How do I specify immediate floating point numbers with inline assembly?

user_3788 0
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When I try to compile this code:

#include <stdio.h>

main(int argc, char *argv[]) {
   double y = 0;

   __asm__ ("fldl $150;"
            "fsqrt;"
            "fstl %0;" : : "g" (y) );

   printf("%f\n", y);


   return 0;
}

I get this error:

sqrt.c: Assembler messages:
sqrt.c:6: Error: suffix or operands invalid for `fld'

Why doesn't this work? Why can't I push the number "150" onto the stack for floating point operations?

x87 inline-assembly assembly gcc c
5 Answers

t constraint

According to the GCC docs https://gcc.gnu.org/onlinedocs/gcc/Machine-Constraints.html#Machine-Constraints

t

Top of 80387 floating-point stack (%st(0)).

So we can do:

#include <assert.h>

int main(void) {
    double io = 4.0;
    __asm__ (
        "fsqrt"
        : "+t" (io)
        :
        :
    );
    assert(io == 2.0);
    return 0;
}

GitHub upstream.

Refresher: + means that io will be used both as input and output.

Tested in Ubuntu 19.04.

GNU GAS ARM assembly supports it

E.g. in ARMv8:

main.c

#include <assert.h>

int main(void) {
    float my_float = 1.5;
    __asm__ (
        "fmov s0, 1.0;"
        "fadd %s[my_float], %s[my_float], s0;"
        : [my_float] "+w" (my_float)
        :
        : "s0"
    );
    assert(my_float == 2.5);
}

GitHub upstream.

Compile and run:

aarch64-linux-gnu-gcc -o main.out -static -std=gnu99 main.c
qemu-aarch64 ./main.out

The %s modifier is mentioned at: ARMv8 floating point output inline assembly

It also works on ARMv7.

However, for some reason, it only works for floating point instructions such as fmov, e.g. the following ARMv7 attempt fails to assemble:

mov r0, 1.5

with error:

Error: garbage following instruction -- `mov r0,1.5'

presumably because it uses the mov instruction, which acts on general purpose registers instead of floating point ones.

However perhaps this doesn't matter much, as for the most part you just want to do your floating operations on your floating point registers, and then do an fcmp followed by vmrs as in:

vmov s0, 1.5
vmov s1, 2.5
fadds s2, s0, s1
vmov s3, 4.0
/* Compare two floating point registers. Stores results in fpscr:
 * (floating point status and control register).
 */
vcmp.f32 s2, s3
/* Move the nzcv bits from fpscr to apsr */
vmrs apsr_nzcv, fpscr
/* This branch uses the Z bit of apsr, which was set accordingly. */
beq theyre_equal

GitHub upstream.

It never ceases to amuse me how GNU GAS has subtly different syntax for every arch!

I could not however find a hex float literal syntax: How to use hexadecimal floating point literals in GNU GAS?

Tested on Ubuntu 18.04.

user_91607620

You can bypass many assemblers refusing to support float literals by having PHP preprocess the literal for you. (rawSingleHex taken from here). In a perfect world C preprocessor would be enough, but that's not true at the moment.

<?php
function rawSingleHex($num) {
    return '0x' . strrev(unpack('h*', pack('f', $num))[1]);
}
?>

#include <stdio.h>

int main(int argc, char **argv) {
   float y = 0;

   __asm__ ("pushl $<?php echo rawSingleHex(150);?>\n"
            "fsqrt\n"
            "fstl %0\n" : : "g" (y));

   printf("%f\n", y);


   return 0;
}

run php to generate the c file, and run c compiler to compile your program :P

user_20127150
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