ARM程序armkey效率优化问询：如何提升文本读取性能？

Hey there! Let's break down practical, actionable ways to boost your armkey ARM program's efficiency when reading from key.in. These optimizations are tailored to your specific scenario of handling ASCII text (including 00h-7Fh control characters) with ARM SWI string reads:

SWI/SVC instructions carry inherent overhead—think kernel mode transitions and context switching. Instead of reading line by line, switch to chunk-based reading:

• Allocate a larger buffer (e.g., 4KB or 8KB, aligned to your system's page size) and read entire blocks at once via SWI.
• Handle newline-to-null replacement and line splitting in user space instead of relying on the SWI to do it per line. This reduces the number of syscalls drastically, which is one of the biggest efficiency wins here.

Since you're working on ARM, leverage its hardware capabilities to speed up buffer processing:

• If your target supports NEON vector instructions, use them to batch-replace newline characters (0x0A, 0x0D) with nulls (0x00). NEON can process 8-16 bytes in parallel, which is way faster than scalar loop operations.
• For non-NEON targets, use loop unrolling (unroll 4-8 iterations) to cut down on branch overhead, and use ARM's load/store multiple instructions (LDM, STM) to move data in batches instead of one byte at a time.

• Process input directly in your read buffer instead of copying it to another string. For example, if you need to pass a line to a function, pass a pointer to the start of the line in the buffer plus its length, rather than duplicating the data.
• If your ARM system supports it, use mmap to map the key.in file directly into user memory. This zero-copy technique lets you access file data without any SWI read calls entirely—you'll work with the mapped memory directly.

• Instead of checking for a 0-byte return after every single read, only validate EOF when your buffer isn't filled completely. This reduces the number of conditional checks in your main loop.
• Pre-allocate your buffer once at startup (with alignment to cache line size) instead of reallocating memory per line or read. This cuts out repeated memory allocation overhead.

• Align your buffer to the L1 cache line size (usually 32 or 64 bytes on ARM) using compiler directives like __attribute__((aligned(64))). This prevents buffer accesses from spanning cache lines, which reduces costly cache misses.
• Process the buffer sequentially (don't jump around in memory) to take advantage of cache prefetching—this lets the ARM CPU load upcoming data into cache before you need it.

Here's a rough snippet showing how chunk reading with in-place newline replacement might look in ARM assembly:

; Pre-allocate aligned 4KB buffer
BUFFER      EQU     0x10000000
BUFFER_SIZE EQU     4096

read_loop:
    SWI     0x6               ; Read chunk into BUFFER (adjust SWI number to match your system)
    CMP     R0, #0            ; Check for EOF
    BEQ     exit_loop

    ; Process buffer to replace newlines/carriage returns with nulls (scalar example)
    LDR     R1, =BUFFER
    ADD     R2, R1, R0        ; End of valid data in buffer
process_char:
    LDRB    R3, [R1]
    CMP     R3, #0x0A         ; Check for newline
    STREQB  R3, [R1], #1      ; Replace with null and increment pointer
    CMP     R3, #0x0D         ; Check for carriage return
    STREQB  R3, [R1], #1
    CMP     R1, R2
    BNE     process_char

    ; Pass buffer to your processing logic
    BL      process_buffer
    B       read_loop
exit_loop:
    ; Cleanup code here

Remember to test each optimization incrementally—measure execution time before and after each change to confirm which ones deliver the most gains for your specific use case!

内容的提问来源于stack exchange，提问作者Saahil Bhutwala