summaryrefslogtreecommitdiffstats
path: root/arch/powerpc/lib/strlen_32.S
diff options
context:
space:
mode:
Diffstat (limited to 'arch/powerpc/lib/strlen_32.S')
-rw-r--r--arch/powerpc/lib/strlen_32.S78
1 files changed, 78 insertions, 0 deletions
diff --git a/arch/powerpc/lib/strlen_32.S b/arch/powerpc/lib/strlen_32.S
new file mode 100644
index 0000000000..bbd24feb23
--- /dev/null
+++ b/arch/powerpc/lib/strlen_32.S
@@ -0,0 +1,78 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * strlen() for PPC32
+ *
+ * Copyright (C) 2018 Christophe Leroy CS Systemes d'Information.
+ *
+ * Inspired from glibc implementation
+ */
+#include <linux/export.h>
+#include <asm/ppc_asm.h>
+#include <asm/cache.h>
+
+ .text
+
+/*
+ * Algorithm:
+ *
+ * 1) Given a word 'x', we can test to see if it contains any 0 bytes
+ * by subtracting 0x01010101, and seeing if any of the high bits of each
+ * byte changed from 0 to 1. This works because the least significant
+ * 0 byte must have had no incoming carry (otherwise it's not the least
+ * significant), so it is 0x00 - 0x01 == 0xff. For all other
+ * byte values, either they have the high bit set initially, or when
+ * 1 is subtracted you get a value in the range 0x00-0x7f, none of which
+ * have their high bit set. The expression here is
+ * (x - 0x01010101) & ~x & 0x80808080), which gives 0x00000000 when
+ * there were no 0x00 bytes in the word. You get 0x80 in bytes that
+ * match, but possibly false 0x80 matches in the next more significant
+ * byte to a true match due to carries. For little-endian this is
+ * of no consequence since the least significant match is the one
+ * we're interested in, but big-endian needs method 2 to find which
+ * byte matches.
+ * 2) Given a word 'x', we can test to see _which_ byte was zero by
+ * calculating ~(((x & ~0x80808080) - 0x80808080 - 1) | x | ~0x80808080).
+ * This produces 0x80 in each byte that was zero, and 0x00 in all
+ * the other bytes. The '| ~0x80808080' clears the low 7 bits in each
+ * byte, and the '| x' part ensures that bytes with the high bit set
+ * produce 0x00. The addition will carry into the high bit of each byte
+ * iff that byte had one of its low 7 bits set. We can then just see
+ * which was the most significant bit set and divide by 8 to find how
+ * many to add to the index.
+ * This is from the book 'The PowerPC Compiler Writer's Guide',
+ * by Steve Hoxey, Faraydon Karim, Bill Hay and Hank Warren.
+ */
+
+_GLOBAL(strlen)
+ andi. r0, r3, 3
+ lis r7, 0x0101
+ addi r10, r3, -4
+ addic r7, r7, 0x0101 /* r7 = 0x01010101 (lomagic) & clear XER[CA] */
+ rotlwi r6, r7, 31 /* r6 = 0x80808080 (himagic) */
+ bne- 3f
+ .balign IFETCH_ALIGN_BYTES
+1: lwzu r9, 4(r10)
+2: subf r8, r7, r9
+ and. r8, r8, r6
+ beq+ 1b
+ andc. r8, r8, r9
+ beq+ 1b
+ andc r8, r9, r6
+ orc r9, r9, r6
+ subfe r8, r6, r8
+ nor r8, r8, r9
+ cntlzw r8, r8
+ subf r3, r3, r10
+ srwi r8, r8, 3
+ add r3, r3, r8
+ blr
+
+ /* Missaligned string: make sure bytes before string are seen not 0 */
+3: xor r10, r10, r0
+ orc r8, r8, r8
+ lwzu r9, 4(r10)
+ slwi r0, r0, 3
+ srw r8, r8, r0
+ orc r9, r9, r8
+ b 2b
+EXPORT_SYMBOL(strlen)