From mboxrd@z Thu Jan 1 00:00:00 1970 From: mans@mansr.com (=?iso-8859-1?Q?M=E5ns_Rullg=E5rd?=) Date: Thu, 26 Nov 2015 00:50:08 +0000 Subject: [PATCH v2 2/2] ARM: Replace calls to __aeabi_{u}idiv with udiv/sdiv instructions In-Reply-To: (Nicolas Pitre's message of "Wed, 25 Nov 2015 19:44:50 -0500 (EST)") References: <1448488264-23400-1-git-send-email-sboyd@codeaurora.org> <1448488264-23400-3-git-send-email-sboyd@codeaurora.org> Message-ID: To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org Nicolas Pitre writes: > On Thu, 26 Nov 2015, M?ns Rullg?rd wrote: > >> Nicolas Pitre writes: >> >> > 3) In fact I was wondering if the overhead of the branch and back is >> > really significant compared to the non trivial cost of a idiv >> > instruction and all the complex infrastructure required to patch >> > those branches directly, and consequently if the performance >> > difference is actually worth it versus simply doing (2) alone. >> >> Depending on the operands, the div instruction can take as few as 3 >> cycles on a Cortex-A7. > > Even the current software based implementation can produce a result with > about 5 simple ALU instructions depending on the operands. > > The average cycle count is more important than the easy-way-out case. > And then how significant the two branches around it are compared to idiv > alone from direct patching of every call to it. If not calling the function saves an I-cache miss, the benefit can be substantial. No, I have no proof of this being a problem, but it's something that could happen. Of course, none of this is going to be as good as letting the compiler generate div instructions directly. -- M?ns Rullg?rd mans at mansr.com