Are there unsigned equivalents of the x87 FILD and SSE CVTSI2SD instructions?

Question

I want to implement the equivalent of C\'s uint-to-double cast in the GHC Haskell compiler. We already implement int-to-double

Answer 1

As someone said, "Good Artists Copy; Great Artists Steal". So we can just check how other compiler writers solved this issue. I used a simple snippet:

volatile unsigned int x;
int main()
{
  volatile double  y = x;
  return y;
}

(volatiles added to ensure the compiler does not optimize out the conversions)

Results (irrelevant instructions skipped):

Visual C++ 2010 cl /Ox (x86)

  __real@41f0000000000000 DQ 041f0000000000000r ; 4.29497e+009

  mov   eax, DWORD PTR ?x@@3IC          ; x
  fild  DWORD PTR ?x@@3IC           ; x
  test  eax, eax
  jns   SHORT $LN4@main
  fadd  QWORD PTR __real@41f0000000000000
$LN4@main:
  fstp  QWORD PTR _y$[esp+8]

So basically the compiler is adding an adjustment value in case the sign bit was set.

Visual C++ 2010 cl /Ox (x64)

  mov   eax, DWORD PTR ?x@@3IC          ; x
  pxor  xmm0, xmm0
  cvtsi2sd xmm0, rax
  movsdx    QWORD PTR y$[rsp], xmm0

No need to adjust here because the compiler knows that rax will have the sign bit cleared.

Visual C++ 2012 cl /Ox

  __xmm@41f00000000000000000000000000000 DB 00H, 00H, 00H, 00H, 00H, 00H, 00H
  DB 00H, 00H, 00H, 00H, 00H, 00H, 00H, 0f0H, 'A'

  mov   eax, DWORD PTR ?x@@3IC          ; x
  movd  xmm0, eax
  cvtdq2pd xmm0, xmm0
  shr   eax, 31                 ; 0000001fH
  addsd xmm0, QWORD PTR __xmm@41f00000000000000000000000000000[eax*8]
  movsd QWORD PTR _y$[esp+8], xmm0

This uses branchless code to add 0 or the magic adjustment depending on whether the sign bit was cleared or set.