From mboxrd@z Thu Jan 1 00:00:00 1970 From: Geert Uytterhoeven Subject: Re: [PATCH] bitfield: Use __ffs64(x) to fix missing __ffsdi2() Date: Tue, 10 Oct 2017 09:03:50 +0200 Message-ID: References: <1507538449-22775-1-git-send-email-geert+renesas@glider.be> <20171009155311.17e1c7f8@cakuba.netronome.com> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Return-path: Received: from mail-wm0-f67.google.com ([74.125.82.67]:33806 "EHLO mail-wm0-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752146AbdJJHDw (ORCPT ); Tue, 10 Oct 2017 03:03:52 -0400 In-Reply-To: <20171009155311.17e1c7f8@cakuba.netronome.com> Sender: linux-arch-owner@vger.kernel.org List-ID: To: Jakub Kicinski Cc: Geert Uytterhoeven , Arnd Bergmann , Andrew Morton , kbuild test robot , Linux-Arch , "linux-kernel@vger.kernel.org" Hi Jakub, On Tue, Oct 10, 2017 at 12:53 AM, Jakub Kicinski wrote: > On Mon, 9 Oct 2017 10:40:49 +0200, Geert Uytterhoeven wrote: >> On most architectures[*], gcc turns __builtin_ffsll() into a call to >> __ffsdi2(), which is not provided by any architecture, leading to >> failures like: >> >> rcar-gen3-cpg.c:(.text+0x289): undefined reference to `__ffsdi2' >> >> To fix this, use __ffs64() instead, which is available on all >> architectures. >> >> [*] Known exceptions are some 64-bit architectures like amd64, arm64, >> ia64, powerpc64, and tilegx. >> >> Reported-by: kbuild test robot >> Fixes: 3e9b3112ec74f192 ("add basic register-field manipulation macros") >> Signed-off-by: Geert Uytterhoeven >> --- >> include/linux/bitfield.h | 3 ++- >> 1 file changed, 2 insertions(+), 1 deletion(-) >> >> diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h >> index 8b9d6fff002db113..0a827677372756fa 100644 >> --- a/include/linux/bitfield.h >> +++ b/include/linux/bitfield.h >> @@ -15,6 +15,7 @@ >> #ifndef _LINUX_BITFIELD_H >> #define _LINUX_BITFIELD_H >> >> +#include >> #include >> >> /* >> @@ -46,7 +47,7 @@ >> * reg |= FIELD_PREP(REG_FIELD_C, c); >> */ >> >> -#define __bf_shf(x) (__builtin_ffsll(x) - 1) >> +#define __bf_shf(x) __ffs64(x) > > Hm. The build bot failure made me think. I think rcar-gen3-cpg.c may > be doing something wrong here, could you point me at the patch in > question? I don't see any FIELD_* there in Linus's tree. See series "[PATCH v3 0/6] clk: renesas: r8a779[56]: Add Z and Z2 clock support" (https://www.spinics.net/lists/arm-kernel/msg609499.html). > __bf_shf() is supposed to be used with constant masks only, therefore > the call must be optimized away completely. > >> #define __BF_FIELD_CHECK(_mask, _reg, _val, _pfx) \ >> ({ \ IC. Yes, it looks like __ffs64() can't be optimized away like __builtin_ffsll() :-( Apparently the patch series above uses __bf_shf() directly, to avoid the BUILD_BUG_ON_MSG(), which doesn't work when the call isn't optimized away. Sorry for not noticing that before... One way to fix that (non-)API abuse would be to get rid of __bf_shf(), and open code it as __builtin_ffsll(x) - 1 everywhere... What do you think? Thanks! Gr{oetje,eeting}s, Geert From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wm0-f67.google.com ([74.125.82.67]:33806 "EHLO mail-wm0-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752146AbdJJHDw (ORCPT ); Tue, 10 Oct 2017 03:03:52 -0400 MIME-Version: 1.0 In-Reply-To: <20171009155311.17e1c7f8@cakuba.netronome.com> References: <1507538449-22775-1-git-send-email-geert+renesas@glider.be> <20171009155311.17e1c7f8@cakuba.netronome.com> From: Geert Uytterhoeven Date: Tue, 10 Oct 2017 09:03:50 +0200 Message-ID: Subject: Re: [PATCH] bitfield: Use __ffs64(x) to fix missing __ffsdi2() Content-Type: text/plain; charset="UTF-8" Sender: linux-arch-owner@vger.kernel.org List-ID: To: Jakub Kicinski Cc: Geert Uytterhoeven , Arnd Bergmann , Andrew Morton , kbuild test robot , Linux-Arch , "linux-kernel@vger.kernel.org" Message-ID: <20171010070350._Ag3ou5_xMNJb56rOttzZVjik5f38c7eFvYgrxbmV4Q@z> Hi Jakub, On Tue, Oct 10, 2017 at 12:53 AM, Jakub Kicinski wrote: > On Mon, 9 Oct 2017 10:40:49 +0200, Geert Uytterhoeven wrote: >> On most architectures[*], gcc turns __builtin_ffsll() into a call to >> __ffsdi2(), which is not provided by any architecture, leading to >> failures like: >> >> rcar-gen3-cpg.c:(.text+0x289): undefined reference to `__ffsdi2' >> >> To fix this, use __ffs64() instead, which is available on all >> architectures. >> >> [*] Known exceptions are some 64-bit architectures like amd64, arm64, >> ia64, powerpc64, and tilegx. >> >> Reported-by: kbuild test robot >> Fixes: 3e9b3112ec74f192 ("add basic register-field manipulation macros") >> Signed-off-by: Geert Uytterhoeven >> --- >> include/linux/bitfield.h | 3 ++- >> 1 file changed, 2 insertions(+), 1 deletion(-) >> >> diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h >> index 8b9d6fff002db113..0a827677372756fa 100644 >> --- a/include/linux/bitfield.h >> +++ b/include/linux/bitfield.h >> @@ -15,6 +15,7 @@ >> #ifndef _LINUX_BITFIELD_H >> #define _LINUX_BITFIELD_H >> >> +#include >> #include >> >> /* >> @@ -46,7 +47,7 @@ >> * reg |= FIELD_PREP(REG_FIELD_C, c); >> */ >> >> -#define __bf_shf(x) (__builtin_ffsll(x) - 1) >> +#define __bf_shf(x) __ffs64(x) > > Hm. The build bot failure made me think. I think rcar-gen3-cpg.c may > be doing something wrong here, could you point me at the patch in > question? I don't see any FIELD_* there in Linus's tree. See series "[PATCH v3 0/6] clk: renesas: r8a779[56]: Add Z and Z2 clock support" (https://www.spinics.net/lists/arm-kernel/msg609499.html). > __bf_shf() is supposed to be used with constant masks only, therefore > the call must be optimized away completely. > >> #define __BF_FIELD_CHECK(_mask, _reg, _val, _pfx) \ >> ({ \ IC. Yes, it looks like __ffs64() can't be optimized away like __builtin_ffsll() :-( Apparently the patch series above uses __bf_shf() directly, to avoid the BUILD_BUG_ON_MSG(), which doesn't work when the call isn't optimized away. Sorry for not noticing that before... One way to fix that (non-)API abuse would be to get rid of __bf_shf(), and open code it as __builtin_ffsll(x) - 1 everywhere... What do you think? Thanks! Gr{oetje,eeting}s, Geert -- Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org In personal conversations with technical people, I call myself a hacker. But when I'm talking to journalists I just say "programmer" or something like that. -- Linus Torvalds