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Diffstat (limited to 'arch/ia64/lib/strlen_user.S')
| -rw-r--r-- | arch/ia64/lib/strlen_user.S | 198 |
1 files changed, 198 insertions, 0 deletions
diff --git a/arch/ia64/lib/strlen_user.S b/arch/ia64/lib/strlen_user.S new file mode 100644 index 00000000..c71eded4 --- /dev/null +++ b/arch/ia64/lib/strlen_user.S @@ -0,0 +1,198 @@ +/* + * 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) |