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Re: [Qemu-devel] [PATCH 1/6] target-ppc: add vmul10[u, eu, cu, ecu]q ins


From: Richard Henderson
Subject: Re: [Qemu-devel] [PATCH 1/6] target-ppc: add vmul10[u, eu, cu, ecu]q instructions
Date: Wed, 28 Sep 2016 21:00:51 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.3.0

On 09/28/2016 07:07 PM, David Gibson wrote:
+    tcg_gen_mulu2_i64(t0, t1, cpu_avrl[rA(ctx->opcode)], val);          \

Do you really want to be using an actual mul op, rather than (in << 3)
+ (in << 1)?  Obviously working out al the carries correctly will be a
bit fiddly.

I think it's fine. Modern hardware will do the double-word multiply in 3-5 cycles, which is probably equal to what we could do by hand with shifts.


r~



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