| /* SPDX-License-Identifier: GPL-2.0 */ |
| /* |
| * |
| * 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 <linux/export.h> |
| #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) |
| EXPORT_SYMBOL(strlen) |
