[Vm-dev] Help: Float returns on RISCV

[Boris Shingarov]

If you are interested, we can have a microhackathon where I can show you 
*my* way of doing it (which means, ULD).  Or maybe Jan wants to show you 
his (which means, libgdbs/vdb).  You were in the Pacific timezone, correct?

Jan: How good/bad is the ArchC model for rv64 right now?  I remember you 
were fixing some defects around the register name conventions being all 
wrong -- are those now better or there is more work needed before ULD 
could even pick it up meaningfully? Would you be interested to also 
participate in a GDB microhackathon with Ken?

On 6/30/22 15:23, ken.dickey at whidbey.com wrote:
>
>> Date: Wed, 29 Jun 2022 09:17:29 -0700
>> From: Eliot Miranda <eliot.miranda at gmail.com>
> ..
>> The pattern is straight-forward.  Look at the various implementations
>> of ffiCalloutTo:SpecOnStack:in:. There are two issues, passing
>> arguments and receiving results.
>
> Yes.  I have followed that.  Thanks.
>
> The generated RiscV64FFIPlugin.c is virtually identical to 
> ARM64FFIPlugin.c as
>
> ThreadedARM64FFIPlugin subclass: #ThreadedRiscV64FFIPlugin with almost 
> no overrides,
>
> aside from
>   #nonRegisterStructReturnIsViaImplicitFirstArgument --> true.
>
> However, the post return assignment to floatRet.d which I see in the 
> ARM64 code appears to be absent from the RiscV64 code.
>
> Being very rusty on C and GDB, I am moby confused by this..
> -KenD
>
> vvvvvv======vvvvvv    From SqueakFFIPlugin:
> [Aarch64/Arm64]
>
> 2470            (floatRet.d = 
> dispatchFunctionPointerwithwithwithwithwithwithwithwith(((struct dprr 
> (*)(sqIntptr_t, sqIntptr_t, sqIntptr_t, sqIntptr_t, sqIntptr_t, 
> sqIntptr_t, sqIntptr_t, sqIntptr_t)) (((void *) address))), 
> ((calloutState->integerRegisters))[0], 
> ((calloutState->integerRegisters))[1], 
> ((calloutState->integerRegisters))[2], 
> ((calloutState->integerRegisters))[3], 
> ((calloutState->integerRegisters))[4], 
> ((calloutState->integerRegisters))[5], 
> ((calloutState->integerRegisters))[6], 
> ((calloutState->integerRegisters))[7]));
>    0x0000007ff6b3b6cc <+2004>:    ldr    x8, [x29, #928]
>    0x0000007ff6b3b6d0 <+2008>:    ldr    x0, [x29, #800]
>    0x0000007ff6b3b6d4 <+2012>:    ldr    x9, [x0, #232]
>    0x0000007ff6b3b6d8 <+2016>:    ldr    x0, [x29, #800]
>    0x0000007ff6b3b6dc <+2020>:    ldr    x1, [x0, #240]
>    0x0000007ff6b3b6e0 <+2024>:    ldr    x0, [x29, #800]
>    0x0000007ff6b3b6e4 <+2028>:    ldr    x2, [x0, #248]
>    0x0000007ff6b3b6e8 <+2032>:    ldr    x0, [x29, #800]
>    0x0000007ff6b3b6ec <+2036>:    ldr    x3, [x0, #256]
>    0x0000007ff6b3b6f0 <+2040>:    ldr    x0, [x29, #800]
>    0x0000007ff6b3b6f4 <+2044>:    ldr    x4, [x0, #264]
>    0x0000007ff6b3b6f8 <+2048>:    ldr    x0, [x29, #800]
>    0x0000007ff6b3b6fc <+2052>:    ldr    x5, [x0, #272]
>    0x0000007ff6b3b700 <+2056>:    ldr    x0, [x29, #800]
>    0x0000007ff6b3b704 <+2060>:    ldr    x6, [x0, #280]
>    0x0000007ff6b3b708 <+2064>:    ldr    x0, [x29, #800]
>    0x0000007ff6b3b70c <+2068>:    ldr    x7, [x0, #288]
>    0x0000007ff6b3b710 <+2072>:    add    x0, x29, #0x3b8
>    0x0000007ff6b3b714 <+2076>:    sub    x19, x0, #0x200
>    0x0000007ff6b3b718 <+2080>:    mov    x0, x9
>    0x0000007ff6b3b71c <+2084>:    blr    x8
> ## after return, struct floatRet.d is assigned
>    0x0000007ff6b3b720 <+2088>:    fmov    d5, d0
>    0x0000007ff6b3b724 <+2092>:    fmov    d4, d1
>    0x0000007ff6b3b728 <+2096>:    fmov    d1, d2
>    0x0000007ff6b3b72c <+2100>:    fmov    d0, d3
>    0x0000007ff6b3b730 <+2104>:    str    d5, [x19]
>    0x0000007ff6b3b734 <+2108>:    str    d4, [x19, #8]
>    0x0000007ff6b3b738 <+2112>:    str    d1, [x19, #16]
>    0x0000007ff6b3b73c <+2116>:    str    d0, [x19, #24]
>
> 2471            if (isCalleePopsConvention((calloutState->callFlags))) {
> ^^^^^^======^^^^^^
> vvvvvv======vvvvvv
> [RISCV64]
>
> 2476            (floatRet.d = 
> dispatchFunctionPointerwithwithwithwithwithwithwithwith(((struct dprr 
> (*)(sqIntptr_t, sqIntptr_t, sqIntptr_t, sqIntptr_t, sqIntptr_t, 
> sqIntptr_t, sqIntptr_t, sqIntptr_t)) (((void *) address))), 
> ((calloutState->integerRegisters))[0], 
> ((calloutState->integerRegisters))[1], 
> ((calloutState->integerRegisters))[2], 
> ((calloutState->integerRegisters))[3], 
> ((calloutState->integerRegisters))[4], 
> ((calloutState->integerRegisters))[5], 
> ((calloutState->integerRegisters))[6], 
> ((calloutState->integerRegisters))[7]));
>    0x0000003ff703f95a <+3456>:    ld    t3,-200(s0)
>    0x0000003ff703f95e <+3460>:    ld    a7,-208(s0)
>    0x0000003ff703f962 <+3464>:    ld    a6,-216(s0)
>    0x0000003ff703f966 <+3468>:    ld    a5,-224(s0)
>    0x0000003ff703f96a <+3472>:    ld    a4,-232(s0)
>    0x0000003ff703f96e <+3476>:    ld    a3,-240(s0)
>    0x0000003ff703f972 <+3480>:    ld    a2,-248(s0)
>    0x0000003ff703f976 <+3484>:    ld    a1,-256(s0)
>    0x0000003ff703f97a <+3488>:    addi    s4,s0,-520
>    0x0000003ff703f97e <+3492>:    mv    a0,s4
>    0x0000003ff703f980 <+3494>:    sd    t3,0(sp)
>    0x0000003ff703f982 <+3496>:    jalr    s5
>
> ## NOTA BENE: After return: NO assignment to floatRet.d !?!
>
> 2477            if (isCalleePopsConvention((calloutState->callFlags))) {
>
> 2478                setsp((calloutState->argVector));
> 2479            }
> 2480            ownVM(myThreadIndex);
> => 0x0000003ff703f984 <+3498>:    ld    a5,408(s10)
>    0x0000003ff703f988 <+3502>:    mv    a0,s1
>    0x0000003ff703f98a <+3504>:    jalr    a5
>
> 2481            if (atomicType == FFITypeDoubleFloat) {
>    0x0000003ff703f98c <+3506>:    li    a5,13
>    0x0000003ff703f98e <+3508>:    beq    s2,a5,0x3ff703fd4a 
> <ffiCallArgArrayOrNilNumArgs+4464>
>
> 2482                result = 
> floatObjectOf(((((floatRet.d)).doubles))[0]);
> ^^^^^^======^^^^^^
>