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+/*
+ * arch/alpha/lib/ev6-strncpy_from_user.S
+ * 21264 version contributed by Rick Gorton <rick.gorton@alpha-processor.com>
+ *
+ * Just like strncpy except in the return value:
+ *
+ * -EFAULT if an exception occurs before the terminator is copied.
+ * N if the buffer filled.
+ *
+ * Otherwise the length of the string is returned.
+ *
+ * Much of the information about 21264 scheduling/coding comes from:
+ * Compiler Writer's Guide for the Alpha 21264
+ * abbreviated as 'CWG' in other comments here
+ * ftp.digital.com/pub/Digital/info/semiconductor/literature/dsc-library.html
+ * Scheduling notation:
+ * E - either cluster
+ * U - upper subcluster; U0 - subcluster U0; U1 - subcluster U1
+ * L - lower subcluster; L0 - subcluster L0; L1 - subcluster L1
+ * A bunch of instructions got moved and temp registers were changed
+ * to aid in scheduling. Control flow was also re-arranged to eliminate
+ * branches, and to provide longer code sequences to enable better scheduling.
+ * A total rewrite (using byte load/stores for start & tail sequences)
+ * is desirable, but very difficult to do without a from-scratch rewrite.
+ * Save that for the future.
+ */
+
+
+#include <asm/errno.h>
+#include <asm/regdef.h>
+
+
+/* Allow an exception for an insn; exit if we get one. */
+#define EX(x,y...) \
+ 99: x,##y; \
+ .section __ex_table,"a"; \
+ .long 99b - .; \
+ lda $31, $exception-99b($0); \
+ .previous
+
+
+ .set noat
+ .set noreorder
+ .text
+
+ .globl __strncpy_from_user
+ .ent __strncpy_from_user
+ .frame $30, 0, $26
+ .prologue 0
+
+ .align 4
+__strncpy_from_user:
+ and a0, 7, t3 # E : find dest misalignment
+ beq a2, $zerolength # U :
+
+ /* Are source and destination co-aligned? */
+ mov a0, v0 # E : save the string start
+ xor a0, a1, t4 # E :
+ EX( ldq_u t1, 0(a1) ) # L : Latency=3 load first quadword
+ ldq_u t0, 0(a0) # L : load first (partial) aligned dest quadword
+
+ addq a2, t3, a2 # E : bias count by dest misalignment
+ subq a2, 1, a3 # E :
+ addq zero, 1, t10 # E :
+ and t4, 7, t4 # E : misalignment between the two
+
+ and a3, 7, t6 # E : number of tail bytes
+ sll t10, t6, t10 # E : t10 = bitmask of last count byte
+ bne t4, $unaligned # U :
+ lda t2, -1 # E : build a mask against false zero
+
+ /*
+ * We are co-aligned; take care of a partial first word.
+ * On entry to this basic block:
+ * t0 == the first destination word for masking back in
+ * t1 == the first source word.
+ */
+
+ srl a3, 3, a2 # E : a2 = loop counter = (count - 1)/8
+ addq a1, 8, a1 # E :
+ mskqh t2, a1, t2 # U : detection in the src word
+ nop
+
+ /* Create the 1st output word and detect 0's in the 1st input word. */
+ mskqh t1, a1, t3 # U :
+ mskql t0, a1, t0 # U : assemble the first output word
+ ornot t1, t2, t2 # E :
+ nop
+
+ cmpbge zero, t2, t8 # E : bits set iff null found
+ or t0, t3, t0 # E :
+ beq a2, $a_eoc # U :
+ bne t8, $a_eos # U : 2nd branch in a quad. Bad.
+
+ /* On entry to this basic block:
+ * t0 == a source quad not containing a null.
+ * a0 - current aligned destination address
+ * a1 - current aligned source address
+ * a2 - count of quadwords to move.
+ * NOTE: Loop improvement - unrolling this is going to be
+ * a huge win, since we're going to stall otherwise.
+ * Fix this later. For _really_ large copies, look
+ * at using wh64 on a look-ahead basis. See the code
+ * in clear_user.S and copy_user.S.
+ * Presumably, since (a0) and (a1) do not overlap (by C definition)
+ * Lots of nops here:
+ * - Separate loads from stores
+ * - Keep it to 1 branch/quadpack so the branch predictor
+ * can train.
+ */
+$a_loop:
+ stq_u t0, 0(a0) # L :
+ addq a0, 8, a0 # E :
+ nop
+ subq a2, 1, a2 # E :
+
+ EX( ldq_u t0, 0(a1) ) # L :
+ addq a1, 8, a1 # E :
+ cmpbge zero, t0, t8 # E : Stall 2 cycles on t0
+ beq a2, $a_eoc # U :
+
+ beq t8, $a_loop # U :
+ nop
+ nop
+ nop
+
+ /* Take care of the final (partial) word store. At this point
+ * the end-of-count bit is set in t8 iff it applies.
+ *
+ * On entry to this basic block we have:
+ * t0 == the source word containing the null
+ * t8 == the cmpbge mask that found it.
+ */
+$a_eos:
+ negq t8, t12 # E : find low bit set
+ and t8, t12, t12 # E :
+
+ /* We're doing a partial word store and so need to combine
+ our source and original destination words. */
+ ldq_u t1, 0(a0) # L :
+ subq t12, 1, t6 # E :
+
+ or t12, t6, t8 # E :
+ zapnot t0, t8, t0 # U : clear src bytes > null
+ zap t1, t8, t1 # U : clear dst bytes <= null
+ or t0, t1, t0 # E :
+
+ stq_u t0, 0(a0) # L :
+ br $finish_up # L0 :
+ nop
+ nop
+
+ /* Add the end-of-count bit to the eos detection bitmask. */
+ .align 4
+$a_eoc:
+ or t10, t8, t8
+ br $a_eos
+ nop
+ nop
+
+
+/* The source and destination are not co-aligned. Align the destination
+ and cope. We have to be very careful about not reading too much and
+ causing a SEGV. */
+
+ .align 4
+$u_head:
+ /* We know just enough now to be able to assemble the first
+ full source word. We can still find a zero at the end of it
+ that prevents us from outputting the whole thing.
+
+ On entry to this basic block:
+ t0 == the first dest word, unmasked
+ t1 == the shifted low bits of the first source word
+ t6 == bytemask that is -1 in dest word bytes */
+
+ EX( ldq_u t2, 8(a1) ) # L : load second src word
+ addq a1, 8, a1 # E :
+ mskql t0, a0, t0 # U : mask trailing garbage in dst
+ extqh t2, a1, t4 # U :
+
+ or t1, t4, t1 # E : first aligned src word complete
+ mskqh t1, a0, t1 # U : mask leading garbage in src
+ or t0, t1, t0 # E : first output word complete
+ or t0, t6, t6 # E : mask original data for zero test
+
+ cmpbge zero, t6, t8 # E :
+ beq a2, $u_eocfin # U :
+ bne t8, $u_final # U : bad news - 2nd branch in a quad
+ lda t6, -1 # E : mask out the bits we have
+
+ mskql t6, a1, t6 # U : already seen
+ stq_u t0, 0(a0) # L : store first output word
+ or t6, t2, t2 # E :
+ cmpbge zero, t2, t8 # E : find nulls in second partial
+
+ addq a0, 8, a0 # E :
+ subq a2, 1, a2 # E :
+ bne t8, $u_late_head_exit # U :
+ nop
+
+ /* Finally, we've got all the stupid leading edge cases taken care
+ of and we can set up to enter the main loop. */
+
+ extql t2, a1, t1 # U : position hi-bits of lo word
+ EX( ldq_u t2, 8(a1) ) # L : read next high-order source word
+ addq a1, 8, a1 # E :
+ cmpbge zero, t2, t8 # E :
+
+ beq a2, $u_eoc # U :
+ bne t8, $u_eos # U :
+ nop
+ nop
+
+ /* Unaligned copy main loop. In order to avoid reading too much,
+ the loop is structured to detect zeros in aligned source words.
+ This has, unfortunately, effectively pulled half of a loop
+ iteration out into the head and half into the tail, but it does
+ prevent nastiness from accumulating in the very thing we want
+ to run as fast as possible.
+
+ On entry to this basic block:
+ t1 == the shifted high-order bits from the previous source word
+ t2 == the unshifted current source word
+
+ We further know that t2 does not contain a null terminator. */
+
+ /*
+ * Extra nops here:
+ * separate load quads from store quads
+ * only one branch/quad to permit predictor training
+ */
+
+ .align 4
+$u_loop:
+ extqh t2, a1, t0 # U : extract high bits for current word
+ addq a1, 8, a1 # E :
+ extql t2, a1, t3 # U : extract low bits for next time
+ addq a0, 8, a0 # E :
+
+ or t0, t1, t0 # E : current dst word now complete
+ EX( ldq_u t2, 0(a1) ) # L : load high word for next time
+ subq a2, 1, a2 # E :
+ nop
+
+ stq_u t0, -8(a0) # L : save the current word
+ mov t3, t1 # E :
+ cmpbge zero, t2, t8 # E : test new word for eos
+ beq a2, $u_eoc # U :
+
+ beq t8, $u_loop # U :
+ nop
+ nop
+ nop
+
+ /* We've found a zero somewhere in the source word we just read.
+ If it resides in the lower half, we have one (probably partial)
+ word to write out, and if it resides in the upper half, we
+ have one full and one partial word left to write out.
+
+ On entry to this basic block:
+ t1 == the shifted high-order bits from the previous source word
+ t2 == the unshifted current source word. */
+ .align 4
+$u_eos:
+ extqh t2, a1, t0 # U :
+ or t0, t1, t0 # E : first (partial) source word complete
+ cmpbge zero, t0, t8 # E : is the null in this first bit?
+ nop
+
+ bne t8, $u_final # U :
+ stq_u t0, 0(a0) # L : the null was in the high-order bits
+ addq a0, 8, a0 # E :
+ subq a2, 1, a2 # E :
+
+ .align 4
+$u_late_head_exit:
+ extql t2, a1, t0 # U :
+ cmpbge zero, t0, t8 # E :
+ or t8, t10, t6 # E :
+ cmoveq a2, t6, t8 # E :
+
+ /* Take care of a final (probably partial) result word.
+ On entry to this basic block:
+ t0 == assembled source word
+ t8 == cmpbge mask that found the null. */
+ .align 4
+$u_final:
+ negq t8, t6 # E : isolate low bit set
+ and t6, t8, t12 # E :
+ ldq_u t1, 0(a0) # L :
+ subq t12, 1, t6 # E :
+
+ or t6, t12, t8 # E :
+ zapnot t0, t8, t0 # U : kill source bytes > null
+ zap t1, t8, t1 # U : kill dest bytes <= null
+ or t0, t1, t0 # E :
+
+ stq_u t0, 0(a0) # E :
+ br $finish_up # U :
+ nop
+ nop
+
+ .align 4
+$u_eoc: # end-of-count
+ extqh t2, a1, t0 # U :
+ or t0, t1, t0 # E :
+ cmpbge zero, t0, t8 # E :
+ nop
+
+ .align 4
+$u_eocfin: # end-of-count, final word
+ or t10, t8, t8 # E :
+ br $u_final # U :
+ nop
+ nop
+
+ /* Unaligned copy entry point. */
+ .align 4
+$unaligned:
+
+ srl a3, 3, a2 # U : a2 = loop counter = (count - 1)/8
+ and a0, 7, t4 # E : find dest misalignment
+ and a1, 7, t5 # E : find src misalignment
+ mov zero, t0 # E :
+
+ /* Conditionally load the first destination word and a bytemask
+ with 0xff indicating that the destination byte is sacrosanct. */
+
+ mov zero, t6 # E :
+ beq t4, 1f # U :
+ ldq_u t0, 0(a0) # L :
+ lda t6, -1 # E :
+
+ mskql t6, a0, t6 # E :
+ nop
+ nop
+ nop
+
+ .align 4
+1:
+ subq a1, t4, a1 # E : sub dest misalignment from src addr
+ /* If source misalignment is larger than dest misalignment, we need
+ extra startup checks to avoid SEGV. */
+ cmplt t4, t5, t12 # E :
+ extql t1, a1, t1 # U : shift src into place
+ lda t2, -1 # E : for creating masks later
+
+ beq t12, $u_head # U :
+ mskqh t2, t5, t2 # U : begin src byte validity mask
+ cmpbge zero, t1, t8 # E : is there a zero?
+ nop
+
+ extql t2, a1, t2 # U :
+ or t8, t10, t5 # E : test for end-of-count too
+ cmpbge zero, t2, t3 # E :
+ cmoveq a2, t5, t8 # E : Latency=2, extra map slot
+
+ nop # E : goes with cmov
+ andnot t8, t3, t8 # E :
+ beq t8, $u_head # U :
+ nop
+
+ /* At this point we've found a zero in the first partial word of
+ the source. We need to isolate the valid source data and mask
+ it into the original destination data. (Incidentally, we know
+ that we'll need at least one byte of that original dest word.) */
+
+ ldq_u t0, 0(a0) # L :
+ negq t8, t6 # E : build bitmask of bytes <= zero
+ mskqh t1, t4, t1 # U :
+ and t6, t8, t12 # E :
+
+ subq t12, 1, t6 # E :
+ or t6, t12, t8 # E :
+ zapnot t2, t8, t2 # U : prepare source word; mirror changes
+ zapnot t1, t8, t1 # U : to source validity mask
+
+ andnot t0, t2, t0 # E : zero place for source to reside
+ or t0, t1, t0 # E : and put it there
+ stq_u t0, 0(a0) # L :
+ nop
+
+ .align 4
+$finish_up:
+ zapnot t0, t12, t4 # U : was last byte written null?
+ and t12, 0xf0, t3 # E : binary search for the address of the
+ cmovne t4, 1, t4 # E : Latency=2, extra map slot
+ nop # E : with cmovne
+
+ and t12, 0xcc, t2 # E : last byte written
+ and t12, 0xaa, t1 # E :
+ cmovne t3, 4, t3 # E : Latency=2, extra map slot
+ nop # E : with cmovne
+
+ bic a0, 7, t0
+ cmovne t2, 2, t2 # E : Latency=2, extra map slot
+ nop # E : with cmovne
+ nop
+
+ cmovne t1, 1, t1 # E : Latency=2, extra map slot
+ nop # E : with cmovne
+ addq t0, t3, t0 # E :
+ addq t1, t2, t1 # E :
+
+ addq t0, t1, t0 # E :
+ addq t0, t4, t0 # add one if we filled the buffer
+ subq t0, v0, v0 # find string length
+ ret # L0 :
+
+ .align 4
+$zerolength:
+ nop
+ nop
+ nop
+ clr v0
+
+$exception:
+ nop
+ nop
+ nop
+ ret
+
+ .end __strncpy_from_user