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Re: [Qemu-devel] [PATCH] tcg: fix dead computation for repeated input ar

From: Richard Henderson
Subject: Re: [Qemu-devel] [PATCH] tcg: fix dead computation for repeated input arguments
Date: Tue, 19 May 2015 07:46:04 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0

On 05/19/2015 03:26 AM, Aurelien Jarno wrote:
> @@ -1522,6 +1522,9 @@ static void tcg_liveness_analysis(TCGContext *s)
>                      if (dead_temps[arg]) {
>                          dead_args |= (1 << i);
>                      }
> +                }
> +                for (i = nb_oargs; i < nb_oargs + nb_iargs; i++) {
> +                    arg = args[i];
>                      dead_temps[arg] = 0;
>                  }
>                  s->op_dead_args[oi] = dead_args;

How about another line of commentary for each loop?

Something like

  /* Record arguments that die in this opcode.  */

for the first and

  /* Input arguments are live for preceeding opcodes.  */

for the second.

As for the same loop for calls, you're right that it may well cause us to do a
tiny bit of redundant work, but nothing else bad will happen.  We'll enter
temp_dead more times than necessary.  I'm always skeptical about knowingly
giving a compiler bad information though.  You tend to not know how data is
going to be used in future, and *then* get bad results.


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