Messages in this thread | | | Date | Tue, 13 Nov 2007 12:39:57 +0100 | From | Adrian Bunk <> | Subject | Re: [RFC: 2.6 patch] add -fno-tree-scev-cprop to KBUILD_CFLAGS |
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On Mon, Nov 12, 2007 at 11:58:43AM -0500, Jakub Jelinek wrote: > On Sun, Nov 11, 2007 at 07:48:29AM +0100, Adrian Bunk wrote: > > The gcc from svn that will become gcc 4.3 generates libgcc calls in > > cases like the following (on 32bit architectures): > > > > <-- snip --> > > > > static inline void timespec_add_ns(struct timespec *a, u64 ns) > > { > > ... > > while(ns >= NSEC_PER_SEC) { > > ns -= NSEC_PER_SEC; > > a->tv_sec++; > > } > > ... > > > > <-- snip --> > > Blindly using -fno-tree-scev-cprop just to get rid of one case where > this turns out to be a pessimization when kernel knows ns is usually very > small is IMHO a wrong thing, you'd lose many cases where this optimization > can actually improve performance.
It's not about performance, it's about a build error.
> Instead, for this exact case just > add an optimization barrier to avoid gcc doing this. > Adding asm ("" : "=r" (ns) : "0" (ns)); (or hide it in some macro) into the > loop will do the job just fine.
The problem is that this is very fragile - imagine what might happen when a frequently used struct member gets changed from int to u64.
After all, when looking at current practice, the kernel will support being built with gcc 4.3 until 2013 or 2014.
> Jakub
cu Adrian
--
"Is there not promise of rain?" Ling Tan asked suddenly out of the darkness. There had been need of rain for many days. "Only a promise," Lao Er said. Pearl S. Buck - Dragon Seed
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