Re: [Qemu-devel] [PATCH 00/18] Emulate guest vector operations with host vector operations

[Alex Bennée]

Kirill Batuzov <address@hidden> writes:

> The goal of these patch series is to set up an infrastructure to emulate
> guest vector operations using host vector operations. Preliminary
> experiments show that simply translating loads and stores increases
> performance of x264 video codec by 10%. The performance of a gcc vectorized
> for loop increased 2x.
>
> To be able to emulate guest vector operations using host vector operations,
> several things need to be done.

I see rth has already done a bunch of review so I'll pass on this cycle
but please feel free to add me to the CC list next iteration.

>
> 1. Corresponding vector types should be added to TCG. These series add
> TCG_v128 and TCG_v64. I've made TCG_v64 a different type than TCG_i64
> because it usually needs to be allocated to different registers and
> supports different operations.
>
> 2. Load/store operations for these new types need to be implemented.
>
> 3. For seamless transition from current model to a new one we need to
> handle cases where memory occupied by global variable can be accessed via
> pointer to the CPUArchState structure. A very simple conservative alias
> analysis has been added to do it. This analysis tracks memory loads and
> stores that overlap with fields of CPUArchState and provides this
> information to the register allocator. The allocator then spills and
> reloads affected globals when needed.
>
> 4. Allow overlapping globals. For scalar registers this is a rare case, and
> overlapping registers can ba handled as a single one (ah, al, ax, eax,
> rax). In ARM every Q-register consists of two D-register each consisting of
> two S-registers. Handling 4 S-registers as one because they are parts of
> the same Q-register is way too inefficient.
>
> 5. Add new memory addressing mode to MMU code for large accesses and create
> needed helpers. Only 128-bit vectors have been handled for now.
>
> 6. Create TCG opcodes for vector operations. Only addition has beed handled
> in these series. Each operation has a wrapper that checks if the backend
> supports the corresponding operation or not. In one case the vector opcode
> is generated, in the other the operation is emulated with scalar
> operations. The emulation code is generated inline for performance reasons
> (there is a huge performance difference between inline generation
> and calling a helper). As a positive side effect this will eventually allow
>  to merge similar emulation code for vector instructions from different
> frontends to target-independent implementation.
>
> 7. Use new operations in the frontend (ARM was used in these series).
>
> 8. Support new operations in the backend (x86_64 was used in these series).
>
> For experiments I have used ARM guest on x86_64 host. I wanted some pair of
> different architectures with vector extensions both. ARM and x86_64 pair
> fits well.
>
> Kirill Batuzov (18):
>   tcg: add support for 128bit vector type
>   tcg: add support for 64bit vector type
>   tcg: add ld_v128, ld_v64, st_v128 and st_v64 opcodes
>   tcg: add simple alias analysis
>   tcg: use results of alias analysis in liveness analysis
>   tcg: allow globals to overlap
>   tcg: add vector addition operations
>   target/arm: support access to vector guest registers as globals
>   target/arm: use vector opcode to handle vadd.<size> instruction
>   tcg/i386: add support for vector opcodes
>   tcg/i386: support 64-bit vector operations
>   tcg/i386: support remaining vector addition operations
>   tcg: do not relay on exact values of MO_BSWAP or MO_SIGN in backend
>   tcg: introduce new TCGMemOp - MO_128
>   tcg: introduce qemu_ld_v128 and qemu_st_v128 opcodes
>   softmmu: create helpers for vector loads
>   tcg/i386: add support for qemu_ld_v128/qemu_st_v128 ops
>   target/arm: load two consecutive 64-bits vector regs as a 128-bit
>     vector reg
>
>  cputlb.c                     |   4 +
>  softmmu_template_vector.h    | 266 
> +++++++++++++++++++++++++++++++++++++++++++
>  target/arm/translate.c       |  89 ++++++++++++++-
>  tcg/aarch64/tcg-target.inc.c |   4 +-
>  tcg/arm/tcg-target.inc.c     |   4 +-
>  tcg/i386/tcg-target.h        |  35 +++++-
>  tcg/i386/tcg-target.inc.c    | 245 ++++++++++++++++++++++++++++++++++++---
>  tcg/mips/tcg-target.inc.c    |   4 +-
>  tcg/optimize.c               | 146 ++++++++++++++++++++++++
>  tcg/ppc/tcg-target.inc.c     |   4 +-
>  tcg/s390/tcg-target.inc.c    |   4 +-
>  tcg/sparc/tcg-target.inc.c   |  12 +-
>  tcg/tcg-op.c                 |  20 +++-
>  tcg/tcg-op.h                 | 262 ++++++++++++++++++++++++++++++++++++++++++
>  tcg/tcg-opc.h                |  34 ++++++
>  tcg/tcg.c                    | 146 ++++++++++++++++++++++++
>  tcg/tcg.h                    | 147 +++++++++++++++++++++++-
>  17 files changed, 1385 insertions(+), 41 deletions(-)
>  create mode 100644 softmmu_template_vector.h


--
Alex Bennée