How to convert an unsigned integer to floating-point in x86 (32-bit) assembly?

Question

I need to convert both 32-bit and 64-bit unsigned integers into floating-point values in xmm registers. There are x86 instructions to convert signed integer

Answer 1

Here's what GCC generates. I wrapped them in functions, but you can easily remove the stack handling. Not all of them use SSE to do the actual work (the ulonglong conversions don't), if you find the corresponding instructions, please tell me. Clang generates almost the same.

% cat tofloats.c 
double ulonglong2double(unsigned long long a) {
    return a;
}
float ulonglong2float(unsigned long long a) {
    return a;
}
double uint2double(unsigned int a) {
    return a;
}
float uint2float(unsigned int a) {
    return a;
}
% gcc -msse4.2 -g -Os -c tofloats.c && objdump -d tofloats.o
00000000 :
   0:   55                      push   %ebp
   1:   89 e5                   mov    %esp,%ebp
   3:   83 ec 10                sub    $0x10,%esp
   6:   8b 55 0c                mov    0xc(%ebp),%edx
   9:   8b 45 08                mov    0x8(%ebp),%eax
   c:   89 55 f4                mov    %edx,-0xc(%ebp)
   f:   85 d2                   test   %edx,%edx
  11:   89 45 f0                mov    %eax,-0x10(%ebp)
  14:   df 6d f0                fildll -0x10(%ebp)
  17:   79 06                   jns    1f 
  19:   d8 05 00 00 00 00       fadds  0x0
  1f:   dd 5d f8                fstpl  -0x8(%ebp)
  22:   dd 45 f8                fldl   -0x8(%ebp)
  25:   c9                      leave  
  26:   c3                      ret    

00000027 :
  27:   55                      push   %ebp
  28:   89 e5                   mov    %esp,%ebp
  2a:   83 ec 10                sub    $0x10,%esp
  2d:   8b 55 0c                mov    0xc(%ebp),%edx
  30:   8b 45 08                mov    0x8(%ebp),%eax
  33:   89 55 f4                mov    %edx,-0xc(%ebp)
  36:   85 d2                   test   %edx,%edx
  38:   89 45 f0                mov    %eax,-0x10(%ebp)
  3b:   df 6d f0                fildll -0x10(%ebp)
  3e:   79 06                   jns    46 
  40:   d8 05 00 00 00 00       fadds  0x0
  46:   d9 5d fc                fstps  -0x4(%ebp)
  49:   d9 45 fc                flds   -0x4(%ebp)
  4c:   c9                      leave  
  4d:   c3                      ret    

0000004e :
  4e:   55                      push   %ebp
  4f:   89 e5                   mov    %esp,%ebp
  51:   83 ec 08                sub    $0x8,%esp
  54:   66 0f 6e 45 08          movd   0x8(%ebp),%xmm0
  59:   66 0f d6 45 f8          movq   %xmm0,-0x8(%ebp)
  5e:   df 6d f8                fildll -0x8(%ebp)
  61:   c9                      leave  
  62:   c3                      ret    

00000063 :
  63:   55                      push   %ebp
  64:   89 e5                   mov    %esp,%ebp
  66:   83 ec 08                sub    $0x8,%esp
  69:   66 0f 6e 45 08          movd   0x8(%ebp),%xmm0
  6e:   66 0f d6 45 f8          movq   %xmm0,-0x8(%ebp)
  73:   df 6d f8                fildll -0x8(%ebp)
  76:   c9                      leave  
  77:   c3                      ret

Here are the bonus points (conversion into ints):

% cat toints.c                                      
unsigned long long float2ulonglong(float a) {
    return a;
}
unsigned long long double2ulonglong(double a) {
    return a;
}
unsigned int float2uint(float a) {
    return a;
}
unsigned int double2uint(double a) {
    return a;
}
% gcc -msse4.2 -g -Os -c toints.c && objdump -d toints.o  
00000000 :
   0:   55                      push   %ebp
   1:   89 e5                   mov    %esp,%ebp
   3:   53                      push   %ebx
   4:   83 ec 0c                sub    $0xc,%esp
   7:   d9 45 08                flds   0x8(%ebp)
   a:   d9 05 00 00 00 00       flds   0x0
  10:   d9 c9                   fxch   %st(1)
  12:   db e9                   fucomi %st(1),%st
  14:   73 0d                   jae    23 
  16:   dd d9                   fstp   %st(1)
  18:   dd 4d f0                fisttpll -0x10(%ebp)
  1b:   8b 45 f0                mov    -0x10(%ebp),%eax
  1e:   8b 55 f4                mov    -0xc(%ebp),%edx
  21:   eb 13                   jmp    36 
  23:   de e1                   fsubp  %st,%st(1)
  25:   dd 4d f0                fisttpll -0x10(%ebp)
  28:   8b 55 f4                mov    -0xc(%ebp),%edx
  2b:   8b 45 f0                mov    -0x10(%ebp),%eax
  2e:   8d 8a 00 00 00 80       lea    -0x80000000(%edx),%ecx
  34:   89 ca                   mov    %ecx,%edx
  36:   83 c4 0c                add    $0xc,%esp
  39:   5b                      pop    %ebx
  3a:   5d                      pop    %ebp
  3b:   c3                      ret    

0000003c :
  3c:   55                      push   %ebp
  3d:   89 e5                   mov    %esp,%ebp
  3f:   53                      push   %ebx
  40:   83 ec 0c                sub    $0xc,%esp
  43:   dd 45 08                fldl   0x8(%ebp)
  46:   d9 05 00 00 00 00       flds   0x0
  4c:   d9 c9                   fxch   %st(1)
  4e:   db e9                   fucomi %st(1),%st
  50:   73 0d                   jae    5f 
  52:   dd d9                   fstp   %st(1)
  54:   dd 4d f0                fisttpll -0x10(%ebp)
  57:   8b 45 f0                mov    -0x10(%ebp),%eax
  5a:   8b 55 f4                mov    -0xc(%ebp),%edx
  5d:   eb 13                   jmp    72 
  5f:   de e1                   fsubp  %st,%st(1)
  61:   dd 4d f0                fisttpll -0x10(%ebp)
  64:   8b 55 f4                mov    -0xc(%ebp),%edx
  67:   8b 45 f0                mov    -0x10(%ebp),%eax
  6a:   8d 8a 00 00 00 80       lea    -0x80000000(%edx),%ecx
  70:   89 ca                   mov    %ecx,%edx
  72:   83 c4 0c                add    $0xc,%esp
  75:   5b                      pop    %ebx
  76:   5d                      pop    %ebp
  77:   c3                      ret    

00000078 :
  78:   55                      push   %ebp
  79:   89 e5                   mov    %esp,%ebp
  7b:   83 ec 08                sub    $0x8,%esp
  7e:   d9 45 08                flds   0x8(%ebp)
  81:   dd 4d f8                fisttpll -0x8(%ebp)
  84:   8b 45 f8                mov    -0x8(%ebp),%eax
  87:   c9                      leave  
  88:   c3                      ret    

00000089 :
  89:   55                      push   %ebp
  8a:   89 e5                   mov    %esp,%ebp
  8c:   83 ec 08                sub    $0x8,%esp
  8f:   dd 45 08                fldl   0x8(%ebp)
  92:   dd 4d f8                fisttpll -0x8(%ebp)
  95:   8b 45 f8                mov    -0x8(%ebp),%eax
  98:   c9                      leave  
  99:   c3                      ret    

There functions take input from the stack and return it over the stack. If you need the result in an XMM register by the end of the function, you can use movd/movq to take them from the stack to the XMM. If the function is returning a double, your result is on -0x8(%ebp). If it's a float, result is in -0x4(%ebp). Ulonglongs have the lengths of doubles and ints have the lengths of floats.

fisttpll: Store Integer with Truncation

FISTTP converts the value in ST into a signed integer using truncation (chop) as rounding mode, transfers the result to the destination, and pop ST. FISTTP accepts word, short integer, and long integer destinations.

fucomi: Compare Floating Point Values and Set EFLAGS

Performs an unordered comparison of the contents of registers ST(0) and ST(i) and sets the status flags ZF, PF, and CF in the EFLAGS register according to the results (see the table below). The sign of zero is ignored for comparisons, so that –0.0 is equal to +0.0.