Floating point rounding modes: we should restrict them slightly

[Michel Fortin]

On 2009-09-10 04:24:38 -0400, Don <nospam at nospam.com> said:

> PROPOSAL:
> Change the spec by adding the line to float.html:
> "If the floating-point rounding mode is changed within a function, it 
> must be restored before the function exits. If this rule is violated 
> (for example, by the use of inline asm), the rounding mode used for 
> subsequent calculations is undefined."
> 
> This slight tightening of semantics can have a significant benefit for 
> all floating-point code.

I perfectly agree. I'm just wondering how to enforce it in the language.

Later in the thread you say:

> ... have an RAII struct to restore the mode automatically. But that's 
> just syntax sugar.

But how do you prevent someone from writing:

	auto s = new roundingMode(...);

The only way you can really make sure it's scoped to a specific 
function is to call it from another function:

	R performWithRoundingMode(R)(flag roundingMode, lazy R result)
	{
		flag oldRoundingMode = getRoundingMode();
		setRoundingMode(roundingMode);
		try
			return result;
		finally
			setRoundingMode(oldRoundingMode);
	}

	auto i = performWithRoundingMode(TOWARDS_ZERO, 12 * 12);

Here you can't escape the scoping requirements.

-- 
Michel Fortin
michel.fortin at michelf.com
http://michelf.com/