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Re: [PATCH] MIPS: MIPS32r2 FP MADD instruction set support


"Maciej W. Rozycki" <macro@codesourcery.com> writes:
>  This issue was originally raised here:
>
> http://gcc.gnu.org/ml/gcc-patches/2012-12/msg00863.html
>
>  We have a shortcoming in GCC in that we only allow the use half of the FP 
> MADD instruction subset (MADD.fmt and MSUB.fmt) in the 64-bit/32-register 
> mode (CP0.Status.FR == 1) on MIPS32r2 processors.  Furthermore we never 
> enable the other half (NMADD.fmt and NMSUB.fmt) on those processors.  
> However this whole instruction subset is always available on MIPS32r2 FPUs 
> regardless of the mode selected, just as it always has been on FPUs of the 
> 64-bit ISA line from MIPS IV up.

Hmm, this was discussed here:

    http://gcc.gnu.org/ml/gcc-patches/2006-11/msg00488.html
    http://gcc.gnu.org/ml/gcc-patches/2006-11/msg00492.html

The footnote for COP1X instructions on page 12 of volume 1 of the MIPS32 ISA
(v2.50) says:

1. In Release 1 of the Architecture, these instructions are legal only
   with a MIPS64 processor with 64-bit operations enabled (they are, in
   effect, actually MIPS64 instructions). In Release 2 of the Architecture,
   these instructions are legal with either a MIPS32 or MIPS64 processor
   _which includes a 64-bit floating point unit_.

(my emphasis).  "which" rather than "that" makes this a bit ambiguous,
but various comments in the rest of the manual imply that MIPS32r2 allows
an implementation choice between 32-bit and 64-bit FPUs.  E.g. page 8 says:

  Support for 64-bit coprocessors with 32-bit CPUs: These changes allow
  a 64-bit coprocessor (including an FPU) to be attached to a 32-bit
  CPU. This enhancement is optional in a Release 2 implementation.

and page 45 says:

  In addition to an Instruction Set Architecture, the MIPS architecture
  definition includes processing resources such as the set of
  coprocessor general registers. In Release 1 of the Architecture, the
  32-bit registers in MIPS32 were enlarged to 64-bits in MIPS64;
  however, these 64-bit FPU registers are not backwards
  compatible. Instead, processors implementing the MIPS64 Architecture
  provide a mode bit to select either the 32-bit or 64-bit register
  model. In Release 2 of the Architecture, a 32-bit CPU _may_ include a
  full 64-bit coprocessor, including a floating point unit which
  implements the same mode bit to select 32-bit or 64-bit FPU register
  model.

On page 322 of volume 2, the footnote for "Table A-20 MIPS64 COP1X
Encoding of Function Field" uses slightly different wording:

  COP1X instructions are legal only if 64-bit floating point operations
  are enabled.

So was this all a big misunderstanding on my part?  The TARGET_FLOAT64
condition came from MIPS themselves, and when challenged they seemed
pretty adamant that it was correct.  If I was wrong to be convinced
by the explanation, I hope you can at least see why I was convinced. :-)

If it wasn't a misunderstanding, then the point is that we can't tell
from ISA_MIPS32R2 alone whether the target has a 32-bit or 64-bit FPU,
but we know that it must have a 64-bit FPU if using TARGET_FLOAT64.

>  Also, according to MIPS IV ISA documentation these operations are only 
> fused (i.e. don't match original IEEE 754-1985 accuracy requirements) on 
> the original MIPS IV R8000 CPU, and MIPS architecture specs don't mention 
> any limitations of these instructions either, so I have updated the GCC 
> manual to document that on non-R8000 CPUs (which are ones we really care 
> about) they are numerically equivalent to computations made with 
> corresponding individual operations.

This part is OK, thanks, and is probably the only bit that's suitable for
4.8 at this stage.  Would you mind applying it separately?

>  Finally, while at it, I found it interesting that we have separate 
> conditions to cover MADD.fmt/MSUB.fmt (ISA_HAS_FP_MADD4_MSUB4) and 
> NMADD.fmt/NMADD.fmt (ISA_HAS_NMADD4_NMSUB4) while all the four 
> instructions need to be implemented as a whole group per data format 
> supported and cannot be separated (the MIPS architecture specification 
> explicitly forbids subsetting).  The difference between the two conditions 
> is the former expands to ISA_HAS_FP4, that is enables the subsubset for 
> any MIPS IV and up FPU while the latter has an extra "&& (!TARGET_MIPS5400 
> || TARGET_MAD)" qualifier.
>
>  I went ahead and checked available NEC VR54xx documentation and here's 
> what I came up with:
>
> 1. "VR5400 MIPS RISC Microprocessor Family" datasheet (NEC doc #13362) 
>    says:
>
>    "The VR5400 processor family complies with the MIPS IV instruction set 
>    and IEEE-754 floating-point and IEEE-1149.1/1149.1a JTAG specification, 
>    [...]"
>
> 2. "VR5432 MIPS RISC Microprocessor User's Manual, Volume 2" (NEC doc 
>    #13751) lists all the individual MADD.fmt, MSUB.fmt, NMADD.fmt and
>    NMSUB.fmt instructions in Chapter 18 "Floating-Point Unit Instruction 
>    Set" with no restrictions as to their availability (the only other 
>    member of the VR54xx family I know of is the VR5464 that is a 
>    high-performance version of the VR5432 and is fully software 
>    compatible).
>
>  Further to that TARGET_MAD controls whether to "Use PMC-style 'mad' 
> instructions" that are all CPU rather than FPU instructions.  The VR5432 
> indeed supports extra integer multiply-accumulate instructions, as 
> documented in #2 above; these are the MACC/MACCHI/MACCHIU/MACCU and 
> MSAC/MSACHI/MSACHIU/MSACU instructions as roughly covered by our 
> ISA_HAS_MACC, ISA_HAS_MSAC and ISA_HAS_MACCHI knobs (the latter is not 
> implied for TARGET_MIPS5400, perhaps because the family does not support 
> the doubleword variants).
>
>  All in all it looks to me like a misplaced hunk.  It was introduced in 
> rev. 56471 (you were named as one of the contributors on that commit, so 
> you may be able to remember and/or correct me if I am wrong here anywhere) 
> and it looks to me it should have been applied to the ISA_HAS_MADD_MSUB 
> macro instead that's still just a few lines above ISA_HAS_NMADD4_NMSUB4 
> (and was even closer to ISA_HAS_NMADD_NMSUB as the latter was then called; 
> the bodies were close enough back then for a hunk to apply cleanly to 
> either).

I was named in that commit but the VR54xx stuff wasn't mine.  I do remember
that Mike put a lot of effort into tuning the VR54xx madd stuff though,
because of the difficulty of having multiply-accumulate instructions
that force the use of HI/LO on an architecture that also has efficient
three-operand multiplies.  So I'm pretty sure that this worked correctly
in the Cygnus devo tree, and your explanation of a misplaced hunk seems
very convincing.

Richard


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