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<p class="MsoNormal" style="margin-bottom:0in;margin-bottom:.0001pt;line-height:normal">
The AMX registers are complicated. The single configuration register (which is mostly used implicitly, similar to MXCSR for floating point) controls the shape of all the tile registers, and if you change the tile configuration every single tile register is
cleared. In practice, if we have to change the the configuration while any of the tile registers are live, performance is going to be terrible. We need to handle this case for correctness, but users of this programming interface will need to have enough awareness
of the performance issues and the hardware details to prevent this. We’ll also want a diagnostic that lets the user know when this has happened.<o:p></o:p></p>
<p class="MsoNormal" style="margin-bottom:0in;margin-bottom:.0001pt;line-height:normal">
<o:p> </o:p></p>
<p class="MsoNormal" style="margin-bottom:0in;margin-bottom:.0001pt;line-height:normal">
When the tile configuration is set, the shape of each tile is locked in, so the individual tile registers aren’t interchangeable at that point. If a function needs 2x4 tiles, 4x2 tiles, and 4x4 tiles, the configuration needs to be set with this in mind. The
shape isn’t explicit in every instruction and intrinsic. It must be deduced. And again, we’ll need a way to tell the user when efficient allocation can’t be done. In practice, I don’t expect any function to be using more than three tile shapes.<o:p></o:p></p>
<p class="MsoNormal" style="margin-bottom:0in;margin-bottom:.0001pt;line-height:normal">
<o:p> </o:p></p>
<p class="MsoNormal" style="margin-bottom:0in;margin-bottom:.0001pt;line-height:normal">
The implication of all this is that I don’t think the greedy register allocator is well suited to figure all of this out. We need a special pass to pre-allocate these registers. If the function is written in a way that makes good performance possible, it should
be a relatively simple task to allocate everything with minimal spilling. If it isn’t possible to get good performance, we don’t need to do anything especially clever. We can just do something straightforward that is correct and let the user know that they
aren’t going to be happy with the results.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">-Andy<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<div>
<div style="border:none;border-top:solid #E1E1E1 1.0pt;padding:3.0pt 0in 0in 0in">
<p class="MsoNormal" style="margin-bottom:0in;margin-bottom:.0001pt;line-height:normal">
<b>From:</b> Philip Reames <listmail@philipreames.com> <br>
<b>Sent:</b> Friday, August 14, 2020 8:29 PM<br>
<b>To:</b> Luo, Yuanke <yuanke.luo@intel.com>; llvm-dev@lists.llvm.org; florian_hahn@apple.com; Kaylor, Andrew <andrew.kaylor@intel.com>; Topper, Craig <craig.topper@intel.com>; Lu, Hongjiu <hongjiu.lu@intel.com><br>
<b>Subject:</b> Re: [llvm-dev] Intel AMX programming model discussion.<o:p></o:p></p>
</div>
</div>
<p class="MsoNormal"><o:p> </o:p></p>
<p>I find your answer unconvincing. I'm not going to debate it as I don't wish to take the time to build the appropriate context, but my initial response is skepticism.<o:p></o:p></p>
<p>Philip<o:p></o:p></p>
<div>
<p class="MsoNormal">On 8/14/20 4:49 PM, Luo, Yuanke wrote:<o:p></o:p></p>
</div>
<blockquote style="margin-top:5.0pt;margin-bottom:5.0pt">
<p class="MsoNormal">[Yuanke] AMX register is special. It needs to be configured before use and the config instruction is expensive. To avoid unnecessary tile configure, we collect the tile shape information as much as possible and combine them into one ldtilecfg
instruction. The ldtilecfg instruction should dominate any AMX instruction that access tile register. On the other side, the ldtilecfg should post-dominated the instruction that define the tile shape. For tile register spill, it should avoid re-config due
to the different tile shape, the spilled register should be reloaded to the register that share the same tile shape. Since tile register allocation is special and it may allocate general virtual register to configure tile register, we can add a sperate pass
to do it before general register allocation pass. After register allocation, the tile shape information is not needed anymore, so we can transform the pseudo AMX instruction to real AMX instruction by removing the row and column operands.<o:p></o:p></p>
<p>[Philip]<o:p></o:p></p>
<p>This seems complicated.<o:p></o:p></p>
<p>Reading through the documentation, there appears to be a single global tile config for all tile registers at any time.<o:p></o:p></p>
<p>Why not simply model this tile config as a designated special register and the tile instructions as having an implicit use of this register? That would seem to ensure that the register allocator has all the constraints needed. You'd need to teach it how
to spill the special registers with the appropriate instructions, but that seems a lot more straight forward?<o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:10.5pt;line-height:105%">[Yuanke] In that case user need to configure the tile register by themselves. Spilling configure register is very expensive, because it clears all the tile data register to zero. In our proposal,
compiler is responsible to deduce the shape for virtual of tile data register, allocate physical registers for them and then configure those physical register. We may build the dependency as you proposed and it can be used for machine IR check to ensure tile
data register is configured before use. </span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:10.5pt;line-height:105%"> </span><o:p></o:p></p>
<div>
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<p class="MsoNormal" style="margin-bottom:0in;margin-bottom:.0001pt;line-height:normal">
<b>From:</b> Philip Reames <a href="mailto:listmail@philipreames.com"><listmail@philipreames.com></a>
<br>
<b>Sent:</b> Saturday, August 15, 2020 1:17 AM<br>
<b>To:</b> Luo, Yuanke <a href="mailto:yuanke.luo@intel.com"><yuanke.luo@intel.com></a>;
<a href="mailto:llvm-dev@lists.llvm.org">llvm-dev@lists.llvm.org</a>; <a href="mailto:florian_hahn@apple.com">
florian_hahn@apple.com</a>; Kaylor, Andrew <a href="mailto:andrew.kaylor@intel.com">
<andrew.kaylor@intel.com></a>; Topper, Craig <a href="mailto:craig.topper@intel.com">
<craig.topper@intel.com></a>; Lu, Hongjiu <a href="mailto:hongjiu.lu@intel.com"><hongjiu.lu@intel.com></a><br>
<b>Subject:</b> Re: [llvm-dev] Intel AMX programming model discussion.<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 8/14/20 6:27 AM, Luo, Yuanke via llvm-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">Intel Advanced Matrix Extensions (Intel AMX) is a new programming paradigm consisting of two components: a set of 2-dimensional registers (tiles) representing sub-arrays from a larger 2-dimensional memory image, and accelerators able to
operate on tiles. Capability of Intel AMX implementation is enumerated by palettes. Two palettes are supported: palette 0 represents the initialized state and palette 1 consists of 8 tile registers of up to 1 KB size, which is controlled by a tile control
register.<o:p></o:p></p>
<p class="MsoNormal">The instruction manual is posted at <a href="https://software.intel.com/content/www/us/en/develop/download/intel-architecture-instruction-set-extensions-programming-reference.html">
https://software.intel.com/content/www/us/en/develop/download/intel-architecture-instruction-set-extensions-programming-reference.html</a>.<o:p></o:p></p>
<p class="MsoNormal">The AMX abi proposal is posted at <a href="https://groups.google.com/g/x86-64-abi/c/NRejFm7pwb4">
https://groups.google.com/g/x86-64-abi/c/NRejFm7pwb4</a>.<o:p></o:p></p>
<p class="MsoNormal">This email is to discuss the programming model for AMX. Florian has introduced the matrix type and intrinsics in LLVM community. We’d like to adopt some ideas from it.<o:p></o:p></p>
<p class="MsoNormal">Here is what we propose for the AMX programming model.<o:p></o:p></p>
<p class="MsoListParagraph" style="margin-left:.5in;text-indent:-.25in;mso-list:l0 level1 lfo2">
<![if !supportLists]><span style="mso-list:Ignore">1.<span style="font:7.0pt "Times New Roman"">
</span></span><![endif]> Data type. <o:p></o:p></p>
<p class="MsoNormal">We’d like to have fixed vector type for AMX. Since the shape to AMX register can be configurable, the vector size is the maximum size of AMX register. That means the vector size is 1024 bytes.<o:p></o:p></p>
<p class="MsoNormal">The C code may look like this.<o:p></o:p></p>
<p class="MsoNormal">typedef int _tile_data __attribute__((__vector_size__(1024), __aligned__(64)));<o:p></o:p></p>
<p class="MsoNormal">_tile_data tile;<o:p></o:p></p>
<p class="MsoNormal">And the LLVM IR may look like this.<o:p></o:p></p>
<p class="MsoNormal">@tile = dso_local local_unnamed_addr global <256 x i32> zeroinitializer, align 64<o:p></o:p></p>
<p class="MsoNormal">For llvm IR, it is nice to have a new type x86_amxtile that can be mapped to AMX registers.<o:p></o:p></p>
<p class="MsoListParagraph" style="margin-left:.5in;text-indent:-.25in;mso-list:l0 level1 lfo2">
<![if !supportLists]><span style="mso-list:Ignore">2.<span style="font:7.0pt "Times New Roman"">
</span></span><![endif]>AMX Intrinsics. <o:p></o:p></p>
<p class="MsoNormal">The internal intrinsics are 1:1 mapped to AMX instructions. The parameter m, n, k identifies the shape of the tile. The shape can be variable, but it cannot exceed the size that AMX HW can support. Compiler can deduce shape of the tile
from the AMX intrinsics.<o:p></o:p></p>
<p class="MsoNormal" style="text-indent:5.5pt">_tile_data _tile_loadd_internal(char m, short n, const void *base, int stride);<o:p></o:p></p>
<p class="MsoNormal">_tile_data _tile_dpbssd_internal(char m, short n, short k, _tile_data dst, _tile_data src1, _tile_data src2);<o:p></o:p></p>
<p class="MsoNormal">_tile_data _tile_dpbf16ps_internal(char m, short n, short k, _tile_data dst, _tile_data src1, _tile_data src2);<o:p></o:p></p>
<p class="MsoNormal">void _tile_stored_internal(char m, short n, void *base, int stride, _tile_data tile);<o:p></o:p></p>
<p class="MsoListParagraph" style="margin-left:.5in;text-indent:-.25in;mso-list:l0 level1 lfo2">
<![if !supportLists]><span style="mso-list:Ignore">3.<span style="font:7.0pt "Times New Roman"">
</span></span><![endif]>User interfaces.<o:p></o:p></p>
<p class="MsoNormal">The tile shape and tile data are combined into a struct in C language. The shape of the tile is only allowed to be initialized once. The user interface looks as this.<o:p></o:p></p>
<p class="MsoNormal"> 3 #define __DEFAULT_FN_AMX \<o:p></o:p></p>
<p class="MsoNormal"> 4 __attribute__((__always_inline__, __nodebug__, __target__("amx-int8")))<o:p></o:p></p>
<p class="MsoNormal"> 9 typedef struct __tile_str {<o:p></o:p></p>
<p class="MsoNormal">10 const char row;<o:p></o:p></p>
<p class="MsoNormal">11 const short col;<o:p></o:p></p>
<p class="MsoNormal">12 _tile_data tile;<o:p></o:p></p>
<p class="MsoNormal">13 }__tile;<o:p></o:p></p>
<p class="MsoNormal">14<o:p></o:p></p>
<p class="MsoNormal">15 __DEFAULT_FN_AMX<o:p></o:p></p>
<p class="MsoNormal">16 void __tile_loadd(__tile *dst, const void *base, long stride) {<o:p></o:p></p>
<p class="MsoNormal">17 dst->tile = _tile_loadd_internal(dst->row, dst->col, base, stride);<o:p></o:p></p>
<p class="MsoNormal">18 }<o:p></o:p></p>
<p class="MsoNormal">19<o:p></o:p></p>
<p class="MsoNormal">20 __DEFAULT_FN_AMX<o:p></o:p></p>
<p class="MsoNormal">21 void __tile_dpbsud(__tile *dst, __tile src1, __tile src2) {<o:p></o:p></p>
<p class="MsoNormal">22 dst->tile = _tile_dpbssd_internal(src1.row, src2.col, src1.col, dst->tile, src1.tile, src2.tile);<o:p></o:p></p>
<p class="MsoNormal">23 }<o:p></o:p></p>
<p class="MsoNormal">24<o:p></o:p></p>
<p class="MsoNormal">25 __DEFAULT_FN_AMX<o:p></o:p></p>
<p class="MsoNormal">26 void __tile_stored(void *base, long stride, __tile src) {<o:p></o:p></p>
<p class="MsoNormal">27 _tile_stored_internal(src.row, src.col, base, stride, src.tile);<o:p></o:p></p>
<p class="MsoNormal">28 }<o:p></o:p></p>
<p class="MsoNormal"> <o:p></o:p></p>
<p class="MsoListParagraph" style="margin-left:.5in;text-indent:-.25in;mso-list:l0 level1 lfo2">
<![if !supportLists]><span style="mso-list:Ignore">4.<span style="font:7.0pt "Times New Roman"">
</span></span><![endif]>Example code<o:p></o:p></p>
<p class="MsoNormal">The example shows how to use the user interface in a function.
<o:p></o:p></p>
<p class="MsoNormal"> 51 void api(int cond, short row, short col) {<o:p></o:p></p>
<p class="MsoNormal">52 __tile a = {row, col};<o:p></o:p></p>
<p class="MsoNormal">53 __tile b = {row, col};<o:p></o:p></p>
<p class="MsoNormal">54 __tile c = {row, col};<o:p></o:p></p>
<p class="MsoNormal">55<o:p></o:p></p>
<p class="MsoNormal">56 if(cond) {<o:p></o:p></p>
<p class="MsoNormal">57 __tile_loadd(&a, buf, STRIDE);<o:p></o:p></p>
<p class="MsoNormal">58 __tile_loadd(&b, buf, STRIDE);<o:p></o:p></p>
<p class="MsoNormal">59 __tile_loadd(&c, buf, STRIDE);<o:p></o:p></p>
<p class="MsoNormal">60 } else {<o:p></o:p></p>
<p class="MsoNormal">61 __tile_loadd(&a, buf2, STRIDE);<o:p></o:p></p>
<p class="MsoNormal">62 __tile_loadd(&b, buf2, STRIDE);<o:p></o:p></p>
<p class="MsoNormal">63 __tile_loadd(&c, buf2, STRIDE);<o:p></o:p></p>
<p class="MsoNormal">64 }<o:p></o:p></p>
<p class="MsoNormal"><span lang="IT">65 __tile_dpbsud(&c, a, b);</span><o:p></o:p></p>
<p class="MsoNormal">66 __tile_stored(buf, STRIDE, c);<o:p></o:p></p>
<p class="MsoNormal">67 }<o:p></o:p></p>
<p class="MsoListParagraph" style="margin-left:.5in;text-indent:-.25in;mso-list:l0 level1 lfo2">
<![if !supportLists]><span style="mso-list:Ignore">5.<span style="font:7.0pt "Times New Roman"">
</span></span><![endif]>LLVM IR<o:p></o:p></p>
<p class="MsoNormal">The LLVM intrinsics IR take the row and column information as the input parameter, so that compiler can deduce the shape of tile data. The remaining parameters are what AMX instructions require. This is the LLVM IR corresponding to the
example code.<o:p></o:p></p>
<p class="MsoNormal">12 define dso_local void @api(i32 %cond, i16 signext %row, i16 signext %col) local_unnamed_addr #2 {<o:p></o:p></p>
<p class="MsoNormal">13 entry:<o:p></o:p></p>
<p class="MsoNormal">14 %tobool = icmp eq i32 %cond, 0<o:p></o:p></p>
<p class="MsoNormal">15 %sext = shl i16 %col, 8<o:p></o:p></p>
<p class="MsoNormal">16 %conv.i31 = ashr exact i16 %sext, 8<o:p></o:p></p>
<p class="MsoNormal">17 br i1 %tobool, label %if.else, label %if.then<o:p></o:p></p>
<p class="MsoNormal">18<o:p></o:p></p>
<p class="MsoNormal">19 if.then: ; preds = %entry<o:p></o:p></p>
<p class="MsoNormal">20 %0 = tail call <256 x i32> @llvm.x86.tileloadd64(i16 %row, i16 %conv.i31, i8* getelementptr inbounds ([1024 x i8], [1024 x i8]* @buf, i64 0, i64 0), i64 32) #3<o:p></o:p></p>
<p class="MsoNormal">21 %1 = tail call <256 x i32> @llvm.x86.tileloadd64(i16 %row, i16 %conv.i31, i8* getelementptr inbounds ([1024 x i8], [1024 x i8]* @buf, i64 0, i64 0), i64 32) #3<o:p></o:p></p>
<p class="MsoNormal">22 %2 = tail call <256 x i32> @llvm.x86.tileloadd64(i16 %row, i16 %conv.i31, i8* getelementptr inbounds ([1024 x i8], [1024 x i8]* @buf, i64 0, i64 0), i64 32) #3<o:p></o:p></p>
<p class="MsoNormal">23 br label %if.end<o:p></o:p></p>
<p class="MsoNormal">24<o:p></o:p></p>
<p class="MsoNormal">25 if.else: ; preds = %entry<o:p></o:p></p>
<p class="MsoNormal">26 %3 = tail call <256 x i32> @llvm.x86.tileloadd64(i16 %row, i16 %conv.i31, i8* getelementptr inbounds ([1024 x i8], [1024 x i8]* @buf2, i64 0, i64 0), i64 32) #3<o:p></o:p></p>
<p class="MsoNormal">27 %4 = tail call <256 x i32> @llvm.x86.tileloadd64(i16 %row, i16 %conv.i31, i8* getelementptr inbounds ([1024 x i8], [1024 x i8]* @buf2, i64 0, i64 0), i64 32) #3<o:p></o:p></p>
<p class="MsoNormal">28 %5 = tail call <256 x i32> @llvm.x86.tileloadd64(i16 %row, i16 %conv.i31, i8* getelementptr inbounds ([1024 x i8], [1024 x i8]* @buf2, i64 0, i64 0), i64 32) #3<o:p></o:p></p>
<p class="MsoNormal">29 br label %if.end<o:p></o:p></p>
<p class="MsoNormal">30<o:p></o:p></p>
<p class="MsoNormal">31 if.end: ; preds = %if.else, %if.then<o:p></o:p></p>
<p class="MsoNormal">32 %a.sroa.1186.0 = phi <256 x i32> [ %3, %if.else ], [ %0, %if.then ]<o:p></o:p></p>
<p class="MsoNormal">33 %b.sroa.1068.0 = phi <256 x i32> [ %4, %if.else ], [ %1, %if.then ]<o:p></o:p></p>
<p class="MsoNormal">34 %c.sroa.1149.0 = phi <256 x i32> [ %5, %if.else ], [ %2, %if.then ]<o:p></o:p></p>
<p class="MsoNormal">35 %6 = tail call <256 x i32> @llvm.x86.tdpbssd(i16 %row, i16 %conv.i31, i16 %conv.i31, <256 x i32> %c.sroa.1149.0, <256 x i32> %a.sroa.1186.0, <256 x i32> %b.sroa.1068.0) #3<o:p></o:p></p>
<p class="MsoNormal">36 tail call void @llvm.x86.tilestored64(i16 %row, i16 %conv.i31, i8* getelementptr inbounds ([1024 x i8], [1024 x i8]* @buf, i64 0, i64 0), i64 32, <256 x i32> %6) #3<o:p></o:p></p>
<p class="MsoNormal">37 ret void<o:p></o:p></p>
<p class="MsoNormal">38 }<o:p></o:p></p>
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<![if !supportLists]><span style="mso-list:Ignore">6.<span style="font:7.0pt "Times New Roman"">
</span></span><![endif]>Shape propagation<o:p></o:p></p>
<p class="MsoNormal">When in -O0 build, some general load/store for tile vector is generated by front-end. We need to root from AMX intrinsics to propagate the shape information to the virtual tile register. If the an AMX intrinsic use the result of load instruction,
the shape is propagated to the load and the load is transformed to tile load intrinsic. If the store instruction uses any result of AMX intrinsic, the shape is propagated to store instruction and the store is transformed to tile store intrinsic<o:p></o:p></p>
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<![if !supportLists]><span style="mso-list:Ignore">7.<span style="font:7.0pt "Times New Roman"">
</span></span><![endif]>Machine IR<o:p></o:p></p>
<p class="MsoNormal">Since the AMX intrinsics take the row and column as the input parameters, we can create a pseudo instruction corresponding to it. The AMX intrinsics are lowered to the pseudo AMX instruction which has extra row and column operands corresponding
to AMX intrinsic. The real AMX instructions don’t need the row and column operands. The row and column information should be configured by ldtilecfg before executing any AMX instruction.<o:p></o:p></p>
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<![if !supportLists]><span style="mso-list:Ignore">8.<span style="font:7.0pt "Times New Roman"">
</span></span><![endif]>Register allocation<o:p></o:p></p>
<p class="MsoNormal">AMX register is special. It needs to be configured before use and the config instruction is expensive. To avoid unnecessary tile configure, we collect the tile shape information as much as possible and combine them into one ldtilecfg instruction.
The ldtilecfg instruction should dominate any AMX instruction that access tile register. On the other side, the ldtilecfg should post-dominated the instruction that define the tile shape. For tile register spill, it should avoid re-config due to the different
tile shape, the spilled register should be reloaded to the register that share the same tile shape. Since tile register allocation is special and it may allocate general virtual register to configure tile register, we can add a sperate pass to do it before
general register allocation pass. After register allocation, the tile shape information is not needed anymore, so we can transform the pseudo AMX instruction to real AMX instruction by removing the row and column operands.<o:p></o:p></p>
</blockquote>
<p>This seems complicated.<o:p></o:p></p>
<p>Reading through the documentation, there appears to be a single global tile config for all tile registers at any time.<o:p></o:p></p>
<p>Why not simply model this tile config as a designated special register and the tile instructions as having an implicit use of this register? That would seem to ensure that the register allocator has all the constraints needed. You'd need to teach it how
to spill the special registers with the appropriate instructions, but that seems a lot more straight forward?<o:p></o:p></p>
<blockquote style="margin-top:5.0pt;margin-bottom:5.0pt">
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<![if !supportLists]><span style="mso-list:Ignore">9.<span style="font:7.0pt "Times New Roman"">
</span></span><![endif]>Use recommendation <o:p></o:p></p>
<p class="MsoNormal">Due to the shape configure issue, we recommend user to define the tile shape at the entry of the function entry and inline function as much as possible. The AMX instructions focus on computation instead of storage, so global variable for
tile data is not recommended.<o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:10.5pt;line-height:105%"> </span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:10.5pt;line-height:105%">Thanks</span><o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:10.5pt;line-height:105%">Yuanke</span><o:p></o:p></p>
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<br>
<br>
<br>
<o:p></o:p></p>
<pre>_______________________________________________<o:p></o:p></pre>
<pre>LLVM Developers mailing list<o:p></o:p></pre>
<pre><a href="mailto:llvm-dev@lists.llvm.org">llvm-dev@lists.llvm.org</a><o:p></o:p></pre>
<pre><a href="https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev">https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev</a><o:p></o:p></pre>
</blockquote>
</blockquote>
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