From: Joshua Harlan Lifton Date: Fri, 8 Feb 2019 03:19:37 +0000 (-0800) Subject: Copy edit update 005 X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;ds=sidebyside;h=75589f95cc8683c052e96a091188e8337dcb292b;p=crowdsupply.git Copy edit update 005 --- diff --git a/updates/005_2018dec14_simd_without_simd.mdwn b/updates/005_2018dec14_simd_without_simd.mdwn index 6548729..b33158f 100644 --- a/updates/005_2018dec14_simd_without_simd.mdwn +++ b/updates/005_2018dec14_simd_without_simd.mdwn @@ -1,160 +1,166 @@ -# Microarchitectural Design by Osmosis - -In a series of different descriptions and evaluations, a picture is -beginning to emerge of a suitable microarchitecture, as the process -of talking on [videos](https://youtu.be/DoZrGJIltgU), and -[writing out thoughts](https://groups.google.com/forum/#!topic/comp.arch/2kYGFU4ppow) -and then talking about the resultant feedback -[elsewhere](http://lists.libre-riscv.org/pipermail/libre-riscv-dev/2018-December/000261.html) -begins to crystallise, without overloading any one group of people. - -There are several things to remember about this design: the primary being -that it is not explicitly intended as a discrete GPU (although one could -be made), it is primarily for a battery-operated efficient hand-held device, -where it happens to just about pass on, say, a low to mid-range chromebook. -Power consumption *for the entire chip* is targetted at 2.5 watts. - -We learned quite quickly that, paradoxically, even a mobile embedded 3D -GPU *requires* extreme numbers of registers (128 floating-point registers) -because it is handling vectors (or quads as they are called), and even -pixel data in floating-point format is also 4 32-bit numbers (including -the transparency). So where a "normal" RISC processor has 32 registers, -a GPU typically has to have 4 times that amount simply because it is -dealing with 4 lots of numbers simultaneously. If you don't do this, -then that data has to go back down to memory (even to L1 cache), and, as the -L1 cache runs a CAM, it's guaranteed to be power-hungry. - -128 registers brings some unique challenges not normally faced by general -purpose CPUs, and when it becomes possible (or a requirement) to access -even down to the byte level of those 64-bit registers as "elements" in -a vector operation, it is even more challenging. Recall Mitch Alsup's -scoreboard dependency floorplan (reproduced with kind permission, here): - -{{mitch_ld_st_augmentation.jpg}} - -There are two key Dependency Matrices here: on the left is the Function -Unit (rows) to Register File (columns), where you can see at the bottom -in the CDC 6600 the Register File is divided down into A, B and X. -On the right is the Function Unit to Function Unit dependency matrix, -which ensures that each FU only starts its arithmetic operations when -its dependent FUs have created the results it needs. Thus, that Matrix -expresses source register to destination register dependencies. - -Now let's do something hair-raising. Let's do two crazed things at once: +Spread over various [videos](https://youtu.be/DoZrGJIltgU), +[writings](https://groups.google.com/forum/#!topic/comp.arch/2kYGFU4ppow), +and [mailing list +discussions](http://lists.libre-riscv.org/pipermail/libre-riscv-dev/2018-December/000261.html), +a picture is beginning to emerge of a suibable microarchitecture. + +There are several things to remember about this design, the primary +being that it is not explicitly intended as a discrete GPU (although +one could be made). Instead, it is primarily for a battery-operated, +power-efficient hand-held device, where it happens to just about pass +on, say, a low to mid-range chromebook. Power consumption *for the +entire chip* is targetted at 2.5 watts. + +We learned quite quickly that, paradoxically, even a mobile embedded +3D GPU *requires* extreme an number of registers (128 floating-point +registers) because it is handling vectors (or quads as they are +called), and even pixel data, in floating-point format, which means +four 32-bit numbers (including the transparency). So, where a +"normal" RISC processor has 32 registers, a GPU typically has to have +four times that many simply because it is dealing with four lots of +numbers simultaneously. If you don't do this, then that data has to +go back down to memory (even to L1 cache), and, as the L1 cache runs a +CAM, it's guaranteed to be power-hungry. + +Dealing with 128 registers brings some unique challenges not normally +faced by general purpose CPUs, and when it becomes possible (or a +requirement) to access even down to the byte level of those 64-bit +registers as "elements" in a vector operation, it is even more +challenging. Recall Mitch Alsup's scoreboard dependency floorplan +(reproduced with kind permission, here): + +{mitch-ld-st-augmentation | link} + +There are two key dependency matrices here: on the left is the +function unit (rows) to register file (columns), where you can see at +the bottom in the CDC 6600 the register file is divided down into A, B +and X. On the right is the function unit to function unit dependency +matrix, which ensures that each function unit only starts its +arithmetic operations when its dependent function units have created +the results it needs. Thus, that matrix expresses source register to +destination register dependencies. + +Now, let's do something hair-raising. Let's do two crazed things at once: increase the number of registers to a whopping 256 total (128 floating point and 128 integer), and at the same time allow those 64-bit registers to be broken down into **eight** separate 8-bit values... *and allow -Function Unit dependencies to exist on them*! - -What would happen if we did not properly take this into account in the -design is that an 8-bit ADD would require us to "lock" say Register R5 -(all 64 bits of it), absolutely preventing and prohibiting the other 7 -bytes of R5 from being used, until such time as that extremely small -8-bit ADD had completed. Such a design would be laughed at, its -performance would be so low. Only one 8-bit ADD per clock cycle, when -Intel has recently added 512-bit SIMD?? - -So this is a diagram of a proposed solution. What if, when an 8-bit -operation needs to do a calculation to go into the 1st byte, the other -7 bytes have their own **completely separate** dependency lines, in -the Register and Function Unit Matrices? It looks like this: - -{{reorder_alias_bytemask_scheme.png}} - -So if you recall from the previous updates about Scoreboards, it's not -the "scoreboard" that's the key, it's these Register to Function Unit -and FU to FU Dependency Matrices that are the misunderstood key. -So let's explain this diagram. Firstly, in purple in the bottom left -is a massive matrix of FU to FU, just as with the standard CDC 6600, -except now there are separate 32-bit FUs, 16-bit FUs, and 8-bit FUs. -In this way, we can have 32-bit ADD depending on and waiting for -an 8-bit computation, or 16-bit MUL on a 32-bit SQRT and so on. Nothing -immediately obviously different there. +function unit dependencies to exist on them*! + +If we didn't properly take this into account in the design, then an +8-bit ADD would require us to "lock", say, Register R5 (all 64 bits of +it), absolutely preventing and prohibiting the other seven bytes of R5 +from being used, until such time as that extremely small 8-bit ADD had +completed. Such a design would be laughed at, its performance would +be so low. Only one 8-bit ADD per clock cycle, when Intel has +recently added 512-bit [SIMD](https://en.wikipedia.org/wiki/SIMD)? + +Here's a proposed solution. What if, when an 8-bit operation needs to +do a calculation to go into the first byte, the other seven bytes have +their own **completely separate** dependency lines in the register and +function unit matrices? It looks like this: + +{reorder-alias-bytemask-scheme | link} + +If you recall from the [previous updates about +scoreboards](https://www.crowdsupply.com/libre-risc-v/m-class/updates), +it's not the "scoreboard" that's the key, it's these register to +function unit and function unit to function unit dependency matrices +that are the misunderstood key. Let's explain the above diagram. +Firstly, in purple in the bottom left, is a massive matrix of function +units to function units, just as with the standard CDC 6600, except +now there are separate 32-bit function units, 16-bit function units, +and 8-bit function units. In this way, we can have a 32-bit ADD +depending on and waiting for an 8-bit computation, or a 16-bit MUL on +a 32-bit SQRT and so on. Nothing obviously different there. Likewise, in the bottom right, in red, we see matrices that have -FU along rows, and Registers along the columns, exactly again as with -the CDC 6600 standard scoreboard: however, again, we note that -because there are separate 32-bit FUs and separate 16-bit and 8-bit -FUs, there are *three* separate sets of FU-to-Register Matrices. -Also, note that these are separate, where they would be expected -to be grouped together. Except, they're *not* independent, and that's -where the diagram at the top (middle) comes in. +function units along rows, and registers along the columns, exactly +again as with the CDC 6600 standard scoreboard. However, again, we +note that because there are separate 32-bit function units and +separate 16-bit and 8-bit function units, there are *three* separate +sets of function unit to register matrices. Also, note that these are +separate, where they would be expected to be grouped together. +Except, they're *not* independent, and that's where the diagram at the +top (middle) comes in. The diagram at the top says, in words, "if you need a 32-bit register -for an operation (using a 32-bit Function Unit), the 16-bit and 8-bit -Function Units *also* connected to that exact same register **must** -be prevented from occuring. Also, if you need 8 bits of a register, +for an operation (using a 32-bit function unit), the 16-bit and 8-bit +function units *also* connected to that exact same register **must** +be prevented from occurring. Also, if you need eight bits of a register, whilst it does not prevent the other bytes of the register from being used, it *does* prevent the overlapping 16-bit portion **and the 32-bit -and the 64-bit** portions of that same named register from being used". +and the 64-bit** portions of that same named register from being used." -This is absolutely essential to understand, this "cascading" relationship. -Need Register R1 (all of it), you **cannot** go and allocate any of that -register for use in any 32-bit, 16-bit or 8-bit operations. This is -common sense! However, if you use the lowest byte (byte 1), you can still -use the top three 16-bit portions of R1, and you can also still use byte 2. -This is also common sense! +This "cascading" relationship is absolutely essential to understand. +If you need register R1 (all of it), you **cannot** go and allocate +any of that register for use in any 32-bit, 16-bit, or 8-bit +operations. This is common sense! However, if you use the lowest +byte (byte 1), you can still use the top three 16-bit portions of R1, +and you can also still use byte 2. This is also common sense! So in fact, it's actually quite simple, and this "cascade" is simply and -easily propagated down to the Function Unit Dependency Matrices, stopping +easily propagated down to the function unit dependency matrices, stopping 32-bit operations from overwriting 8-bit and vice-versa. -# Virtual Registers - -The fourth part is the grid in green, in the top left corner. This is -a "virtual" to "real" one-bit table. It's here because the size of -these matrices is so enormous that there is deep concern about the line -driver strength, as well as the actual size. 128 registers means -that one single gate, when it goes high or low, has to "drive" the -input of 128 other gates. That takes longer and longer to do, the higher -the number of gates, so it becomes a critical factor in determining the -maximum speed of the entire processor. We will have to keep an eye -on this. - -So, to keep the FU to Register matrix size down, this "virtual" register -concept was introduced. Only one bit in each row of the green table -may be active: it says, for example, "IR1 actually represents that there -is an instruction being executed using R3". This does mean however that -if this table is not high enough (not enough IRs), the processor has to -stall until an instruction is completed, so that one register becomes -free. Again, another thing to keep an eye on, in simulations. - -# Refinements - -The second major concern is the purple matrix: the FU-to-FU one. Basically -where previously we would have FU1 cover all ADDs, FU2 would cover all MUL -operations, FU3 covers BRANCH and so on, now we have to multiply those -numbers by **four** (64-bit ops, 32-bit ops, 16-bit and 8), which in turn -means that the size of the FU-to-FU Matrix has gone up by a staggering -**sixteen** times. This is not really acceptable, so we have to do something -different. - -So the refinement is based on an observation that 16-bit operations of -course may be constructed from 8-bit values, and that 64-bit operations -can be constructed from 32-bit ones. So, what if we skipped the -cascade on 64 and 16 bit, and made the cascade out of just 32-bit and 8-bit? -Then, very simply, the top half of a 64-bit source register is allocated -to one Function Unit, the bottom half to the one next to it, and when it -comes to actually passing the source registers to the relevant ALU, take -from *both* FUs. - -The primary focus is on 32-bit (single-precision floating-point) performance -anyway, for 3D, so if 64-bit operations happen to have half the number of -Reservation Stations / Function Units, and block more often, we actually -don't mind so much. Also, we can still apply the same "banks" trick on -the Register File, except this time with 4-way multiplexing on 32-bit -wide banks, and 4x4 crossbars on the bytes as well: - -{{register_file_multiplexing.jpg}} - -To cope with 16-bit operations, pairs of 8-bit values in adjacent Function -Units are reserved. Likewise for 64-bit operations, the 8-bit crossbars +### Virtual Registers + +The fourth part of the above diagram is the grid in green, in the top +left corner. This is a "virtual" to "real" one-bit table. It's here +because the size of these matrices is so enormous that there is deep +concern about the line driver strength, as well as the actual size. +128 registers means that one single gate, when it goes high or low, +has to "drive" the input of 128 other gates. That takes longer and +longer to do, the higher the number of gates, so it becomes a critical +factor in determining the maximum speed of the entire processor. We +will have to keep an eye on this. + +So, to keep the function unit to register matrix size down, this +"virtual" register concept was introduced. Only one bit in each row +of the green table may be active: it says, for example, "IR1 actually +represents that there is an instruction being executed using R3." +This does mean, however, that if this table is not high enough (not +enough IRs), the processor has to stall until an instruction is +completed, so that one register becomes free. Again, another thing to +keep an eye on, in simulations. + +### Refinements + +The second major concern is the purple matrix, the function unit to +function unit one. Basically, where previously we would have FU1 +cover all ADDs, FU2 would cover all MUL operations, FU3 covers BRANCH, +and so on, now we have to multiply those numbers by **four** (64-bit +ops, 32-bit ops, 16-bit, and 8), which in turn means that the size of +the FU-to-FU matrix has gone up by a staggering **sixteen** times. +This is not really acceptable, so we have to do something different. + +The refinement is based on an observation that 16-bit operations of +course may be constructed from 8-bit values, and that 64-bit +operations can be constructed from 32-bit ones. So, what if we +skipped the cascade on 64 and 16 bit, and made the cascade out of just +32-bit and 8-bit? Then, very simply, the top half of a 64-bit source +register is allocated to one function unit, the bottom half to the one +next to it, and when it comes to actually passing the source registers +to the relevant ALU, take from *both* function units. + +For 3D, the primary focus is on 32-bit (single-precision +floating-point) performance anyway, so if 64-bit operations happen to +have half the number of reservation stations / function units, and +block more often, we actually don't mind so much. Also, we can still +apply the same "banks" trick on the register file, except this time +with four-way multiplexing on 32-bit wide banks, and 4 x 4 crossbars +on the bytes as well: + +{register-file-multiplexing | link} + +To cope with 16-bit operations, pairs of 8-bit values in adjacent function +units are reserved. Likewise for 64-bit operations, the 8-bit crossbars are not used, and pairs of 32-bit source values in adjacent Function Units -in the *32-bit* FU area are reserved. +in the *32-bit* function unit area are reserved. -However, the gate count in such a staggered crossbar arrangement is insane: -bear in mind that this will be 3R1W or 2R1W (2 or 3 reads, 1 write per -register), and that means **three** sets of crossbars, comprising **four** -banks, with effectively 16 byte to 16 byte routing. +However, the gate count in such a staggered crossbar arrangement is +insane: bear in mind that this will be 3R1W or 2R1W (2 or 3 reads, 1 +write per register), and that means **three** sets of crossbars, +comprising **four** banks, with effectively 16 byte to 16 byte +routing. It's too much - so in later updates, this will be explored further.