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Thu, 25 Aug 2022 09:49:32 -0700 (PDT) X-Google-Smtp-Source: AA6agR6qNIbJJqUamZSoycZ35sURKLz5U/mMoQM6zahUyQ2kY7PpaXvrCLo7oMCL6g+zP1LLwKJ2EQ== X-Received: by 2002:a5d:59a8:0:b0:225:6292:dfc9 with SMTP id p8-20020a5d59a8000000b002256292dfc9mr2900599wrr.354.1661446171917; Thu, 25 Aug 2022 09:49:31 -0700 (PDT) From: Paolo Bonzini To: qemu-devel@nongnu.org Cc: Paul Brook , =?UTF-8?q?Alex=20Benn=C3=A9e?= Subject: [PATCH] i386: DPPS rounding fix Date: Thu, 25 Aug 2022 18:49:27 +0200 Message-Id: <20220825164927.393060-2-pbonzini@redhat.com> X-Mailer: git-send-email 2.37.1 In-Reply-To: <20220825164927.393060-1-pbonzini@redhat.com> References: <20220825164927.393060-1-pbonzini@redhat.com> MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Received-SPF: pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) client-ip=209.51.188.17; envelope-from=qemu-devel-bounces+importer=patchew.org@nongnu.org; helo=lists.gnu.org; Received-SPF: pass client-ip=170.10.129.124; envelope-from=pbonzini@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -27 X-Spam_score: -2.8 X-Spam_bar: -- X-Spam_report: (-2.8 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 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+importer=patchew.org@nongnu.org Sender: "Qemu-devel" X-ZohoMail-DKIM: pass (identity @redhat.com) X-ZM-MESSAGEID: 1661446545152100001 Content-Type: text/plain; charset="utf-8" The DPPS (Dot Product) instruction is defined to first sum pairs of intermediate results, then sum those values to get the final result. i.e. (A+B)+(C+D) We incrementally sum the results, i.e. ((A+B)+C)+D, which can result in incorrect rouding. For consistency, also change the variable names to the ones used in the Intel SDM and implement DPPD following the manual. Based on a patch by Paul Brook . Signed-off-by: Paolo Bonzini --- target/i386/ops_sse.h | 67 ++++++++++++++++++++++--------------------- 1 file changed, 35 insertions(+), 32 deletions(-) diff --git a/target/i386/ops_sse.h b/target/i386/ops_sse.h index 2524db4c25..b12b271fcd 100644 --- a/target/i386/ops_sse.h +++ b/target/i386/ops_sse.h @@ -1943,56 +1943,59 @@ SSE_HELPER_I(helper_pblendw, W, 8, FBLENDP) =20 void glue(helper_dpps, SUFFIX)(CPUX86State *env, Reg *d, Reg *s, uint32_t = mask) { - float32 iresult =3D float32_zero; + float32 prod1, prod2, temp2, temp3, temp4; =20 + /* + * We must evaluate (A+B)+(C+D), not ((A+B)+C)+D + * to correctly round the intermediate results + */ if (mask & (1 << 4)) { - iresult =3D float32_add(iresult, - float32_mul(d->ZMM_S(0), s->ZMM_S(0), - &env->sse_status), - &env->sse_status); + prod1 =3D float32_mul(d->ZMM_S(0), s->ZMM_S(0), &env->sse_status); + } else { + prod1 =3D float32_zero; } if (mask & (1 << 5)) { - iresult =3D float32_add(iresult, - float32_mul(d->ZMM_S(1), s->ZMM_S(1), - &env->sse_status), - &env->sse_status); + prod2 =3D float32_mul(d->ZMM_S(1), s->ZMM_S(1), &env->sse_status); + } else { + prod2 =3D float32_zero; } + temp2 =3D float32_add(prod1, prod2, &env->sse_status); if (mask & (1 << 6)) { - iresult =3D float32_add(iresult, - float32_mul(d->ZMM_S(2), s->ZMM_S(2), - &env->sse_status), - &env->sse_status); + prod1 =3D float32_mul(d->ZMM_S(2), s->ZMM_S(2), &env->sse_status); + } else { + prod1 =3D float32_zero; } if (mask & (1 << 7)) { - iresult =3D float32_add(iresult, - float32_mul(d->ZMM_S(3), s->ZMM_S(3), - &env->sse_status), - &env->sse_status); + prod2 =3D float32_mul(d->ZMM_S(3), s->ZMM_S(3), &env->sse_status); + } else { + prod2 =3D float32_zero; } - d->ZMM_S(0) =3D (mask & (1 << 0)) ? iresult : float32_zero; - d->ZMM_S(1) =3D (mask & (1 << 1)) ? iresult : float32_zero; - d->ZMM_S(2) =3D (mask & (1 << 2)) ? iresult : float32_zero; - d->ZMM_S(3) =3D (mask & (1 << 3)) ? iresult : float32_zero; + temp3 =3D float32_add(prod1, prod2, &env->sse_status); + temp4 =3D float32_add(temp2, temp3, &env->sse_status); + + d->ZMM_S(0) =3D (mask & (1 << 0)) ? temp4 : float32_zero; + d->ZMM_S(1) =3D (mask & (1 << 1)) ? temp4 : float32_zero; + d->ZMM_S(2) =3D (mask & (1 << 2)) ? temp4 : float32_zero; + d->ZMM_S(3) =3D (mask & (1 << 3)) ? temp4 : float32_zero; } =20 void glue(helper_dppd, SUFFIX)(CPUX86State *env, Reg *d, Reg *s, uint32_t = mask) { - float64 iresult =3D float64_zero; + float64 prod1, prod2, temp2; =20 if (mask & (1 << 4)) { - iresult =3D float64_add(iresult, - float64_mul(d->ZMM_D(0), s->ZMM_D(0), - &env->sse_status), - &env->sse_status); + prod1 =3D float64_mul(d->ZMM_D(0), s->ZMM_D(0), &env->sse_status); + } else { + prod1 =3D float64_zero; } if (mask & (1 << 5)) { - iresult =3D float64_add(iresult, - float64_mul(d->ZMM_D(1), s->ZMM_D(1), - &env->sse_status), - &env->sse_status); + prod2 =3D float64_mul(d->ZMM_D(1), s->ZMM_D(1), &env->sse_status); + } else { + prod2 =3D float64_zero; } - d->ZMM_D(0) =3D (mask & (1 << 0)) ? iresult : float64_zero; - d->ZMM_D(1) =3D (mask & (1 << 1)) ? iresult : float64_zero; + temp2 =3D float64_add(prod1, prod2, &env->sse_status); + d->ZMM_D(0) =3D (mask & (1 << 0)) ? temp2 : float64_zero; + d->ZMM_D(1) =3D (mask & (1 << 1)) ? temp2 : float64_zero; } =20 void glue(helper_mpsadbw, SUFFIX)(CPUX86State *env, Reg *d, Reg *s, --=20 2.37.1