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Mon, 28 Jun 2021 07:50:13 -0700 (PDT) Received: from [192.168.1.11] ([71.212.149.176]) by smtp.gmail.com with ESMTPSA id r30sm4387704pgn.46.2021.06.28.07.50.12 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 28 Jun 2021 07:50:12 -0700 (PDT) Subject: Re: [PATCH v3 13/29] tcg/s390: Support bswap flags To: Peter Maydell References: <20210626063631.2411938-1-richard.henderson@linaro.org> <20210626063631.2411938-14-richard.henderson@linaro.org> From: Richard Henderson Message-ID: Date: Mon, 28 Jun 2021 07:50:11 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Received-SPF: pass client-ip=2607:f8b0:4864:20::532; envelope-from=richard.henderson@linaro.org; helo=mail-pg1-x532.google.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-0.001, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=unavailable autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: qemu-s390x , QEMU Developers Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 6/28/21 7:43 AM, Peter Maydell wrote: > On Sat, 26 Jun 2021 at 07:44, Richard Henderson > wrote: >> >> For INDEX_op_bswap16_i64, use 64-bit instructions so that we can >> easily provide the extension to 64-bits. Drop the special case, >> previously used, where the input is already zero-extended -- the >> minor code size savings is not worth the complication. >> >> Cc: qemu-s390x@nongnu.org >> Signed-off-by: Richard Henderson > >> + case INDEX_op_bswap32_i32: >> tcg_out_insn(s, RRE, LRVR, args[0], args[1]); >> break; > > When we're working with i32s, is the top half of the host register > zero or garbage ? The general case on s390x is that the top half of the host register is unmodified by insns with 32-bit operands. This is true for LRVR. It's also true for LRVRH, with its 16-bit operand. Which is why it's not as useful as it could be. r~