openpower.mdwn

   1 # Evaluation
   2
   3 EULA released! looks good.
   4
   5 Links
   6
   7 * [[openpower/isatables]]
   8 * https://forums.raptorcs.com/
   9 * http://lists.mailinglist.openpowerfoundation.org/mailman/listinfo/openpower-community-dev
  10 * http://lists.mailinglist.openpowerfoundation.org/mailman/listinfo
  11 * http://bugs.libre-riscv.org/show_bug.cgi?id=179
  12 * https://openpowerfoundation.org/?resource_lib=power-isa-version-3-0
  13 * https://openpowerfoundation.org/?resource_lib=ibm-power-isa-version-2-07-b
  14
  15 Summary
  16
  17 * FP32 is converted to FP64. Requires SimpleV to be active.
  18 * FP16 needed
  19 * transcendental FP opcodes needed (sin, cos, atan2, root, log1p)
  20 * FCVT between 16/32/64 needed
  21 * c++11 atomics not very efficient
  22 * no 16/48/64 opcodes, needs a shuffle of opcodes.  TODO investigate Power VLE
  23 * needs escape sequencing (ISAMUX/NS)
  24
  25 # What we are *NOT* doing:
  26
  27 * A processor that is fundamentally incompatible (noncompliant) with Power.
  28 * Opcode 4 Signal Processing (SPE)
  29 * Opcode 4 Vectors or Opcode 60 VSX
  30 * Avoidable legacy opcodes
  31
  32 # SimpleV
  33
  34 see [[simple_v_extension]] - will fit into 48/64/VBLOCK, see below.
  35 SimpleV: a "hardware for-loop" which involves type-casting (both) the
  36 register files to "a sequence of elements".  The **one** instruction
  37 (an unmodified **scalar** instruction) is interpreted as a *hardware
  38 for-loop* that issues **multiple** internal instructions with
  39 sequentially-incrementing register numbers.
  40
  41 Thus it is completely unnecessary to add any vector opcodes - at all -
  42 saving hugely on both hardware and compiler development time when
  43 the concept is dropped on top of a pre-existing ISA.
  44
  45 # Integer Overflow / Saturate
  46
  47 Typically used on vector operations (audio DSP), it makes no sense to have separate opcodes (Opcode 4 SPE).  To be done instead as CSRs / vector-flags on *standard* arithmetic operations.
  48
  49 # atomics
  50
  51 Single instruction on RV, and x86, but multiple on Power. Needs investigation, particularly as to why cache flush exists.
  52
  53 https://www.cl.cam.ac.uk/~pes20/cpp/cpp0xmappings.html
  54
  55 Hot loops contain significant instruction count, really need new c++11 atomics.  To be proposed as new extension because other OpenPower members will need them too
  56
  57 # FP16
  58
  59 Doesn't exist in Power, need to work out suitable opcodes, basically means duplicating the entire range of FP32/64 ops, symmetrically.
  60
  61 Usually done with a fmt field, 2 bit, last one is FP128
  62
  63 idea: rather than add dozens of new opcodes, add "repurposer" instructions that remap FP32 to 16/32/64/128 and FP64 likewise.  can also be done as C instruction, only needs 4 bits to specify.
  64
  65 # Escape Sequencing
  66
  67 aka "ISAMUX/NS".  Absolutely critical, also to have official endorsement
  68 from OpenPower Foundation.
  69
  70 This will allow extending ISA (see ISAMUX/NS) in a clean fashion
  71 (including for and by OpenPower Foundation)
  72
  73 ## Branches in namespaces
  74
  75 Branches are fine as it is up to the compiler to decide whether to let the
  76 ISAMUX/NS/escape-sequence countdown run out.
  77
  78 This is all a software / compiler / ABI issue.
  79
  80 ## Function calls in namespaces
  81
  82 Storing and restoring the state of the page/subpage CSR should be done by the caller. Or, again, let the countdowns run out.
  83
  84 If certain alternative configs are expected, they are part of the function ABI which must be spec'd.
  85
  86 All of this is a software issue (compiler / ABI).
  87
  88 # Compressed, 48, 64, VBLOCK
  89
  90 TODO investigate Power VLE (Freescale doc Ref 314-68105)
  91
  92 Under Esc Seq, move mulli, twi, tdi out of major OP000 then use the
  93 entire row, 2 bits instead of 3.  greatly simplifies decoder.
  94
  95 * OP 000-000 and 000-001 for 16 bit compressed, 11 bit instructions
  96 * OP 000-010 and 000-011 for 48 bit. 11 bits for SVP P48
  97 * OP 000-100 and 000-201 for 64 bit. 11 bits for SVP P64
  98 * OP 000-110 and 000-111 for VBLOCK. 11 bits available.
  99
 100 # Compressed 16
 101
 102 Further "escape-sequencing".
 103
 104 Only 11 bits available. Idea: have "pages" where one instruction selects
 105 the page number. It also specifies for how long that page is activated
 106 (terminated on a branch)
 107
 108 The length to be a maximum of 4 bits, where 0b1111 indicates "permanently active".
 109
 110 Perhaps split OP000-000 and OP000-001 so that 2 pages can be active.
 111
 112 Store activation length in a CSR.
 113
 114 2nd idea: 11 bits can be used for extremely common operations, then length-encoding page selection for further ops, using the full 16 bit range and an entirely new encoding scheme. 1 bit specifies which of 2 pages was selected?
 115
 116 3rd idea: "stack" mechanism. Allow subpages like a stack, to page in new pages.
 117
 118 3 bits for subpage number. 4 bits for length, gives 7 bits. 4x7 is 28, then 3 bits can be used to specify "stack depth".
 119
 120 Requirements are to have one instruction in each subpage which resets all the way back to PowerISA default. The other is a "back up stack by 1".
 121
 122 # RISCV userspace
 123
 124 Dual ISA, RV userspace only. Requires PowerISA to be able to context-switch RV registers and CSRs.
 125
 126 the exception entry point:
 127 https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/powerpc/kernel/exceptions-64s.S?h=v5.4-rc5#n409
 128
 129 the rest of the context switch code is in a different file:
 130 https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/powerpc/kernel/entry_64.S?h=v5.4-rc5#n589