Re: [GD-Windows] VC8.0 and CMOV

[Jan W. <jan...@gm...>]

Hello,

On Fri, 23 Nov 2007 11:45:21 +0100, Alen Ladavac <ale...@cr...> =
 =

wrote:
> Is there any way to make MSVC8.0 to generate CMOV instructions?
> Allegedly, it is able to do so,
Confirmed, I've seen them in VC2005 SP1's output. Not sure whether that =
 =

was due to hand-generated snippets being pasted in by the "optimizer" or=
  =

because the code generator has been improved.
Even after quite some experimentation, no reliable way to provoke CMOV w=
as  =

found.

One thing that did work (surprisingly enough) is a carry flag-based tric=
k:
int mask =3D (condition)? ~0 : 0;
This actually works out to a SBB, which can be used to select one of two=
  =

values or increment/decrement an index, etc. Whether this is useful  =

depends on the exact contents of your if() statements.

> but /O2 doesn't seem to help
Code Generation -> Enhanced Instruction Set will probably be necessary a=
s  =

well.


On Fri, 23 Nov 2007 16:13:21 +0100, Andrew Finkenstadt <pb...@gm...=
>  =

wrote:
> Does using "__asm" help?  You could hand-code the instructions, then.
Unfortunately this approach suffers from VC's inability to  =

peephole-optimize the 'boundaries' of asm code. You'll see back-to-back =
 =

stores and loads of variables you pass to/from asm, which often eats awa=
y  =

any gains attained. CPUs have forwarding hardware for this, but it still=
  =

hurts.

HTH+HAND
Jan Wassenberg