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+/*
+ * Optimized version of the strlen_user() function
+ *
+ * Inputs:
+ * in0 address of buffer
+ *
+ * Outputs:
+ * ret0 0 in case of fault, strlen(buffer)+1 otherwise
+ *
+ * Copyright (C) 1998, 1999, 2001 Hewlett-Packard Co
+ * David Mosberger-Tang <davidm@hpl.hp.com>
+ * Stephane Eranian <eranian@hpl.hp.com>
+ *
+ * 01/19/99 S.Eranian heavily enhanced version (see details below)
+ * 09/24/99 S.Eranian added speculation recovery code
+ */
+
+#include <asm/asmmacro.h>
+
+//
+// int strlen_user(char *)
+// ------------------------
+// Returns:
+// - length of string + 1
+// - 0 in case an exception is raised
+//
+// This is an enhanced version of the basic strlen_user. 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 return an
+// error (ret0=0). Otherwise we return the strlen+1 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
+
+GLOBAL_ENTRY(__strlen_user)
+ .prologue
+ .save ar.pfs, saved_pfs
+ alloc saved_pfs=ar.pfs,11,0,0,8
+
+ .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
+ .save pr, saved_pr
+ mov saved_pr=pr // preserve predicates (rotation)
+ ;;
+
+ .body
+
+ ld8.s v[1]=[src],8 // load the initial 8bytes (must speculate)
+ shl tmp=tmp,3 // multiply by 8bits/byte
+ mov mask=-1 // our mask
+ ;;
+ ld8.s w[1]=[src],8 // load next 8 bytes in 2nd pipeline
+ cmp.eq p6,p0=r0,r0 // sets p6 (required because of // 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
+ chk.s v[1], .recover // if already NaT, then directly skip to recover
+ 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)
+ tnat.nz p6,p7=val1 // test NaT on val1
+(p6) br.cond.spnt .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 .recover // jump to recovery if val2 is NaT
+ ;;
+(p8) mov val1=val2 // val2 contains the value
+(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 origin
+ sub tmp=7,val1 // 7=8-1 because this strlen returns strlen+1
+ 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.many 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
+ //
+ // XXX Fixme
+ // - today we restart from the beginning of the string instead
+ // of trying to continue where we left off.
+ //
+.recover:
+ EX(.Lexit1, ld8 val=[base],8) // load the initial bytes
+ ;;
+ 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:
+ EX(.Lexit1, (p6) ld8 val=[base],8)
+ ;;
+ 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=7,val1 // 7=8-1 because this strlen returns strlen+1
+ 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.many rp // end of successful recovery code
+
+ //
+ // We failed even on the normal load (called from exception handler)
+ //
+.Lexit1:
+ mov ret0=0
+ mov pr=saved_pr,0xffffffffffff0000
+ mov ar.pfs=saved_pfs // because of ar.ec, restore no matter what
+ br.ret.sptk.many rp
+END(__strlen_user)