[LLVMdev] Best way to interface with MSVC _ftol2 runtime function for fptoui?

[Jakob Stoklund Olesen]

On Jan 18, 2012, at 3:50 PM, Joe Groff wrote:

> Hi everyone. On i386--win32 targets, LLVM tries to use the MSVCRT
> routine _ftol2 for floating-point to unsigned conversions, but this
> function has a nonstandard calling convention LLVM doesn't understand.
> It takes its input operand on the x87 stack as ST0, which it pops off
> of the stack before returning. The return value is given in EDX:EAX.
> In effect, I need to call it like this:
> 
> %1 = call i64 asm "call __ftol2",
> "=A,{st},~{dirflag},~{fpsr},~{flags}" (double %x) nounwind
> 
> but with the added consideration that the input operand is popped by
> the call, so the callee can't emit its own fstp instruction afterward.
> LLVM inline asm doesn't appear to be capable of communicating this.

This should work:

%1 = call i64 asm "call __ftol2", "=A,{st},~{dirflag},~{fpsr},~{flags},~{st}" (double %x) nounwind

See http://llvm.org/viewvc/llvm-project/llvm-gcc-4.2/trunk/gcc/reg-stack.c?view=markup
And the INLINEASM handling in http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/X86/X86FloatingPoint.cpp?view=markup

>  In
> #llvm it was suggested to write a custom instruction for the call, but
> it looks like there are a few layers of abstraction in the X86 target
> for dealing with x87, and I can't quite grasp exactly how instruction
> stack effects are communicated. What would be the best approach to
> implement proper support for this runtime call?

If the inline asm works for you, use it.  It is currently the only way of supporting caller-popped arguments. Normal instructions can't do it because it depends on treating the inline asm clobber list differently from otherwise clobbered registers.

/jakob