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[156.19.246.23]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-220d5585fbfsm155095845ad.226.2025.02.22.10.08.42 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Sat, 22 Feb 2025 10:08:43 -0800 (PST) Message-ID: Date: Sat, 22 Feb 2025 10:08:41 -0800 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3 122/162] tcg: Add tcg_gen_addcio_{i32,i64,tl} To: Paolo Bonzini , qemu-devel@nongnu.org References: <20250216231012.2808572-1-richard.henderson@linaro.org> <20250216231012.2808572-123-richard.henderson@linaro.org> <21c3c655-3676-4bd3-9088-8f266d5dfd3a@redhat.com> Content-Language: en-US From: Richard Henderson In-Reply-To: <21c3c655-3676-4bd3-9088-8f266d5dfd3a@redhat.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=2607:f8b0:4864:20::102f; envelope-from=richard.henderson@linaro.org; helo=mail-pj1-x102f.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, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On 2/21/25 07:41, Paolo Bonzini wrote: > Ok, this one definitely caught my eye. :) > > On 2/17/25 00:09, Richard Henderson wrote: >> +        tcg_gen_add_i32(t0, a, b); >> +        tcg_gen_setcond_i32(TCG_COND_LTU, t1, t0, a); > > Compare against b instead?  If there's an immediate (which could even be > zero) it is there. Sure. > >> +        tcg_gen_add_i32(r, t0, ci); >> +        tcg_gen_setcond_i32(TCG_COND_LTU, t0, r, t0); >> +        tcg_gen_or_i32(co, t0, t1); > > setcond + or can become a movcond: For the set of hosts that require this support (mips, riscv, loongarch), this isn't really an improvement -- we'll end up emitting a setcond LTU anyway and using that to feed movcond NE instruction. >> +        if (tcg_op_supported(INDEX_op_addci, TCG_TYPE_I32, 0)) { >> +            TCGv_i32 discard = tcg_temp_ebb_new_i32(); >> +            TCGv_i32 zero = tcg_constant_i32(0); >> +            TCGv_i32 mone = tcg_constant_i32(-1); >> + >> +            tcg_gen_op3_i32(INDEX_op_addco, discard, TCGV_LOW(ci), mone); >> +            tcg_gen_op3_i32(INDEX_op_addcio, discard, TCGV_HIGH(ci), mone); > > This addcio is unnecessary/incorrect.  I think you assume that TCGV_HIGH(ci) = 0, > since that's what you set it below, and then this you can remove it. I am *not* assuming TCGV_HIGH(ci) == 0. I briefly thought about ignoring the high part, because "of course" carry-in is just the low bit. However, the code emitted for a 64-bit host will not ignore the upper 32 bits, so I thought it better to not intentionally change semantics. >> +        } else { >> +            TCGv_i32 t0 = tcg_temp_ebb_new_i32(); >> +            TCGv_i32 c0 = tcg_temp_ebb_new_i32(); >> +            TCGv_i32 c1 = tcg_temp_ebb_new_i32(); >> + >> +            tcg_gen_or_i32(c1, TCGV_LOW(ci), TCGV_HIGH(ci)); >> +            tcg_gen_setcondi_i32(TCG_COND_NE, c1, c1, 0); > > Likewise, this shouldn't be needed and you can just add TCGV_LOW(ci) > below. Again, this is about keeping the semantics the same as x86_64, where a non-zero ci is treated as a single carry-in bit. r~