[PATCH 5/14] i386 / Use early clobber to eliminate rotate in desc

[PATCH 5/14] i386 / Use early clobber to eliminate rotate in desc

[zach]

Use an early clobber on addr to avoid the extra rorl instruction at the
end of _set_tssldt_desc.

Also, get some C type checking on the descriptor struct here.

Patch-base: 2.6.13-rc5-mm1
Patch-keys: i386 desc cleanup optimize
Signed-off-by: Zachary Amsden <zach@vmware.com>
Index: linux-2.6.13/include/asm-i386/desc.h
===================================================================
--- linux-2.6.13.orig/include/asm-i386/desc.h	2005-08-09 18:59:10.000000000 -0700
+++ linux-2.6.13/include/asm-i386/desc.h	2005-08-10 20:42:20.000000000 -0700
@@ -34,17 +34,21 @@
 extern struct desc_struct default_ldt[];
 extern void set_intr_gate(unsigned int irq, void * addr);
 
-#define _set_tssldt_desc(n,addr,limit,type) \
-__asm__ __volatile__ ("movw %w3,0(%2)\n\t" \
-	"movw %w1,2(%2)\n\t" \
-	"rorl $16,%1\n\t" \
-	"movb %b1,4(%2)\n\t" \
-	"movb %4,5(%2)\n\t" \
-	"movb $0,6(%2)\n\t" \
-	"movb %h1,7(%2)\n\t" \
-	"rorl $16,%1" \
-	: "=m"(*(n)) : "q" (addr), "r"(n), "ir"(limit), "i"(type))
-
+#define _set_tssldt_desc(desc,addr,limit,type) \
+do { \
+	unsigned long __tmp; \
+	typecheck(struct desc_struct *, desc); \
+	asm volatile ("movw %w4,0(%3)\n\t" \
+		      "movw %w2,2(%3)\n\t" \
+		      "rorl $16,%2\n\t" \
+		      "movb %b2,4(%3)\n\t" \
+		      "movb %5,5(%3)\n\t" \
+		      "movb $0,6(%3)\n\t" \
+		      "movb %h2,7(%3)\n\t" \
+		      : "=m"(*(desc)), "=&q" (__tmp) \
+		      : "1" (addr), "r"(desc), "ir"(limit), "i"(type)); \
+} while (0)
+  
 #include <mach_desc.h>
 
 #define set_tss_desc(cpu,addr) __set_tss_desc(cpu, GDT_ENTRY_TSS, addr)

Re: [PATCH 5/14] i386 / Use early clobber to eliminate rotate in desc

[Andi Kleen]

On Wed, Aug 10, 2005 at 09:54:11PM -0700, zach@vmware.com wrote:
> Use an early clobber on addr to avoid the extra rorl instruction at the
> end of _set_tssldt_desc.

I would suggest to just use C for this. I do this on x86-64 and 
I don't think there is any reason to use this hard to maintain
code for it.

It's probably a left over from Linus first experiments with inline
assembly, similar to the old string.h and by now so obsolete
it doesn't even stink anymore.

-Andi

Re: [PATCH 5/14] i386 / Use early clobber to eliminate rotate in desc

[Zachary Amsden]

Andi Kleen wrote:

>On Wed, Aug 10, 2005 at 09:54:11PM -0700, zach@vmware.com wrote:
>  
>
>>Use an early clobber on addr to avoid the extra rorl instruction at the
>>end of _set_tssldt_desc.
>>    
>>
>
>I would suggest to just use C for this. I do this on x86-64 and 
>I don't think there is any reason to use this hard to maintain
>code for it.
>

This one in particular is non-optimal looking from C because the 
compiler misses the potential for rotation.  But, composing into 
temporaries and then issuing two writes to memory instead of multiple 
writes within the same word could actually get you a better cycle count, 
and that is something GCC just might be able to do :)

Zach

Re: [PATCH 5/14] i386 / Use early clobber to eliminate rotate in desc

[H. Peter Anvin]

Zachary Amsden wrote:
> 
> This one in particular is non-optimal looking from C because the 
> compiler misses the potential for rotation.  But, composing into 
> temporaries and then issuing two writes to memory instead of multiple 
> writes within the same word could actually get you a better cycle count, 
> and that is something GCC just might be able to do :)
> 

At least i386 and x86-64 gcc should recognize

	((foo << x) + (foo >> (32-x)))

... as a 32-bit rotate; similar for 8-, 16- and 64-bit rotates of 
appropriate sized items.  Also, it seems it could just be an inline 
function instead of a macro.

	-hpa

Re: [PATCH 5/14] i386 / Use early clobber to eliminate rotate in desc

[Alan Cox]

On Maw, 2005-08-16 at 17:06 -0700, H. Peter Anvin wrote:
> At least i386 and x86-64 gcc should recognize
> 
> 	((foo << x) + (foo >> (32-x)))
> 
> ... as a 32-bit rotate

Only for  1 <= x <= 31. For the x = 0 case the code posted is undefined
and at least in some cases gcc will do "interesting" things as a result
such as treating >> 32 as >> 0.

Alan

Re: [PATCH 5/14] i386 / Use early clobber to eliminate rotate in desc

[H. Peter Anvin]

Alan Cox wrote:
> On Maw, 2005-08-16 at 17:06 -0700, H. Peter Anvin wrote:
> 
>>At least i386 and x86-64 gcc should recognize
>>
>>	((foo << x) + (foo >> (32-x)))
>>
>>... as a 32-bit rotate
> 
> 
> Only for  1 <= x <= 31. For the x = 0 case the code posted is undefined
> and at least in some cases gcc will do "interesting" things as a result
> such as treating >> 32 as >> 0.
> 

True, although I thought gcc considered the above an idiom and would 
generate a rotate instruction.  Perhaps it should be | rather than +.

	-hpa