Messages in this thread Patches in this message | | | Date | Tue, 20 Jan 2009 23:05:16 +0100 | From | Ingo Molnar <> | Subject | Re: gcc inlining heuristics was Re: [PATCH -v7][RFC]: mutex: implement adaptive spinning |
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* Linus Torvalds <torvalds@linux-foundation.org> wrote:
> On Tue, 20 Jan 2009, Ingo Molnar wrote: > > > > (Different-type pointer uses are a common pattern: we have a lot of > > places where we have pointers to structures with different types so > > strict-aliasing optimization opportunities apply quite broadly > > already.) > > Yes and no. > > It's true that the kernel in general uses mostly pointers through > structures that can help the type-based thing. > > However, the most common and important cases are actually the very same > structures. In particular, things like <linux/list.h>. Same "struct > list", often embedded into another case of the same struct. > > And that's where "restrict" can actually help. It might be interesting > to see, for example, if it makes any difference to add a "restrict" > qualifier to the "new" pointer in __list_add(). That might give the > compiler the ability to schedule the stores to next->prev and prev->next > differently, and maybe it could matter? > > It probably is not noticeable. The big reason for wanting to do alias > analysis tends to not be thatt kind of code at all, but the cases where > you can do much bigger simplifications, or on in-order machines where > you really want to hoist things like FP loads early and FP stores late, > and alias analysis (and here type-based is more reasonable) shows that > the FP accesses cannot alias with the integer accesses around it.
Hm, GCC uses __restrict__, right?
The patch below makes no difference at all on an x86 defconfig:
vmlinux: text data bss dec hex filename 7253544 1641812 1324296 10219652 9bf084 vmlinux.before 7253544 1641812 1324296 10219652 9bf084 vmlinux.after
not a single instruction was changed:
--- vmlinux.before.asm +++ vmlinux.after.asm @@ -1,5 +1,5 @@ -vmlinux.before: file format elf64-x86-64 +vmlinux.after: file format elf64-x86-64
I'm wondering whether there's any internal tie-up between alias analysis and the __restrict__ keyword - so if we turn off aliasing optimizations the __restrict__ keyword's optimizations are turned off as well. Nope, with aliasing optimizations turned back on there's still no change on the x86 defconfig:
text data bss dec hex filename 7240893 1641796 1324296 10206985 9bbf09 vmlinux.before 7240893 1641796 1324296 10206985 9bbf09 vmlinux.after
GCC 4.3.2. Maybe i missed something obvious?
Ingo
--- include/linux/list.h | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) Index: linux/include/linux/list.h =================================================================== --- linux.orig/include/linux/list.h +++ linux/include/linux/list.h @@ -38,7 +38,7 @@ static inline void INIT_LIST_HEAD(struct * the prev/next entries already! */ #ifndef CONFIG_DEBUG_LIST -static inline void __list_add(struct list_head *new, +static inline void __list_add(struct list_head * __restrict__ new, struct list_head *prev, struct list_head *next) { @@ -48,7 +48,7 @@ static inline void __list_add(struct lis prev->next = new; } #else -extern void __list_add(struct list_head *new, +extern void __list_add(struct list_head * __restrict__ new, struct list_head *prev, struct list_head *next); #endif
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