Re: [Qemu-devel] [ARM - FCVT inst] : Difference in calculated value

[Peter Maydell]

On 19 August 2014 13:39, Gaurav Sharma <gauravs.2...@gmail.com> wrote:
> I am running a simple bare metal program with only the above specified
> instruction
>
> [Instrn]
> fcvt h16 s25
>
> [register values]
> SIMD register [25] = 0x9EA82A22AB98FBA8L
> FPCR = 0x40ae2f54 [with according mask removing the UFE and other
> unnecessary bit]

Thanks. My test case produced from that:

===begin===
/* aarch64-linux-gnu-gcc -g -Wall -o fcvt fcvt.c -static */
#include <stdio.h>
#include <stdint.h>
#include <inttypes.h>

void do_fcvt(uint32_t fpcr, uint64_t a)
{
    uint64_t r;

    printf("a = 0x%016" PRIx64 " fpcr = 0x%08x\n", a, fpcr);

    __asm__ volatile(
        "fmov d25, %[a]\n"
        "mrs x3, FPCR\n"
        "msr FPCR, %[fpcr]\n"
        "isb\n"
        "fcvt h16, s25\n"
        "msr FPCR, x3\n"
        "isb\n"
        "fmov %[r], d16\n"
        : [r] "=r" (r)
        : [a] "r" (a),
          [fpcr] "r" (fpcr)
        : "d16", "d25", "x3"
        );

    printf("result = 0x%016" PRIx64 "\n", r);
}

int main(void)
{
    do_fcvt(0x40ae2f54 & 0x07f70000, 0x9EA82A22AB98FBA8ull);

    return 0;
}
===endit===

which on a Fast Model produces:
root@genericarmv8:~# ./fcvt
a = 0x9ea82a22ab98fba8 fpcr = 0x00a60000
result = 0x0000000000008001

and on QEMU:
e104462:trusty:qemu$ ./build/x86/aarch64-linux-user/qemu-aarch64 /tmp/fcvt
a = 0x9ea82a22ab98fba8 fpcr = 0x40ae2f54
result = 0x0000000000008000

so we are indeed getting the rounding wrong here.

thanks
-- PMM