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/*
*
* 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, 2001 Hewlett-Packard Co
* Stephane Eranian <eranian@hpl.hp.com>
*
* 09/24/99 S.Eranian add speculation recovery code
*/
#include <asm/asmmacro.h>
//
//
// 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
GLOBAL_ENTRY(strlen)
.prologue
.save ar.pfs, saved_pfs
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
.save pr, saved_pr
mov saved_pr=pr // preserve predicates (rotation)
;;
.body
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 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 // 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.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
//
// 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 2b // loop until p6 == 0
;; // (avoid WAW on p63)
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.many rp // end of successful recovery code
END(strlen)
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