Stream: cranelift

Topic: Passing F64 to a C function

osa1 (May 15 2020 at 22:43):

I'm generating a function call where the function's sig is

Signature { params: [AbiParam { value_type: types::I64, purpose: Normal, extension: None, location: Unassigned }, AbiParam { value_type: types::F64, purpose: Normal, extension: None, location: Unassigned }], returns: [AbiParam { value_type: types::I64, purpose: Normal, extension: None, location: Unassigned }], call_conv: SystemV }

or simply, (I64, F64) -> I64. The function is defined like this in C:

int64_t mc_int_of_float_f(FunctionClosure *self, double d) {
    printf("%lf\n", (float)d);
    return (int64_t)llround(d);
}

I checked and double is 8-bytes on my system.

For some reason when I pass 1.23 to this function the print line prints 123.000000. Any ideas what I'm doing wrong?

Relevant code that does the call:

    v0 = f64const 0x1.ec00000000000p6
    v1 = global_value.i64 gv0
    v2 = load.i64 v1
    v3 = global_value.i64 gv1
    v4 = call_indirect sig1, v2(v3, v0)

Dan Gohman (May 15 2020 at 22:46):

To printf a double, use plain %f

Dan Gohman (May 15 2020 at 22:47):

(printf is varargs, and in varargs, floats are always promoted to double, so C uses the same format specifiers for float and double)

osa1 (May 15 2020 at 22:47):

OK thanks. That doesn't solve the issue though, it still prints 123.000000

osa1 (May 15 2020 at 22:48):

I don't really care about the printf line, that's for debugging. The real problem is that the return value is not correct.

Dan Gohman (May 15 2020 at 22:49):

0x1.ec00000000000p6 is 123.000000

osa1 (May 15 2020 at 22:51):

Ahh.. hmm... I'm using f64const with a Rust f64 value which has the correct value. Out of curiosity how did you check that?

Dan Gohman (May 15 2020 at 22:52):

the p6 means "times 2**6" or "times 64", and 1.ec is somewhere between 1 and 2, closer to 2

Dan Gohman (May 15 2020 at 22:53):

On Linux, you can use the printf command at the command-line too; printf "%f" 0x1.ec00000000000p prints the result

Dan Gohman (May 15 2020 at 22:53):

or you can write a C program that does that

osa1 (May 15 2020 at 22:53):

Thanks for the help. I think I found the problem.

Last updated: Dec 23 2024 at 12:05 UTC