Re: [PATCH] tcg: Use memset for large vector byte replication

["Philippe Mathieu-Daudé" <f4bug@amsat.org>]

On 12/15/20 6:48 PM, Richard Henderson wrote:
> In f47db80cc07, we handled odd-sized tail clearing for
> the case of hosts that have vector operations, but did
> not handle the case of hosts that do not have vector ops.
> 
> This was ok until e2e7168a214b, which changed the encoding
> of simd_desc such that the odd sizes are impossible.
> 
> Add memset as a tcg helper, and use that for all out-of-line
> byte stores to vectors.  This includes, but is not limited to,
> the tail clearing operation in question.
> 
> Cc: qemu-stable@nongnu.org
> Buglink: https://bugs.launchpad.net/bugs/1907817
> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
> ---
>  accel/tcg/tcg-runtime.h     | 11 +++++++++++
>  include/exec/helper-proto.h |  4 ++++
>  tcg/tcg-op-gvec.c           | 32 ++++++++++++++++++++++++++++++++
>  3 files changed, 47 insertions(+)
> 
> diff --git a/accel/tcg/tcg-runtime.h b/accel/tcg/tcg-runtime.h
> index 4eda24e63a..2e36d6eb0c 100644
> --- a/accel/tcg/tcg-runtime.h
> +++ b/accel/tcg/tcg-runtime.h
> @@ -28,6 +28,17 @@ DEF_HELPER_FLAGS_1(lookup_tb_ptr, TCG_CALL_NO_WG_SE, ptr, env)
>  
>  DEF_HELPER_FLAGS_1(exit_atomic, TCG_CALL_NO_WG, noreturn, env)
>  
> +#ifndef IN_HELPER_PROTO
> +/*
> + * Pass calls to memset directly to libc, without a thunk in qemu.
> + * Do not re-declare memset, especially since we fudge the type here;
> + * we assume sizeof(void *) == sizeof(size_t), which is true for
> + * all supported hosts.
> + */
> +#define helper_memset memset
> +DEF_HELPER_FLAGS_3(memset, TCG_CALL_NO_RWG, ptr, ptr, int, ptr)

Nice :)

> +#endif /* IN_HELPER_PROTO */
> +
>  #ifdef CONFIG_SOFTMMU
>  
>  DEF_HELPER_FLAGS_5(atomic_cmpxchgb, TCG_CALL_NO_WG,
> diff --git a/include/exec/helper-proto.h b/include/exec/helper-proto.h
> index a0a8d9aa46..659f9298e8 100644
> --- a/include/exec/helper-proto.h
> +++ b/include/exec/helper-proto.h
> @@ -35,11 +35,15 @@ dh_ctype(ret) HELPER(name) (dh_ctype(t1), dh_ctype(t2), dh_ctype(t3), \
>                              dh_ctype(t4), dh_ctype(t5), dh_ctype(t6), \
>                              dh_ctype(t7));
>  
> +#define IN_HELPER_PROTO
> +
>  #include "helper.h"
>  #include "trace/generated-helpers.h"
>  #include "tcg-runtime.h"
>  #include "plugin-helpers.h"
>  
> +#undef IN_HELPER_PROTO
> +
>  #undef DEF_HELPER_FLAGS_0
>  #undef DEF_HELPER_FLAGS_1
>  #undef DEF_HELPER_FLAGS_2
> diff --git a/tcg/tcg-op-gvec.c b/tcg/tcg-op-gvec.c
> index ddbe06b71a..6c42d76f3a 100644
> --- a/tcg/tcg-op-gvec.c
> +++ b/tcg/tcg-op-gvec.c
> @@ -547,6 +547,9 @@ static void do_dup(unsigned vece, uint32_t dofs, uint32_t oprsz,
>          in_c = dup_const(vece, in_c);
>          if (in_c == 0) {
>              oprsz = maxsz;
> +            vece = MO_8;
> +        } else if (in_c == dup_const(MO_8, in_c)) {
> +            vece = MO_8;

Nice optimization.

>          }
>      }
>  
> @@ -628,6 +631,35 @@ static void do_dup(unsigned vece, uint32_t dofs, uint32_t oprsz,
>      /* Otherwise implement out of line.  */
>      t_ptr = tcg_temp_new_ptr();
>      tcg_gen_addi_ptr(t_ptr, cpu_env, dofs);
> +
> +    /*
> +     * This may be expand_clr for the tail of an operation, e.g.
> +     * oprsz == 8 && maxsz == 64.  The size of the clear is misaligned
> +     * wrt simd_desc and will assert.  Simply pass all replicated byte
> +     * stores through to memset.
> +     */
> +    if (oprsz == maxsz && vece == MO_8) {
> +        TCGv_ptr t_size = tcg_const_ptr(oprsz);
> +        TCGv_i32 t_val;
> +
> +        if (in_32) {
> +            t_val = in_32;
> +        } else if (in_64) {
> +            t_val = tcg_temp_new_i32();
> +            tcg_gen_extrl_i64_i32(t_val, in_64);
> +        } else {
> +            t_val = tcg_const_i32(in_c);
> +        }
> +        gen_helper_memset(t_ptr, t_ptr, t_val, t_size);
> +
> +        tcg_temp_free_ptr(t_ptr);
> +        tcg_temp_free_ptr(t_size);
> +        if (!in_32) {
> +            tcg_temp_free_i32(t_val);
> +        }
> +        return;

LGTM but I'd rather have another R-b:
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>

BTW (nitpicking) I'd rewrite the epilogue as:

        if (!in_32) {
            tcg_temp_free_i32(t_val);
        }
        tcg_temp_free_ptr(t_size);

        tcg_temp_free_ptr(t_ptr);
        return;

t_val first, because the !in_32 allocs are few lines earlier,
t_ptr last because allocated in prologue, so keep close to
'return'. Matter of taste ;)

> +    }
> +
>      t_desc = tcg_const_i32(simd_desc(oprsz, maxsz, 0));
>  
>      if (vece == MO_64) {
>