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Re: [Qemu-ppc] [Qemu-devel] [PATCH 04/14] target/ppc: introduce GEN_VSX_

From: Richard Henderson
Subject: Re: [Qemu-ppc] [Qemu-devel] [PATCH 04/14] target/ppc: introduce GEN_VSX_HELPER_X3 macro to fpu_helper.c
Date: Sun, 5 May 2019 07:49:36 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.6.1

On 5/5/19 2:52 AM, Mark Cave-Ayland wrote:
> Right, it looks like VSX_CMP is the culprit here. Am I right in thinking that 
> it's
> best to remove the opc parameter from GEN_VSX_HELPER_X3 above, and then have a
> separate gen and helper function for just the VSX_CMP instructions? 
> Presumably this
> reduces of the overhead at both translation and execution time for the 
> instructions
> that don't require it.


I think the best fix for VSX_CMP is to return the value that is to be assigned
to cr[6], and let it assign like so:

  gen_helper_foo(cpu_crf[6], other, arguments);

(Or if the opcode bit is unset,

  TCGv_i32 ignored = tcg_temp_new_i32();
  gen_helper_foo(ignored, other arguments);

at which point these functions do not modify tcg globals, so the decl
can be improved to

  DEF_HELPER_FLAGS_2(xvcmpeqdp, TCG_CALL_NO_RWG, i32, ptr, ptr, ptr)

To keep the assignment vs exception order, you remove the direct call to
do_float_check_status and use gen_helper_float_check_status.


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