pg_crc32c_armv8_choose.c

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/*-------------------------------------------------------------------------
 *
 * pg_crc32c_armv8_choose.c
 *    Choose between ARMv8 and software CRC-32C implementation.
 *
 * On first call, checks if the CPU we're running on supports the ARMv8
 * CRC Extension. If it does, use the special instructions for CRC-32C
 * computation. Otherwise, fall back to the pure software implementation
 * (slicing-by-8).
 *
 * Portions Copyright (c) 1996-2024, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/port/pg_crc32c_armv8_choose.c
 *
 *-------------------------------------------------------------------------
 */
 
#ifndef FRONTEND
#include "postgres.h"
#else
#include "postgres_fe.h"
#endif
 
#include <setjmp.h>
#include <signal.h>
 
#include "port/pg_crc32c.h"
 
 
static sigjmp_buf illegal_instruction_jump;
 
/*
 * Probe by trying to execute pg_comp_crc32c_armv8().  If the instruction
 * isn't available, we expect to get SIGILL, which we can trap.
 */
static void
illegal_instruction_handler(SIGNAL_ARGS)
{
    siglongjmp(illegal_instruction_jump, 1);
}
 
static bool
pg_crc32c_armv8_available(void)
{
    uint64      data = 42;
    int         result;
 
    /*
     * Be careful not to do anything that might throw an error while we have
     * the SIGILL handler set to a nonstandard value.
     */
    pqsignal(SIGILL, illegal_instruction_handler);
    if (sigsetjmp(illegal_instruction_jump, 1) == 0)
    {
        /* Rather than hard-wiring an expected result, compare to SB8 code */
        result = (pg_comp_crc32c_armv8(0, &data, sizeof(data)) ==
                  pg_comp_crc32c_sb8(0, &data, sizeof(data)));
    }
    else
    {
        /* We got the SIGILL trap */
        result = -1;
    }
    pqsignal(SIGILL, SIG_DFL);
 
#ifndef FRONTEND
    /* We don't expect this case, so complain loudly */
    if (result == 0)
        elog(ERROR, "crc32 hardware and software results disagree");
 
    elog(DEBUG1, "using armv8 crc32 hardware = %d", (result > 0));
#endif
 
    return (result > 0);
}
 
/*
 * This gets called on the first call. It replaces the function pointer
 * so that subsequent calls are routed directly to the chosen implementation.
 */
static pg_crc32c
pg_comp_crc32c_choose(pg_crc32c crc, const void *data, size_t len)
{
    if (pg_crc32c_armv8_available())
        pg_comp_crc32c = pg_comp_crc32c_armv8;
    else
        pg_comp_crc32c = pg_comp_crc32c_sb8;
 
    return pg_comp_crc32c(crc, data, len);
}
 
pg_crc32c   (*pg_comp_crc32c) (pg_crc32c crc, const void *data, size_t len) = pg_comp_crc32c_choose;