[swift-evolution] floating point numbers implicit conversion

Stephen Canon scanon at apple.com
Mon Jun 19 17:08:44 CDT 2017

> On Jun 19, 2017, at 5:43 PM, David Sweeris <davesweeris at mac.com> wrote:
> Sent from my iPhone
> On Jun 19, 2017, at 13:44, John McCall via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>> On Jun 19, 2017, at 1:58 PM, Stephen Canon via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>> On Jun 19, 2017, at 11:46 AM, Ted F.A. van Gaalen via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>> var result: Float = 0.0
>>>> result = float * integer * uint8 +  double   
>>>> // here, all operands should be implicitly promoted to Double before the complete expression evaluation.
>>> You would have this produce different results than:
>>> 	let temp = float * integer * uint8
>>> 	result = temp + double
>>> That would be extremely surprising to many unsuspecting users.
>>> Don’t get me wrong; I *really want* implicit promotions (I proposed one scheme for them  way back when Swift was first unveiled publicly).
>> I don't!  At least not for floating point.  It is important for both reliable behavior and performance that programmers understand and minimize the conversions they do between different floating-point types.
> How expensive is it?

On most contemporary hardware, it’s comparable to a floating-point add or multiply. On current generation Intel, it’s actually a little bit more expensive than that. Not catastrophic, but expensive enough that you are throwing away half or more of your performance if you incur spurious conversions on every operation.

This is really common in C and C++ where a naked floating-point literal like 1.2 is double:

	float x;
	x *= 1.2;

Instead of a bare multiplication (current generation x86 hardware: 1 µop and 4 cycles latency) this produces a convert-to-double, multiplication, and convert-to-float (5 µops and 14 cycles latency per Agner Fog).

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