| /* |
| * strcmp - compare two strings |
| * |
| * Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. |
| * See https://llvm.org/LICENSE.txt for license information. |
| * SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception |
| */ |
| |
| /* Assumptions: |
| * |
| * ARMv8-a, AArch64 |
| */ |
| |
| #include "../asmdefs.h" |
| |
| #define REP8_01 0x0101010101010101 |
| #define REP8_7f 0x7f7f7f7f7f7f7f7f |
| #define REP8_80 0x8080808080808080 |
| |
| /* Parameters and result. */ |
| #define src1 x0 |
| #define src2 x1 |
| #define result x0 |
| |
| /* Internal variables. */ |
| #define data1 x2 |
| #define data1w w2 |
| #define data2 x3 |
| #define data2w w3 |
| #define has_nul x4 |
| #define diff x5 |
| #define syndrome x6 |
| #define tmp1 x7 |
| #define tmp2 x8 |
| #define tmp3 x9 |
| #define zeroones x10 |
| #define pos x11 |
| |
| /* Start of performance-critical section -- one 64B cache line. */ |
| ENTRY (__strcmp_aarch64) |
| eor tmp1, src1, src2 |
| mov zeroones, #REP8_01 |
| tst tmp1, #7 |
| b.ne L(misaligned8) |
| ands tmp1, src1, #7 |
| b.ne L(mutual_align) |
| /* NUL detection works on the principle that (X - 1) & (~X) & 0x80 |
| (=> (X - 1) & ~(X | 0x7f)) is non-zero iff a byte is zero, and |
| can be done in parallel across the entire word. */ |
| L(loop_aligned): |
| ldr data1, [src1], #8 |
| ldr data2, [src2], #8 |
| L(start_realigned): |
| sub tmp1, data1, zeroones |
| orr tmp2, data1, #REP8_7f |
| eor diff, data1, data2 /* Non-zero if differences found. */ |
| bic has_nul, tmp1, tmp2 /* Non-zero if NUL terminator. */ |
| orr syndrome, diff, has_nul |
| cbz syndrome, L(loop_aligned) |
| /* End of performance-critical section -- one 64B cache line. */ |
| |
| L(end): |
| #ifndef __AARCH64EB__ |
| rev syndrome, syndrome |
| rev data1, data1 |
| /* The MS-non-zero bit of the syndrome marks either the first bit |
| that is different, or the top bit of the first zero byte. |
| Shifting left now will bring the critical information into the |
| top bits. */ |
| clz pos, syndrome |
| rev data2, data2 |
| lsl data1, data1, pos |
| lsl data2, data2, pos |
| /* But we need to zero-extend (char is unsigned) the value and then |
| perform a signed 32-bit subtraction. */ |
| lsr data1, data1, #56 |
| sub result, data1, data2, lsr #56 |
| ret |
| #else |
| /* For big-endian we cannot use the trick with the syndrome value |
| as carry-propagation can corrupt the upper bits if the trailing |
| bytes in the string contain 0x01. */ |
| /* However, if there is no NUL byte in the dword, we can generate |
| the result directly. We can't just subtract the bytes as the |
| MSB might be significant. */ |
| cbnz has_nul, 1f |
| cmp data1, data2 |
| cset result, ne |
| cneg result, result, lo |
| ret |
| 1: |
| /* Re-compute the NUL-byte detection, using a byte-reversed value. */ |
| rev tmp3, data1 |
| sub tmp1, tmp3, zeroones |
| orr tmp2, tmp3, #REP8_7f |
| bic has_nul, tmp1, tmp2 |
| rev has_nul, has_nul |
| orr syndrome, diff, has_nul |
| clz pos, syndrome |
| /* The MS-non-zero bit of the syndrome marks either the first bit |
| that is different, or the top bit of the first zero byte. |
| Shifting left now will bring the critical information into the |
| top bits. */ |
| lsl data1, data1, pos |
| lsl data2, data2, pos |
| /* But we need to zero-extend (char is unsigned) the value and then |
| perform a signed 32-bit subtraction. */ |
| lsr data1, data1, #56 |
| sub result, data1, data2, lsr #56 |
| ret |
| #endif |
| |
| L(mutual_align): |
| /* Sources are mutually aligned, but are not currently at an |
| alignment boundary. Round down the addresses and then mask off |
| the bytes that preceed the start point. */ |
| bic src1, src1, #7 |
| bic src2, src2, #7 |
| lsl tmp1, tmp1, #3 /* Bytes beyond alignment -> bits. */ |
| ldr data1, [src1], #8 |
| neg tmp1, tmp1 /* Bits to alignment -64. */ |
| ldr data2, [src2], #8 |
| mov tmp2, #~0 |
| #ifdef __AARCH64EB__ |
| /* Big-endian. Early bytes are at MSB. */ |
| lsl tmp2, tmp2, tmp1 /* Shift (tmp1 & 63). */ |
| #else |
| /* Little-endian. Early bytes are at LSB. */ |
| lsr tmp2, tmp2, tmp1 /* Shift (tmp1 & 63). */ |
| #endif |
| orr data1, data1, tmp2 |
| orr data2, data2, tmp2 |
| b L(start_realigned) |
| |
| L(misaligned8): |
| /* Align SRC1 to 8 bytes and then compare 8 bytes at a time, always |
| checking to make sure that we don't access beyond page boundary in |
| SRC2. */ |
| tst src1, #7 |
| b.eq L(loop_misaligned) |
| L(do_misaligned): |
| ldrb data1w, [src1], #1 |
| ldrb data2w, [src2], #1 |
| cmp data1w, #1 |
| ccmp data1w, data2w, #0, cs /* NZCV = 0b0000. */ |
| b.ne L(done) |
| tst src1, #7 |
| b.ne L(do_misaligned) |
| |
| L(loop_misaligned): |
| /* Test if we are within the last dword of the end of a 4K page. If |
| yes then jump back to the misaligned loop to copy a byte at a time. */ |
| and tmp1, src2, #0xff8 |
| eor tmp1, tmp1, #0xff8 |
| cbz tmp1, L(do_misaligned) |
| ldr data1, [src1], #8 |
| ldr data2, [src2], #8 |
| |
| sub tmp1, data1, zeroones |
| orr tmp2, data1, #REP8_7f |
| eor diff, data1, data2 /* Non-zero if differences found. */ |
| bic has_nul, tmp1, tmp2 /* Non-zero if NUL terminator. */ |
| orr syndrome, diff, has_nul |
| cbz syndrome, L(loop_misaligned) |
| b L(end) |
| |
| L(done): |
| sub result, data1, data2 |
| ret |
| |
| END (__strcmp_aarch64) |