/* * * 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)