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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4538238198asm12803765e9.39.2025.06.25.01.34.16 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 25 Jun 2025 01:34:16 -0700 (PDT) Date: Wed, 25 Jun 2025 09:34:15 +0100 From: David Laight To: Radim =?UTF-8?B?S3LEjW3DocWZ?= Cc: "Palmer Dabbelt" , , , "Paul Walmsley" , , "Alexandre Ghiti" , "Atish Patra" , , , , , "Jeff Law" Subject: Re: [PATCH v2 3/2] RISC-V: sbi: remove sbi_ecall tracepoints Message-ID: <20250625093415.53fd21cf@pumpkin> In-Reply-To: References: <20250619190315.2603194-4-rkrcmar@ventanamicro.com> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable On Wed, 25 Jun 2025 09:51:45 +0200 Radim Kr=C4=8Dm=C3=A1=C5=99 wrote: > 2025-06-24T15:09:09+02:00, Radim Kr=C4=8Dm=C3=A1=C5=99 : > > For another example, let's have the following function: > > > > struct sbiret some_sbi_ecall(uintptr_t a0, uintptr_t a1) > > { > > return sbi_ecall(123, 456, a0, a1); > > } > > ... >=20 > GCC 15.1 still leaves "mv" outside the branch, but at least seems to be > on the right track (undesired overhead is marked with leading stars): >=20 > 0xffffffff800236e8 <+0>: addi sp,sp,-48 > 0xffffffff800236ea <+2>: sd s0,32(sp) > 0xffffffff800236ec <+4>: sd ra,40(sp) > 0xffffffff800236ee <+6>: addi s0,sp,48 > * 0xffffffff800236f0 <+8>: mv a4,a0 > * 0xffffffff800236f2 <+10>: mv a5,a1 > 0xffffffff800236f4 <+12>: nop > * 0xffffffff800236f8 <+16>: mv a0,a4 > * 0xffffffff800236fa <+18>: mv a1,a5 > 0xffffffff800236fc <+20>: li a7,123 > 0xffffffff80023700 <+24>: li a6,456 > 0xffffffff80023704 <+28>: ecall > * 0xffffffff80023708 <+32>: mv a5,a0 > * 0xffffffff8002370a <+34>: mv a2,a1 > 0xffffffff8002370c <+36>: nop > 0xffffffff80023710 <+40>: ld ra,40(sp) > 0xffffffff80023712 <+42>: ld s0,32(sp) > * 0xffffffff80023714 <+44>: mv a0,a5 > * 0xffffffff80023716 <+46>: mv a1,a2 > 0xffffffff80023718 <+48>: addi sp,sp,48 > 0xffffffff8002371a <+50>: ret > [Tracing goes to +126] How much do a few register moves/spills matter compared to the cost of the called code? There will but much worse things out there if you look. David