From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pg1-x533.google.com (mail-pg1-x533.google.com [IPv6:2607:f8b0:4864:20::533]) by sourceware.org (Postfix) with ESMTPS id 2E37F3858D32 for ; Wed, 5 Oct 2022 05:56:45 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org 2E37F3858D32 Authentication-Results: sourceware.org; dmarc=pass (p=none dis=none) header.from=gmail.com Authentication-Results: sourceware.org; spf=pass smtp.mailfrom=gmail.com Received: by mail-pg1-x533.google.com with SMTP id a23so6789573pgi.10 for ; Tue, 04 Oct 2022 22:56:45 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20210112; h=cc:to:subject:message-id:date:from:in-reply-to:references :mime-version:from:to:cc:subject:date; bh=AhvK4Lgn4ZrzeoYoleuQEdirjitFvpYNw+jdZJNKz/A=; b=QJVMe6DRAbaMHR/2jYnkchWOpcmx+DNkK6z7YxL41Knf/KHQ/Ym2+e/i2jlVUA9822 YNCyuqm9/PEVG65uxjkSVwe2cbqfB6h9md3dvhaYN3GvPOL/HE5tb3xekvF6qCwH3Cox 8TeIEJxm+TwZ/vEA4tJq396Gf1giMHRIz/JIEhBeztjIIpNE3TyvxvRfv2wpM7HwgJf5 nr8zX1eHeBlgtiGhF629LGD1+g2OpCgN6AvQ2iLgirPWJi/Op7rPEMgW4h4JoFgEqK4f fqg1XKITuV/etECQHE4FqjtOx4kH13e78D5B0kVeiHhp/4iVRlAeFrDH+TfgSTbYdblM LKRQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=cc:to:subject:message-id:date:from:in-reply-to:references :mime-version:x-gm-message-state:from:to:cc:subject:date; bh=AhvK4Lgn4ZrzeoYoleuQEdirjitFvpYNw+jdZJNKz/A=; b=yKKpQzvYiIPiruocxynZlx4U0exM5msd9DOP3C7+MpT1wSHyT9hffvYuIYkNu/z7fP OZtuw175st/n4UmjWs3cLQKJ8DpBAh/PL71m4WYJNY8ewrOw6nmumzrCP31Jtcok/SFD UQj73JjsKYO4mr6Oa5DdRcK7vIFI16hYOmRu2jQahx26BEGOTHsaMatDIBpSax8lxbyZ FFEakLlgkk8NM4oJQmFoSBW7hK15ZxU29ddyyVMzg5G2zbFDb4gsipSTEUytreV6LWXf AnPP+J7hGEd5sc2o3DMxJKxNmC9X6ZbY6tOkSr+EW39ikDR+MwDOFK9kcWukcBG6AD4o 3vEA== X-Gm-Message-State: ACrzQf2TSzfsEUQq5gAoCFQNZn7PvGwi/hgUq6FXZO9dq1lR/n8jbHAn coeyCx2T4TW2+D6RM8gODxhngJkApmUJfo3rJ9g= X-Google-Smtp-Source: AMsMyM7gDIsEB3ilddv6HylYMbcwjPnwtjV0LAhWSKVm+ZJDiZeisBr5gqblz3SZdeuE3dKh7Wg2aPWw3LqWhlG0o4A= X-Received: by 2002:a05:6a00:35c6:b0:561:ff65:6a2c with SMTP id dc6-20020a056a0035c600b00561ff656a2cmr3185704pfb.55.1664949404091; Tue, 04 Oct 2022 22:56:44 -0700 (PDT) MIME-Version: 1.0 References: <20221005035446.30054-1-palmer@rivosinc.com> In-Reply-To: <20221005035446.30054-1-palmer@rivosinc.com> From: Andrew Pinski Date: Tue, 4 Oct 2022 22:56:31 -0700 Message-ID: Subject: Re: [PATCH] RISC-V: Default to tuning for the thead-c906 To: Palmer Dabbelt Cc: gcc-patches@gcc.gnu.org Content-Type: text/plain; charset="UTF-8" X-Spam-Status: No, score=-7.6 required=5.0 tests=BAYES_00,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,FREEMAIL_FROM,GIT_PATCH_0,KAM_NUMSUBJECT,RCVD_IN_DNSWL_NONE,SPF_HELO_NONE,SPF_PASS,TXREP autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on server2.sourceware.org List-Id: On Tue, Oct 4, 2022 at 8:55 PM Palmer Dabbelt wrote: > > The C906 is by far the most widely available RISC-V processor, so let's > default to tuning for it. > > gcc/ChangeLog > > * config/riscv/riscv.h (RISCV_TUNE_STRING_DEFAULT): Change to > thead-c906. > * doc/invoke.texi (RISC-V -mtune): Change the default to > thead-c906. > > --- I am ok with this as --with-tune and --with-arch works as ways of changing the default still. Thanks, Andrew > > This has come up a handful of times, most recently during the Cauldron. > It seems like a grey area to me: we're changing the behavior of some > command-line arguments (ie, everything that doesn't specify -mtune), but > we sort of change that anyway as the tuning parameters change between > releases. > > I'm not really seeing much of a precedent from the other ports. It > looks like aarch64 sort of changed the default in 02fdbd5beb0 > ("[AArch64] [-mtune cleanup 2/5] Tune for Cortex-A53 by default.") but I > think at that point -mtune=generic and -mtune=cortex-a53 were equivalent > so I'm not sure that counts. I can't quite sort out if the default x86 > tuning has ever changed, but the tuning parameters have changed. I > don't see any way around having the tuning parameters change as they're > pretty tightly coupled to the GCC internals, but changing to a different > tuning target is a bit bigger of a change. > > We also have a bit of a special case here: -mtune is in theory only a > performance issue, but this change will emit a lot more misaligned > accesses and we've seen those trigger bugs in the trap handlers before. > Those bugs are elsewhere so it's sort of not a GCC problem, but I'm sure > there's still users out there with broken firmware and this may cause > visible fallout. We can just tell those users their systems were always > broken, but that's never a fun way to do things. > > I figured the easiest way to talk about this would be to just send the > patch, but I definitely don't plan on committing it without some > discussion. > --- > gcc/config/riscv/riscv.h | 2 +- > gcc/doc/invoke.texi | 2 +- > 2 files changed, 2 insertions(+), 2 deletions(-) > > diff --git a/gcc/config/riscv/riscv.h b/gcc/config/riscv/riscv.h > index 363113c6511..1d9379fa5ee 100644 > --- a/gcc/config/riscv/riscv.h > +++ b/gcc/config/riscv/riscv.h > @@ -40,7 +40,7 @@ along with GCC; see the file COPYING3. If not see > #endif > > #ifndef RISCV_TUNE_STRING_DEFAULT > -#define RISCV_TUNE_STRING_DEFAULT "rocket" > +#define RISCV_TUNE_STRING_DEFAULT "thead-c906" > #endif > > extern const char *riscv_expand_arch (int argc, const char **argv); > diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi > index e0c2c57c9b2..2a9ea3455f6 100644 > --- a/gcc/doc/invoke.texi > +++ b/gcc/doc/invoke.texi > @@ -28529,7 +28529,7 @@ particular CPU name. Permissible values for this option are: @samp{rocket}, > @samp{thead-c906}, @samp{size}, and all valid options for @option{-mcpu=}. > > When @option{-mtune=} is not specified, use the setting from @option{-mcpu}, > -the default is @samp{rocket} if both are not specified. > +the default is @samp{thead-c906} if both are not specified. > > The @samp{size} choice is not intended for use by end-users. This is used > when @option{-Os} is specified. It overrides the instruction cost info > -- > 2.34.1 >