patch-2.3.43 linux/arch/ia64/lib/strlen.S

• Lines: 198
• Date: Wed Feb 9 19:45:43 2000
• Orig file: v2.3.42/linux/arch/ia64/lib/strlen.S
• Orig date: Wed Dec 31 16:00:00 1969

diff -u --recursive --new-file v2.3.42/linux/arch/ia64/lib/strlen.S linux/arch/ia64/lib/strlen.S
@@ -0,0 +1,197 @@
+/*
+ *
+ * Optimized version of the standard strlen() function
+ *
+ *
+ * Inputs:
+ *	in0	address of string
+ * 
+ * Outputs: 
+ * 	ret0	the number of characters in the string (0 if empty string)
+ *         	does not count the \0
+ *
+ * Copyright (C) 1999 Hewlett-Packard Co
+ * Copyright (C) 1999 Stephane Eranian <eranian@hpl.hp.com>
+ *
+ * 09/24/99 S.Eranian add speculation recovery code
+ */
+
+//
+//
+// This is an enhanced version of the basic strlen. it includes a combination
+// of compute zero index (czx), parallel comparisons, speculative loads and
+// loop unroll using rotating registers.
+//
+// General Ideas about the algorithm:
+//	  The goal is to look at the string in chunks of 8 bytes.
+//	  so we need to do a few extra checks at the beginning because the
+//	  string may not be 8-byte aligned. In this case we load the 8byte
+//	  quantity which includes the start of the string and mask the unused
+//	  bytes with 0xff to avoid confusing czx.
+//	  We use speculative loads and software pipelining to hide memory 
+//	  latency and do read ahead safely. This way we defer any exception.
+//
+//	  Because we don't want the kernel to be relying on particular
+//	  settings of the DCR register, we provide recovery code in case
+//	  speculation fails. The recovery code is going to "redo" the work using
+//	  only normal loads. If we still get a fault then we generate a
+//	  kernel panic. Otherwise we return the strlen as usual.
+//
+//	  The fact that speculation may fail can be caused, for instance, by
+//	  the DCR.dm bit being set. In this case TLB misses are deferred, i.e.,
+//	  a NaT bit will be set if the translation is not present. The normal
+//	  load, on the other hand, will cause the translation to be inserted 
+//	  if the mapping exists.
+//
+//	  It should be noted that we execute recovery code only when we need
+//	  to use the data that has been speculatively loaded: we don't execute
+//	  recovery code on pure read ahead data.
+//
+// Remarks:
+//	- the cmp r0,r0 is used as a fast way to initialize a predicate 
+//	  register to 1. This is required to make sure that we get the parallel
+//	  compare correct.
+//
+//	- we don't use the epilogue counter to exit the loop but we need to set
+//	  it to zero beforehand.
+//
+//	- after the loop we must test for Nat values because neither the 
+//	  czx nor cmp instruction raise a NaT consumption fault. We must be
+//	  careful not to look too far for a Nat for which we don't care. 
+//	  For instance we don't need to look at a NaT in val2 if the zero byte
+//	  was in val1.
+//
+//	- Clearly performance tuning is required.
+//
+// 
+//
+#define saved_pfs	r11
+#define	tmp		r10
+#define base		r16
+#define orig		r17
+#define saved_pr	r18
+#define src		r19
+#define mask		r20
+#define val		r21
+#define val1		r22
+#define val2		r23
+
+
+	.text
+	.psr abi64
+	.psr lsb
+	.lsb
+
+	.align 32
+	.global strlen
+	.proc strlen
+strlen:
+	alloc saved_pfs=ar.pfs,11,0,0,8 // rotating must be multiple of 8
+
+	.rotr v[2], w[2]	// declares our 4 aliases
+
+	extr.u tmp=in0,0,3	// tmp=least significant 3 bits
+	mov orig=in0		// keep trackof initial byte address
+	dep src=0,in0,0,3	// src=8byte-aligned in0 address
+	mov saved_pr=pr		// preserve predicates (rotation)
+	;;
+	ld8 v[1]=[src],8	// must not speculate: can fail here
+	shl tmp=tmp,3		// multiply by 8bits/byte
+	mov mask=-1		// our mask
+	;;
+	ld8.s w[1]=[src],8	// speculatively load next
+	cmp.eq p6,p0=r0,r0	// sets p6 to true for cmp.and
+	sub tmp=64,tmp		// how many bits to shift our mask on the right
+	;;
+	shr.u	mask=mask,tmp	// zero enough bits to hold v[1] valuable part
+	mov ar.ec=r0		// clear epilogue counter (saved in ar.pfs)
+	;;
+	add base=-16,src	// keep track of aligned base
+	or v[1]=v[1],mask	// now we have a safe initial byte pattern
+	;;
+1:
+	ld8.s v[0]=[src],8	// speculatively load next 
+	czx1.r val1=v[1]	// search 0 byte from right 
+	czx1.r val2=w[1]	// search 0 byte from right following 8bytes
+	;;
+	ld8.s w[0]=[src],8	// speculatively load next to next
+	cmp.eq.and p6,p0=8,val1	// p6 = p6 and val1==8
+	cmp.eq.and p6,p0=8,val2	// p6 = p6 and mask==8
+(p6)	br.wtop.dptk.few 1b	// loop until p6 == 0
+	;;
+	//
+	// We must return try the recovery code iff
+	// val1_is_nat || (val1==8 && val2_is_nat)
+	//
+	// XXX Fixme
+	//	- there must be a better way of doing the test
+	//
+	cmp.eq  p8,p9=8,val1	// p6 = val1 had zero (disambiguate)
+#ifdef notyet
+	tnat.nz p6,p7=val1	// test NaT on val1
+#else
+	tnat.z p7,p6=val1	// test NaT on val1
+#endif
+(p6)	br.cond.spnt.few recover// jump to recovery if val1 is NaT
+	;;
+	//
+	// if we come here p7 is true, i.e., initialized for // cmp
+	//
+	cmp.eq.and  p7,p0=8,val1// val1==8?
+	tnat.nz.and p7,p0=val2	// test NaT if val2
+(p7)	br.cond.spnt.few recover// jump to recovery if val2 is NaT
+	;;
+(p8)	mov val1=val2		// the other test got us out of the loop
+(p8)	adds src=-16,src	// correct position when 3 ahead
+(p9)	adds src=-24,src	// correct position when 4 ahead
+	;;
+	sub ret0=src,orig	// distance from base
+	sub tmp=8,val1		// which byte in word
+	mov pr=saved_pr,0xffffffffffff0000
+	;;
+	sub ret0=ret0,tmp	// adjust 
+	mov ar.pfs=saved_pfs	// because of ar.ec, restore no matter what
+	br.ret.sptk.few rp	// end of normal execution
+
+	//
+	// Outlined recovery code when speculation failed
+	//
+	// This time we don't use speculation and rely on the normal exception
+	// mechanism. that's why the loop is not as good as the previous one
+	// because read ahead is not possible
+	//
+	// IMPORTANT:
+	// Please note that in the case of strlen() as opposed to strlen_user()
+	// we don't use the exception mechanism, as this function is not 
+	// supposed to fail. If that happens it means we have a bug and the 
+	// code will cause of kernel fault.
+	//
+	// XXX Fixme
+	//	- today we restart from the beginning of the string instead
+	//	  of trying to continue where we left off.
+	//
+recover:
+	ld8 val=[base],8	// will fail if unrecoverable fault
+	;;
+	or val=val,mask		// remask first bytes
+	cmp.eq p0,p6=r0,r0	// nullify first ld8 in loop
+	;;
+	//
+	// ar.ec is still zero here
+	//
+2:
+(p6)	ld8 val=[base],8	// will fail if unrecoverable fault
+	;;
+	czx1.r val1=val		// search 0 byte from right 
+	;;
+	cmp.eq p6,p0=8,val1	// val1==8 ?
+(p6)	br.wtop.dptk.few 2b	// loop until p6 == 0
+	sub ret0=base,orig	// distance from base
+	sub tmp=8,val1
+	mov pr=saved_pr,0xffffffffffff0000
+	;;
+	sub ret0=ret0,tmp	// length=now - back -1
+	mov ar.pfs=saved_pfs	// because of ar.ec, restore no matter what
+	br.ret.sptk.few rp	// end of sucessful recovery code
+
+	.endp strlen