Re: gcc inlining heuristics was Re: [PATCH -v7][RFC]: mutex: implement adaptive spinning

Ingo Molnar
2009-01-20T22:05:16Z

* 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