Re: Integer Overflow/Saturation Operations


Mikael
 

As Cohen responded. The examples provided only work, as you rightly noted, if you have implied guard bits in the system.
This is fine, with the caveat that the ISA definition doesn't provision for the users to have manually manage guard bits in their choice of math to fully utilize the vector engine.
The open question is, what would a user of the vector ISA expect the results to be of int64 += int32 * int32? fixedPoint64 += fixedPoint32 * fixedPoint32?

From our perspective we are looking at the ISA extension as defined. Which is without guard bits in the accumulation, and from this its easy to demonstrate that even for signed widening MAC operations overflow happens quite easily. I think we all agree on this point.
I would also like to stress here that we are not trying to build an argument for added guard bits a special accumulation registers, as this would break the vector register model.

Now the challenges we find is the only way out of this conundrum, as you point out, is to use the optional quad widening operation.
As you point out, for DSP operations, integer/fixed point, this is an issue that is unresolved.
Fundamentally this boils down to a realization that the vector ISA as defined is not well suited for DSP operations, unless float is used. Which is the point we are trying to get across.

Saturation is a short-cut to at least detect when an overflow happens. This has one caveat, which is saturation in a DSP filter is a sign of issues. You really do not want the innards of a DSP filter to saturate.
As currently defined. The vector ISA either mandates the use of the optional quad widening instruction for DSP math, which implies the DSP math is optional to begin with, or DSP math has to be performed in floating point, which defeats the purpose of supporting fixed point DSP math to begin with.

There are alternatives involve utilizing block float style fixed point coding. This does introduce extra overhead for each fixed point math operation as well as a need to keep track of various decimal points throughout the signal path.

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