patch-2.4.2 linux/arch/alpha/lib/ev6-copy_page.S

• Lines: 204
• Date: Thu Feb 8 12:56:29 2001
• Orig file: v2.4.1/linux/arch/alpha/lib/ev6-copy_page.S
• Orig date: Wed Dec 31 16:00:00 1969

diff -u --recursive --new-file v2.4.1/linux/arch/alpha/lib/ev6-copy_page.S linux/arch/alpha/lib/ev6-copy_page.S
@@ -0,0 +1,203 @@
+/*
+ * arch/alpha/lib/ev6-copy_page.S
+ *
+ * Copy an entire page.
+ */
+
+/* The following comparison of this routine vs the normal copy_page.S
+   was written by an unnamed ev6 hardware designer and forwarded to me
+   via Steven Hobbs <hobbs@steven.zko.dec.com>.
+ 
+   First Problem: STQ overflows.
+   -----------------------------
+
+	It would be nice if EV6 handled every resource overflow efficiently,
+	but for some it doesn't.  Including store queue overflows.  It causes
+	a trap and a restart of the pipe.
+
+	To get around this we sometimes use (to borrow a term from a VSSAD
+	researcher) "aeration".  The idea is to slow the rate at which the
+	processor receives valid instructions by inserting nops in the fetch
+	path.  In doing so, you can prevent the overflow and actually make
+	the code run faster.  You can, of course, take advantage of the fact
+	that the processor can fetch at most 4 aligned instructions per cycle.
+
+	I inserted enough nops to force it to take 10 cycles to fetch the
+	loop code.  In theory, EV6 should be able to execute this loop in
+	9 cycles but I was not able to get it to run that fast -- the initial
+	conditions were such that I could not reach this optimum rate on
+	(chaotic) EV6.  I wrote the code such that everything would issue
+	in order. 
+
+   Second Problem: Dcache index matches.
+   -------------------------------------
+
+	If you are going to use this routine on random aligned pages, there
+	is a 25% chance that the pages will be at the same dcache indices.
+	This results in many nasty memory traps without care.
+
+	The solution is to schedule the prefetches to avoid the memory
+	conflicts.  I schedule the wh64 prefetches farther ahead of the
+	read prefetches to avoid this problem.
+
+   Third Problem: Needs more prefetching.
+   --------------------------------------
+
+	In order to improve the code I added deeper prefetching to take the
+	most advantage of EV6's bandwidth.
+
+	I also prefetched the read stream. Note that adding the read prefetch
+	forced me to add another cycle to the inner-most kernel - up to 11
+	from the original 8 cycles per iteration.  We could improve performance
+	further by unrolling the loop and doing multiple prefetches per cycle.
+
+   I think that the code below will be very robust and fast code for the
+   purposes of copying aligned pages.  It is slower when both source and
+   destination pages are in the dcache, but it is my guess that this is
+   less important than the dcache miss case.  */
+
+
+	.text
+	.align 4
+	.global copy_page
+	.ent copy_page
+copy_page:
+	.prologue 0
+
+	/* Prefetch 5 read cachelines; write-hint 10 cache lines.  */
+	wh64	($16)
+	ldl	$31,0($17)
+	ldl	$31,64($17)
+	lda	$1,1*64($16)
+
+	wh64	($1)
+	ldl	$31,128($17)
+	ldl	$31,192($17)
+	lda	$1,2*64($16)
+
+	wh64	($1)
+	ldl	$31,256($17)
+	lda	$18,118
+	lda	$1,3*64($16)
+
+	wh64	($1)
+	nop
+	lda	$1,4*64($16)
+	lda	$2,5*64($16)
+
+	wh64	($1)
+	wh64	($2)
+	lda	$1,6*64($16)
+	lda	$2,7*64($16)
+
+	wh64	($1)
+	wh64	($2)
+	lda	$1,8*64($16)
+	lda	$2,9*64($16)
+
+	wh64	($1)
+	wh64	($2)
+	lda	$19,10*64($16)
+	nop
+
+	/* Main prefetching/write-hinting loop.  */
+1:	ldq	$0,0($17)
+	ldq	$1,8($17)
+	unop
+	unop
+
+	unop
+	unop
+	ldq	$2,16($17)
+	ldq	$3,24($17)
+
+	ldq	$4,32($17)
+	ldq	$5,40($17)
+	unop
+	unop
+
+	unop
+	unop
+	ldq	$6,48($17)
+	ldq	$7,56($17)
+
+	ldl	$31,320($17)
+	unop
+	unop
+	unop
+
+	/* This gives the extra cycle of aeration above the minimum.  */
+	unop			
+	unop
+	unop
+	unop
+
+	wh64	($19)
+	unop
+	unop
+	unop
+
+	stq	$0,0($16)
+	subq	$18,1,$18
+	stq	$1,8($16)
+	unop
+
+	unop
+	stq	$2,16($16)
+	addq	$17,64,$17
+	stq	$3,24($16)
+
+	stq	$4,32($16)
+	stq	$5,40($16)
+	addq	$19,64,$19
+	unop
+
+	stq	$6,48($16)
+	stq	$7,56($16)
+	addq	$16,64,$16
+	bne	$18, 1b
+
+	/* Prefetch the final 5 cache lines of the read stream.  */
+	lda	$18,10
+	ldl	$31,320($17)
+	ldl	$31,384($17)
+	ldl	$31,448($17)
+
+	ldl	$31,512($17)
+	ldl	$31,576($17)
+	nop
+	nop
+
+	/* Non-prefetching, non-write-hinting cleanup loop for the
+	   final 10 cache lines.  */
+2:	ldq	$0,0($17)
+	ldq	$1,8($17)
+	ldq	$2,16($17)
+	ldq	$3,24($17)
+
+	ldq	$4,32($17)
+	ldq	$5,40($17)
+	ldq	$6,48($17)
+	ldq	$7,56($17)
+
+	stq	$0,0($16)
+	subq	$18,1,$18
+	stq	$1,8($16)
+	addq	$17,64,$17
+
+	stq	$2,16($16)
+	stq	$3,24($16)
+	stq	$4,32($16)
+	stq	$5,40($16)
+
+	stq	$6,48($16)
+	stq	$7,56($16)
+	addq	$16,64,$16
+	bne	$18, 2b
+
+	ret
+	nop
+	unop
+	nop
+
+	.end copy_page