
</span><o:p></o:p></p>
<p class="MsoNormal"><span style="color:#1F497D"> </span><o:p></o:p></p>
<p class="MsoNormal"><span style="color:#1F497D">Cheers,</span><o:p></o:p></p>
<p class="MsoNormal"><span style="color:#1F497D">Sjoerd.</span><o:p></o:p></p>
<p class="MsoNormal"><span style="color:#1F497D"> </span><o:p></o:p></p>
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<p class="MsoNormal"><b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif";color:windowtext;mso-fareast-language:EN-GB">From:</span></b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif";color:windowtext;mso-fareast-language:EN-GB">
Martin J. O'Riordan [<a href="mailto:martin.oriordan@movidius.com">mailto:martin.oriordan@movidius.com</a>]
<br>
<b>Sent:</b> 10 May 2017 14:13<br>
<b>To:</b> 'Hal Finkel'; Sjoerd Meijer<br>
<b>Cc:</b> 'clang developer list'<br>
<b>Subject:</b> RE: [cfe-dev] [RFC] implementation of _Float16</span><o:p></o:p></p>
</div>
</div>
<p class="MsoNormal"> <o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:12.0pt">Our Out-of-Tree target implements fully native FP16 operations based on ‘__fp16’ (scalar and SIMD vector), so is the issue for ARM that ‘__fp16’ is already used to implement a storage-only type and that another
type is needed to differentiate between a native and a storage-only type? Once the ‘f16’ type appears in the IR (and the vector variants) the code-generation is straightforward enough.</span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:12.0pt"> </span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:12.0pt">Certainly we have had to make many changes to CLang and to LLVM to fully implement this including suppression of implicit conversion to ‘double’, but nothing scary or obscure. Many of these changes are simply
to enable something that is already normal for OpenCL, but to do so for C and C++.</span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:12.0pt"> </span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:12.0pt">More controversially we also added a “synonym” for this using ‘short float’ rather than ‘_Float16’ (or OpenCL’s ‘half’), and created a parallel set of the ISO C library functions using ‘s’ to suffix the usual
names (e.g. ‘tan’, ‘tanf’, ‘tanl’ plus ‘tans’). The ‘s’ suffix was unambiguous (though we actually use the double-underscore prefix, e.g. ‘__tans’ to avoid conflict with the user’s names) and the type ‘short float’ was available too without breaking anything.
Enabling the ‘h’ suffix for FP constants (again from OpenCL) makes the whole fit smoothly with the normal FP types.</span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:12.0pt"> </span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:12.0pt">However, for variadic functions (such as ‘printf’) we do promote to ‘double’ because there are no formatting specifiers available for ‘half’ any more than there is support for ‘float’ - it is also consistent
with ‘va_arg’ usage for ‘char’ and ‘short’ as ‘int’. My feeling is that using implementation defined types ‘float’, ‘double’ and ‘long double’ can be extended to include ‘short float’ without dictating that they have any particular bit-sizes (e.g. FP16 for
‘half’).</span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:12.0pt"> </span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:12.0pt">This solution has worked very well over the past few years and is symmetric with the other floating-point data types.</span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:12.0pt"> </span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:12.0pt">There are some issues with C++ and overloading because ‘__fp16’ to other FP types (and INT types) is not ranked in exactly the same way as for example ‘float’ is to other FP types; but this is really only
because it is not a 1<sup>st</sup> class citizen of the type-system and the rules would need to be specified to make this valid. I have not tried to fix this as it works reasonably well as it is, and it would really be an issue for the C++ committee to decide
if they ever choose to adopt another FP data type. I did add it to the traits in the C++ library though so that it is considered legal for floating-point types.</span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:12.0pt"> </span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:12.0pt">I’d love to see this adopted as a formal type in a future version of ISO C and ISO C++.</span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:12.0pt"> </span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:12.0pt"> MartinO</span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:12.0pt"> </span><o:p></o:p></p>
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<p class="MsoNormal"><b><span lang="EN-US" style="color:windowtext;mso-fareast-language:EN-IE">From:</span></b><span lang="EN-US" style="color:windowtext;mso-fareast-language:EN-IE"> cfe-dev [<a href="mailto:cfe-dev-bounces@lists.llvm.org">mailto:cfe-dev-bounces@lists.llvm.org</a>]
<b>On Behalf Of </b>Hal Finkel via cfe-dev<br>
<b>Sent:</b> 10 May 2017 11:39<br>
<b>To:</b> Sjoerd Meijer <<a href="mailto:Sjoerd.Meijer@arm.com">Sjoerd.Meijer@arm.com</a>>;
<a href="mailto:cfe-dev@lists.llvm.org">cfe-dev@lists.llvm.org</a><br>
<b>Subject:</b> Re: [cfe-dev] [RFC] implementation of _Float16</span><o:p></o:p></p>
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<p class="MsoNormal"> <o:p></o:p></p>
<div>
<p class="MsoNormal">On 05/10/2017 05:18 AM, Sjoerd Meijer via cfe-dev wrote:<o:p></o:p></p>
</div>
<blockquote style="margin-top:5.0pt;margin-bottom:5.0pt">
<p class="MsoNormal">Hi,<o:p></o:p></p>
<p class="MsoNormal"> <o:p></o:p></p>
<p class="MsoNormal">ARMv8.2-A introduces as an optional extension half-precision data-processing instructions for Advanced SIMD and floating-point in both AArch64 and AArch32 states [1], and we are looking into implementing C/C++-language support for these
new ARMv8.2-A half-precision instructions.<o:p></o:p></p>
<p class="MsoNormal"> <o:p></o:p></p>
<p class="MsoNormal">We would like to introduce a new Clang type. The reason is that we e.g. cannot use type __fp16 (defined in the ARM C Language Extensions [2]) because it is a storage type only. This means when using standard C operators values of __fp16
type promote to float when used in arithmetic operations, which we would like to avoid for the ARMv8.2-A half-precision instructions. Please note that the LLVM IR already has a half precision type, onto which for example __fp16 is mapped, so there are no changes
or additions required for the LLVM IR.<o:p></o:p></p>
<p class="MsoNormal"> <o:p></o:p></p>
<p class="MsoNormal">As a new Clang type we would like to propose _Float16 as defined in a C11 extension, see [3]. Arithmetic is well defined, it is not only a storage type as __fp16. Our question is whether a partial implementation, just implementing this
type and not claiming (full) C11 conformance is acceptable?<o:p></o:p></p>
</blockquote>
<p class="MsoNormal"><span style="font-size:12.0pt"><br>
I would very much like to see fp16 as a first-class floating-point type in Clang and LLVM (i.e. handling that is not just a storage type). Doing this in Clang in a way that is specified by C11 seems like the right approach. I don't see why implementing this
would be predicated on implementing other parts of C11.<br>
<br>
-Hal</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-size:12.0pt"><br>
<br>
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</span><o:p></o:p></p>
<pre>-- <o:p></o:p></pre>
<pre>Hal Finkel<o:p></o:p></pre>
<pre>Lead, Compiler Technology and Programming Languages<o:p></o:p></pre>
<pre>Leadership Computing Facility<o:p></o:p></pre>
<pre>Argonne National Laboratory<o:p></o:p></pre>
<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Times New Roman","serif";mso-fareast-language:EN-GB"><br>
<br>
<o:p></o:p></span></p>
<pre>-- <o:p></o:p></pre>
<pre>Hal Finkel<o:p></o:p></pre>
<pre>Lead, Compiler Technology and Programming Languages<o:p></o:p></pre>
<pre>Leadership Computing Facility<o:p></o:p></pre>
<pre>Argonne National Laboratory<o:p></o:p></pre>
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