Re: [RFC PATCH 32/42] docker: Add gentoo-mipsr5900el-cross image

[Fredrik Noring]

On Friday, 12 March 2021, Maciej W. Rozycki wrote:
> On Fri, 12 Mar 2021, Philippe Mathieu-Daudé wrote:
> 
> > >>> Is there any way we can do this with a distro that isn't Gentoo
> > >>> so that we can get a container build that is fast enough to be
> > >>> useful for CI ?
> > 
> > Using the Debian cross image I get:
> > 
> > /home/phil/source/qemu/tests/docker/docker.py --engine auto cc --cc
> > mips64el-linux-gnuabi64-gcc -i qemu/debian-mips64el-cross -s
> > /home/phil/source/qemu --   -Wall -Werror -O0 -g -fno-strict-aliasing
> > -mabi=n32 -march=r5900
> > /home/phil/source/qemu/tests/tcg/mips/test-r5900-dmult.c -o
> > test-r5900-dmult  -static
> > cc1: error: unsupported combination: -march=r5900 -mhard-float
> > -mdouble-float
> > 
> > No clue what is setting '-mhard-float -mdouble-float' yet.
> 
>  The R5900 has an FPU that only supports the single floating-point format.  
> It's also not an IEEE 754 format.  The Linux kernel ABI does support the 
> double and also the single floating-point format, both compliant with IEEE 
> 754.
> 
>  In the absence of a suitable FPU emulation code included with the kernel 
> will handle the missing instructions (you can use the `nofpu' kernel 
> parameter to force that in the presence of an FPU too).  Beware however 
> that a recent change to the Linux kernel made FPU emulation code optional 
> to suit some deeply embedded applications known never to use FPU machine 
> instructions.
> 
>  NB the presence of emulation is always required for MIPS ISA compliance 
> if FPU machine instructions are ever to be used in a given application, 
> because operations are allowed to trap regardless and rely on emulation.
> 
>  I don't know what you are trying to achieve,

I believe Philippe is trying to compile

https://lists.gnu.org/archive/html/qemu-devel/2021-03/msg04565.html

testing this:

        The R5900 reports itself as MIPS III but does not implement DMULT.
        Verify that DMULT is emulated properly in user mode by multiplying
        two 64-bit numbers to produce a 128-bit number.

with this piece of code (notice the mips3 ISA directive for DMULT):

/*
 * Test DMULT.
 */

#include <stdio.h>
#include <inttypes.h>
#include <assert.h>

struct hi_lo { int64_t hi; uint64_t lo; };

static struct hi_lo dmult(int64_t rs, int64_t rt)
{
    int64_t hi;
    uint64_t lo;

    /*
     * The R5900 reports itself as MIPS III but does not implement DMULT.
     * Verify that DMULT is emulated properly in user mode.
     */
    __asm__ __volatile__ (
            "    .set  mips3\n"
            "    dmult %2, %3\n"
            "    mfhi  %0\n"
            "    mflo  %1\n"
            : "=r" (hi), "=r" (lo)
            : "r" (rs), "r" (rt));

    return (struct hi_lo) { .hi = hi, .lo = lo };
}

int main()
{
    /* Verify that multiplying two 64-bit numbers yields a 128-bit number. */
    struct hi_lo r = dmult(2760727302517, 5665449960167);

    assert(r.hi == 847887);
    assert(r.lo == 7893651516417804947);

    return 0;
}

> but your two options to choose from are:
> 
> 1. Build for the soft-float ABI (`-msoft-float') where any FP calculations 
>    are compiled such as to be made by the CPU using integer arithmetic.
> 
> 2. Build for a generic MIPS ISA, for the R5900/n32 that would be MIPS III
>    (`-march=mips3'), and rely on the kernel FPU emulation.  Note that some 
>    integer MIPS III operations are missing too from the R5900 and have to 
>    be emulated by the kernel for MIPS/Linux n32 psABI compliance (an 
>    implementation can be pinched from an old libgcc version that was still 
>    under GNU GPLv2 or another algorithm reused, e.g. my `__div64_32' piece
>    easily adapted).

So qemu/tests/tcg/mips/Makefile.target is patched with

# r5900 is only 64 bit little-endian
ifneq ($(findstring 64el,$(TARGET_NAME)),)
MIPS_TESTS += test-r5900-dmult
test-r5900-dmult: CFLAGS += -mabi=n32 -march=r5900
endif

I didn't have issues with the -mhard-float -mdouble-float flags at the time,
and I didn't use mips64el-linux-gnuabi64-gcc for Debian.

Fredrik