From: David Daney <ddaney@caviumnetworks.com> To: Scott Wood <scottwood@freescale.com> Cc: Coly Li <bosong.ly@taobao.com>, Wang Cong <xiyou.wangcong@gmail.com>, Jeremy Fitzhardinge <jeremy@goop.org>, linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org, Yong Zhang <yong.zhang0@gmail.com> Subject: Re: [PATCH 2/7] PowerPC: add unlikely() to BUG_ON() Date: Thu, 27 Jan 2011 12:32:02 -0800 [thread overview] Message-ID: <4D41D642.9020603@caviumnetworks.com> (raw) In-Reply-To: <20110127140420.2c727f36@udp111988uds.am.freescale.net> On 01/27/2011 12:04 PM, Scott Wood wrote: > On Thu, 27 Jan 2011 09:57:39 -0800 > David Daney<ddaney@caviumnetworks.com> wrote: > >> On 01/27/2011 04:12 AM, Coly Li wrote: >>> diff --git a/arch/powerpc/include/asm/bug.h b/arch/powerpc/include/asm/bug.h >>> index 065c590..10889a6 100644 >>> --- a/arch/powerpc/include/asm/bug.h >>> +++ b/arch/powerpc/include/asm/bug.h >>> @@ -2,6 +2,7 @@ >>> #define _ASM_POWERPC_BUG_H >>> #ifdef __KERNEL__ >>> >>> +#include<linux/compiler.h> >>> #include<asm/asm-compat.h> >>> >>> /* >>> @@ -71,7 +72,7 @@ >>> unreachable(); \ >>> } while (0) >>> >>> -#define BUG_ON(x) do { \ >>> +#define __BUG_ON(x) do { \ >>> if (__builtin_constant_p(x)) { \ >>> if (x) \ >>> BUG(); \ >>> @@ -85,6 +86,8 @@ >>> } \ >>> } while (0) >>> >>> +#define BUG_ON(x) __BUG_ON(unlikely(x)) >>> + >> >> This is the same type of frobbing you were trying to do to MIPS. >> >> I will let the powerpc maintainers weigh in on it, but my opinion is >> that, as with MIPS, BUG_ON() is expanded to a single machine >> instruction, and this unlikely() business will not change the generated >> code in any useful way. It is thus gratuitous code churn and >> complexification. > > What about just doing this: > > #define BUG() __builtin_trap() > > #define BUG_ON(x) do { \ > if (x) \ > BUG(); \ > } while (0) > > GCC should produce better code than forcing it into twnei. A simple > BUG_ON(x != y) currently generates something like this (GCC 4.3): > > xor r0,r11,r0 > addic r10,r0,-1 > subfe r9,r10,r0 > twnei r9,0 > > Or this (GCC 4.5): > > xor r0,r11,r0 > cntlzw r0,r0 > srwi r0,r0,5 > xori r0,r0,1 > twnei r0,0 > > Instead of: > > twne r0,r11 > > And if GCC doesn't treat code paths that lead to __builtin_trap() as > unlikely (which could make a difference with complex expressions, > even with a conditional trap instruction), that's something that could > and should be fixed in GCC. > > The downside is that GCC says, "The mechanism used may vary from > release to release so you should not rely on any particular > implementation." However, some architectures (sparc, m68k, ia64) > already use __builtin_trap() for this, it seems unlikely that future GCC > versions would switch away from using the trap instruction[1], and there > doesn't seem to be a better-defined way to make GCC generate trap > instructions intelligently. > This is good in theory, however powerpc has this _EMIT_BUG_ENTRY business that wouldn't work with __builtin_trap(). David Daney > -Scott > > [1] A more likely possibility is that an older compiler just generates a > call to abort() or similar, and later versions implemented trap -- need > to check what the oldest supported GCC does. >
WARNING: multiple messages have this Message-ID (diff)
From: David Daney <ddaney@caviumnetworks.com> To: Scott Wood <scottwood@freescale.com> Cc: Jeremy Fitzhardinge <jeremy@goop.org>, Coly Li <bosong.ly@taobao.com>, linux-kernel@vger.kernel.org, Yong Zhang <yong.zhang0@gmail.com>, Wang Cong <xiyou.wangcong@gmail.com>, linuxppc-dev@lists.ozlabs.org Subject: Re: [PATCH 2/7] PowerPC: add unlikely() to BUG_ON() Date: Thu, 27 Jan 2011 12:32:02 -0800 [thread overview] Message-ID: <4D41D642.9020603@caviumnetworks.com> (raw) In-Reply-To: <20110127140420.2c727f36@udp111988uds.am.freescale.net> On 01/27/2011 12:04 PM, Scott Wood wrote: > On Thu, 27 Jan 2011 09:57:39 -0800 > David Daney<ddaney@caviumnetworks.com> wrote: > >> On 01/27/2011 04:12 AM, Coly Li wrote: >>> diff --git a/arch/powerpc/include/asm/bug.h b/arch/powerpc/include/asm/bug.h >>> index 065c590..10889a6 100644 >>> --- a/arch/powerpc/include/asm/bug.h >>> +++ b/arch/powerpc/include/asm/bug.h >>> @@ -2,6 +2,7 @@ >>> #define _ASM_POWERPC_BUG_H >>> #ifdef __KERNEL__ >>> >>> +#include<linux/compiler.h> >>> #include<asm/asm-compat.h> >>> >>> /* >>> @@ -71,7 +72,7 @@ >>> unreachable(); \ >>> } while (0) >>> >>> -#define BUG_ON(x) do { \ >>> +#define __BUG_ON(x) do { \ >>> if (__builtin_constant_p(x)) { \ >>> if (x) \ >>> BUG(); \ >>> @@ -85,6 +86,8 @@ >>> } \ >>> } while (0) >>> >>> +#define BUG_ON(x) __BUG_ON(unlikely(x)) >>> + >> >> This is the same type of frobbing you were trying to do to MIPS. >> >> I will let the powerpc maintainers weigh in on it, but my opinion is >> that, as with MIPS, BUG_ON() is expanded to a single machine >> instruction, and this unlikely() business will not change the generated >> code in any useful way. It is thus gratuitous code churn and >> complexification. > > What about just doing this: > > #define BUG() __builtin_trap() > > #define BUG_ON(x) do { \ > if (x) \ > BUG(); \ > } while (0) > > GCC should produce better code than forcing it into twnei. A simple > BUG_ON(x != y) currently generates something like this (GCC 4.3): > > xor r0,r11,r0 > addic r10,r0,-1 > subfe r9,r10,r0 > twnei r9,0 > > Or this (GCC 4.5): > > xor r0,r11,r0 > cntlzw r0,r0 > srwi r0,r0,5 > xori r0,r0,1 > twnei r0,0 > > Instead of: > > twne r0,r11 > > And if GCC doesn't treat code paths that lead to __builtin_trap() as > unlikely (which could make a difference with complex expressions, > even with a conditional trap instruction), that's something that could > and should be fixed in GCC. > > The downside is that GCC says, "The mechanism used may vary from > release to release so you should not rely on any particular > implementation." However, some architectures (sparc, m68k, ia64) > already use __builtin_trap() for this, it seems unlikely that future GCC > versions would switch away from using the trap instruction[1], and there > doesn't seem to be a better-defined way to make GCC generate trap > instructions intelligently. > This is good in theory, however powerpc has this _EMIT_BUG_ENTRY business that wouldn't work with __builtin_trap(). David Daney > -Scott > > [1] A more likely possibility is that an older compiler just generates a > call to abort() or similar, and later versions implemented trap -- need > to check what the oldest supported GCC does. >
next prev parent reply other threads:[~2011-01-27 20:32 UTC|newest] Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top 2011-01-27 12:12 [PATCH 0/6] use BUG_ON correctly, v2 Coly Li 2011-01-27 12:12 ` [PATCH 1/7] MIPS: add unlikely() to BUG_ON() Coly Li 2011-01-27 17:50 ` David Daney 2011-01-28 10:41 ` Coly Li 2011-01-27 12:12 ` [PATCH 2/7] PowerPC: " Coly Li 2011-01-27 17:57 ` David Daney 2011-01-27 17:57 ` David Daney 2011-01-27 20:04 ` Scott Wood 2011-01-27 20:04 ` Scott Wood 2011-01-27 20:32 ` David Daney [this message] 2011-01-27 20:32 ` David Daney 2011-01-28 9:05 ` David Laight 2011-01-28 9:05 ` David Laight [not found] ` <AE90C24D6B3A694183C094C60CF0A2F6D8AC2D__37237.0892241181$1296205746$gmane$org@saturn3.aculab.com> 2011-01-28 10:14 ` Andreas Schwab 2011-01-28 10:14 ` Andreas Schwab 2011-01-28 11:02 ` Coly Li 2011-01-28 11:02 ` Coly Li 2011-01-27 12:12 ` [PATCH 3/7] dma: use BUG_ON correctly in iop-adma.c Coly Li 2011-01-27 12:12 ` [PATCH 4/7] dma: use BUG_ON correctly in mv_xor.c Coly Li 2011-01-27 12:12 ` [PATCH 5/7] dma: use BUG_ON correctly in ppc4xx/adam.c Coly Li 2011-01-27 12:12 ` [PATCH 6/7] wl_cfg80211.c: use BUG_ON correctly Coly Li 2011-01-27 12:12 ` [PATCH 7/7] scsi_lib.c: " Coly Li
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