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<p class="MsoNormal"><span style="color:#1F497D">Hi Sam,<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 agree the original implementation of sampler type in LLVM isn’t ideal.<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">Your approach seems sensible to me. Would this mean that all sampler variables will be transformed to local by Clang?<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">Thanks,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">Anastasia <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"">From:</span></b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> Liu, Yaxun (Sam) [mailto:Yaxun.Liu@amd.com]
<br>
<b>Sent:</b> 09 June 2016 21:18<br>
<b>To:</b> cfe-dev (cfe-dev@lists.llvm.org); Anastasia Stulova; Bader, Alexey (alexey.bader@intel.com); Pan, Xiuli<br>
<b>Cc:</b> Tom Stellard; Sumner, Brian; Tye, Tony<br>
<b>Subject:</b> [RFC][OpenCL] New representation for sampler_t and its literal initializer<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><span lang="EN-US">Hi,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Currently Clang use int32 to represent sampler_t, which have been a source of issue for some backends, because in some backends sampler_t cannot be represented by int32. They have to depend on kernel argument metadata
and use IPA to find the sampler arguments and global variables and transform them to target specific sampler type.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Khronos Clang spirv-1.0 branch (<a href="https://github.com/KhronosGroup/SPIR/tree/spirv-1.0">https://github.com/KhronosGroup/SPIR/tree/spirv-1.0</a> ) represents sampler_t as an opaque struct pointer. Also it represents
sampler literal as a concrete struct type, and cast its pointer to a pointer to the opaque struct type for sampler_t. However there are two issues with the way how sampler literal is represented:<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt"><span lang="EN-US">1.</span><span lang="EN-US" style="font-size:7.0pt;font-family:"Times New Roman","serif"">
</span><span lang="EN-US">Backends still need to translate the sampler initializer struct to target specific sampler initializer, and this transformation cannot be implemented by library functions. Instead, it needs to be done by passes.<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt"><span lang="EN-US">2.</span><span lang="EN-US" style="font-size:7.0pt;font-family:"Times New Roman","serif"">
</span><span lang="EN-US">Optimizer may try to optimize the sampler global variable initialized with a sampler initializer struct. The optimizer will assume the sampler contains the bits of the initializer struct and may do memory optimizations on it, which
will cause difficulty for backends to transform the samplers.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">We think representing sampler as an opaque type is a good idea, which we suggest Clang trunk to adopt. And we propose a another way to represent sampler literal. Basically in Clang codegen we generate a function call
for each reference of a sampler global variable initialized with a sampler literal, e.g.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">sampler_t s = 0;<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">void f() {<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> g(s);<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">}<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt"><span lang="EN-US" style="font-family:Wingdings">ð</span><span lang="EN-US" style="font-size:7.0pt;font-family:"Times New Roman","serif"">
</span><span lang="EN-US">Llvm bitcode equivalent to (assuming __sampler is the opaque struct type to represent sampler_t):<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">constant __sampler *__attribute__((always_inline)) __initialize_sampler(int); // a builtin function for initialize a sampler<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">void f() {<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> constant __sampler *_s = __initialize_sampler(0);<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> g(_s);<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">}<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Each builtin library can implement its own __initialize_sampler(). Since the real sampler type tends to be architecture dependent, allowing it to be initialized by a library function simplifies backend design. A typical
implementation of __initialize_sampler could be a table lookup of real sampler literal values. Since its argument is always a literal, the returned pointer is known at compile time and easily optimized to finally become some literal values directly put into
image read instructions.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">The advantage of this representation is:<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt"><span lang="EN-US">1.</span><span lang="EN-US" style="font-size:7.0pt;font-family:"Times New Roman","serif"">
</span><span lang="EN-US">Robust - can be optimized without losing information<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt"><span lang="EN-US">2.</span><span lang="EN-US" style="font-size:7.0pt;font-family:"Times New Roman","serif"">
</span><span lang="EN-US">Easy to implement – can be implemented by library instead of pass<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Your feedbacks are welcome. Thanks.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Sam<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
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