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AJvYcCVUQrAvX81M2VoytWvv0UGgXfBQeEGoFVBw4CtIX5gVPhoGZUYQLmTeEhkfTHt72RxW0283FMTic59Y0AE=@vger.kernel.org X-Gm-Message-State: AOJu0Yw1DXCmLrbuCP3vljS6c716gDmO08aXY0ZPr97h/L6Wmvb0Bt4/ jPCndesrpDzYKxi8Ef/ZAJ6zwZVdgVYURtnbtKtHLLTxZS+KDi33jbihihtrW55SCZKGt/KNMBD sdbKJj8LwVROGizi1MBH8NIF9NujYL6U= X-Gm-Gg: ASbGnctgvbpC373hC0i6F8b23aY8AzKSrKux+31RjP5xHQ7lzUJokQkfaYsHcxJjgix flNHoJ4ZrAv2iCV6NEU4snAeXILzQqyAhA987PX0WELHa8RH9Zq/sV3x7nU+W7Qk15htlhWUDZ5 FjQniH7R2KbBR/cKzzV4wi+YzvFo78PJvP X-Google-Smtp-Source: AGHT+IHtyAA75K0NgEjgRbDcsqE4SXagGOb8xzzlF3VJGOkTbh1v4g11tN0y9l0THW1J9PBFK/bF/H1Dh4p76kdd75k= X-Received: by 2002:a05:651c:1989:b0:32a:62a2:f73e with SMTP id 38308e7fff4ca-32adfbbe389mr5005261fa.4.1749190437684; Thu, 05 Jun 2025 23:13:57 -0700 (PDT) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 References: <20250605164733.737543-1-mjguzik@gmail.com> <20250605190054.GH30486@noisy.programming.kicks-ass.net> In-Reply-To: <20250605190054.GH30486@noisy.programming.kicks-ass.net> From: Uros Bizjak Date: Fri, 6 Jun 2025 08:13:46 +0200 X-Gm-Features: AX0GCFvpYd8kzCzsE_bJGrCUIOVfM5HdcONbpcfdkPDh9SLLBtH4pKiPicYESV4 Message-ID: Subject: Re: [PATCH v2] x86: prevent gcc from emitting rep movsq/stosq for inlined ops To: Peter Zijlstra Cc: Mateusz Guzik , torvalds@linux-foundation.org, mingo@redhat.com, x86@kernel.org, linux-kernel@vger.kernel.org Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Thu, Jun 5, 2025 at 9:00=E2=80=AFPM Peter Zijlstra wrote: > > On Thu, Jun 05, 2025 at 06:47:33PM +0200, Mateusz Guzik wrote: > > gcc is over eager to use rep movsq/stosq (starts above 40 bytes), which > > comes with a significant penalty on CPUs without the respective fast > > short ops bits (FSRM/FSRS). > > I don't suppose there's a magic compiler toggle to make it emit prefix > padded 'rep movs'/'rep stos' variants such that they are 5 bytes each, > right? > > Something like: > > 2e 2e 2e f3 a4 cs cs rep movsb %ds:(%rsi),%es:(%rdi) This won't fly, because gas complains: z.s: Assembler messages: z.s:1: Error: same type of prefix used twice Uros.