[M3commit] CVS Update: cm3

[Jay K]

What hardware traps?
 
Sure, maybe integer divide by zero.
 
 
Unlikely I think for integer overflow.
I agree out of line inefficient.
Probably want "check the carry flag" generated inline,
in whatever machine-specific way that is done, may
or may not require extra work in m3cg.
 
 
I might try this soon though, where a command line option (for now)
uses function calls and raises exceptions.
My slightly refined thinking is that the generate code will look like:
 
 
a := b + c * d;
Foo()
d := a + b * c;
Bar()
 
 
overflow = 0;
a = add(b, mult(c, d), &overflow);
if (overflow) Raise...
Foo();
overflow = 0;
a = add(a, mult(b, c), &overflow);
if (overflow) Raise...
 
 
That is -- the overflow flag need only be checked "occasionally", such as before any function call, maybe at the start of any loop.
Or at the very least, if a statement contains no function calls, only at the end of the statement, not after evaluating each term in an expression.
 
 
Once overflow occurs, there's little requirement on how the statement proceeds. Though divide by zero shouldn't result. Probably if an expression contains any division, check overflow at each step.
?
 
 
But maybe "check the overflow flag" is easy enough and right surely far more efficient.
 
 
 
 - Jay



________________________________
> From: hosking at cs.purdue.edu
> Date: Tue, 12 Jan 2010 13:18:27 -0500
> To: jkrell at elego.de
> CC: m3commit at elegosoft.com
> Subject: Re: [M3commit] CVS Update: cm3
>
>
>
> Do we really want to do this via compilation and out-of-line functions? Surely we would prefer to implement the FloatMode interface to catch hardware traps... I haven't looked at the details but that seems to be the intent...
>
>
>
> On 12 Jan 2010, at 12:16, Jay Krell wrote:
>
> CVSROOT: /usr/cvs
> Changes by: jkrell at birch. 10/01/12 12:16:59
>
> Modified files:
> cm3/m3-libs/m3core/src/Csupport/Common/: hand.c
>
> Log message:
> add portable possible functions for implementing overflow checking of add/sub/mult of int, int64, uint, uint64
>