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Rozycki" Cc: Paul Walmsley , Palmer Dabbelt , Huacai Chen , WANG Xuerui , Thomas Gleixner , Peter Zijlstra , Andy Lutomirski , Alexandre Ghiti , linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org, loongarch@lists.linux.dev Subject: Re: [PATCH 1/4] riscv: entry: Convert ret_from_fork() to C Message-ID: References: <20250122-riscv_optimize_entry-v1-0-4ee95559cfd0@rivosinc.com> <20250122-riscv_optimize_entry-v1-1-4ee95559cfd0@rivosinc.com> 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=us-ascii Content-Disposition: inline In-Reply-To: On Mon, Jan 27, 2025 at 08:20:56PM +0000, Maciej W. Rozycki wrote: > On Sun, 26 Jan 2025, Charlie Jenkins wrote: > > > > Wouldn't it make sense to save a jump here and make it a tail call, i.e.: > > > > > > la ra, ret_from_exception > > > tail ret_from_fork > > > > > > > I don't believe so due to the return address stack. It was shown in this > > patch [1] that a 7% performance improvement can be seen on existing > > riscv hardware by performing the extra jump. Doing tail calls should be > > avoided on riscv since the hardware can be expected to predict the > > return address incorrectly every time if the return address is manually > > changed. > > Fair enough, thanks. Though no link to said patch given. Oops! Here it is: https://lore.kernel.org/linux-riscv/20240607061335.2197383-1-cyrilbur@tenstorrent.com/. > > Maciej