Not sure if this is valgrind, me or DMD so thought I should report it. Program runs, so could be just valgrind. 

It causes valgrind grief when casting a mutable double variable to a ubyte variable. Casting a literal floating point value worked, as did an immutable double variable.

------------------------
void main()
{
    double dval = 123.456;
    ubyte ubvalFAIL = cast(ubyte)(dval);    // Causes the problem.
    ubyte ubvalOK = cast(ubyte)(123.456); // This cast works  if above line commented out.

}
------------------------
Given the second cast worked I tried the following and it worked also:

void main() {
    immutable double dval = 123.456;
    ubyte ubvalOK = cast(ubyte)(dval);    // Happy valgrind.
}

=============================================
$ objdump -d quicktest | grep "41652e" -n8
427-  41650c:	f2 48 0f 10 05 ab 77 	rex.W movsd 0x177ab(%rip),%xmm0        # 42dcc0 <__dso_handle+0x8>
428-  416513:	01 00 
429-  416515:	f2 48 0f 11 45 f0    	rex.W movsd %xmm0,-0x10(%rbp)
430-  41651b:	dd 45 f0             	fldl   -0x10(%rbp)
431-  41651e:	50                   	push   %rax
432-  41651f:	d9 7c 24 02          	fnstcw 0x2(%rsp)
433-  416523:	66 c7 44 24 04 bf 0f 	movw   $0xfbf,0x4(%rsp)
434-  41652a:	d9 6c 24 04          	fldcw  0x4(%rsp)
435:  41652e:	48 df 1c 24          	rex.W fistp (%rsp)       <<<< HERE
436-  416532:	48 d9 6c 24 02       	rex.W fldcw 0x2(%rsp)
437-  416537:	58                   	pop    %rax
438-  416538:	88 45 f8             	mov    %al,-0x8(%rbp)
439-  41653b:	31 c0                	xor    %eax,%eax
440-  41653d:	c9                   	leaveq 
441-  41653e:	c3                   	retq   
442-  41653f:	90                   	nop
443-  416540:	c8 00 00 00          	enterq $0x0,$0x0
============================================================================
VALGRIND OUTPUT (abbrev.)
------------------------
vex amd64->IR: unhandled instruction bytes: 0x48 0xDF 0x1C 0x24 0x48 0xD9 0x6C 0x24
vex amd64->IR:   REX=1 REX.W=1 REX.R=0 REX.X=0 REX.B=0
vex amd64->IR:   VEX=0 VEX.L=0 VEX.nVVVV=0x0 ESC=NONE
vex amd64->IR:   PFX.66=0 PFX.F2=0 PFX.F3=0
==2323== valgrind: Unrecognised instruction at address 0x4176fe.
==2323==    at 0x4176FE: _Dmain (/tmp/quicktest.d:18)
...
...
==2323== Your program just tried to execute an instruction that Valgrind
==2323== did not recognise.  There are two possible reasons for this.
==2323== 1. Your program has a bug and erroneously jumped to a non-code
==2323==    location.  If you are running Memcheck and you just saw a
==2323==    warning about a bad jump, it's probably your program's fault.
==2323== 2. The instruction is legitimate but Valgrind doesn't handle it,
==2323==    i.e. it's Valgrind's fault.  If you think this is the case or
==2323==    you are not sure, please let us know and we'll try to fix it.
==2323== Either way, Valgrind will now raise a SIGILL signal which will
==2323== probably kill your program.
===========================

Thanks.