-/*
- * 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>
-#include <asm/export.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)
-EXPORT_SYMBOL(__strlen_user)
git.samba.org / sfrench / cifs-2.6.git / commitdiff