Soft Float on x86_64

Question

I work on software for an embedded system which does not have an FPU. I would like to be able to build this software with a software floating point library on my desktop in orde

Answer 1

GCC can not do it out-of-box without some additional libraries. Basically, -msoft-float just generate calls for floating point libraries but the required libraries are not part of GCC.

To link the soft-fp libraries you can use the switch -lsoft-fp.

X86_64 architecture contains SSE extensions, so the compiler will try to generate SSE-code for basic operations like + - * /. We will use -mno-sse switch suppress this unauthorized behavior.

It may look as follows:

gcc -g -msoft-float -mno-sse -m64 -lsoft-fp

For the code:

int main()
{
    float a = 10;
    float b = 20;

    float c = a * b;

    return 0;
}

The resulting assembly will be:

    .file   "12.cpp"
    .def    __main; .scl    2;  .type   32; .endef
    .def    __mulsf3;   .scl    2;  .type   32; .endef
    .text
    .globl  main
    .def    main;   .scl    2;  .type   32; .endef
    .seh_proc   main
main:
    pushq   %rbp
    .seh_pushreg    %rbp
    movq    %rsp, %rbp
    subq    $48, %rsp
    .seh_stackalloc 48
    .seh_setframe   %rbp, 48
    .seh_endprologue
    call    __main
    movl    .LC0(%rip), %eax
    movl    %eax, -4(%rbp)
    movl    .LC1(%rip), %eax
    movl    %eax, -8(%rbp)
    movl    -8(%rbp), %edx
    movl    -4(%rbp), %ecx
    call    __mulsf3
    movl    %eax, -12(%rbp)
    movl    $0, %eax
    addq    $48, %rsp
    popq    %rbp
    ret
    .seh_endproc
    .section .rdata,"dr"
    .align 4
.LC0:
    .long   1092616192
    .align 4
.LC1:
    .long   1101004800
    .ident  "GCC: (GNU) 4.8.0 20120624 (experimental)"

No SSE instructions were generated. Note the call to __mulsf3.

Some interesting ideas can be found in this question: Using software floating point on x86 linux