On 4/6/2016 7:43 PM, Manu via Digitalmars-d wrote:
1. This has been characterized as a blocker, it is not, as it does not
impede writing code that takes advantage of various SIMD code generation at
compile time.
It's sufficiently blocking that I have not felt like working any
further without this feature present. I can't feel like it 'works' or
it's 'done', until I can demonstrate this functionality.
Perhaps we can call it a psychological blocker, and I am personally
highly susceptible to those.
I can understand that it might be demotivating for you, but that is not a
blocker. A blocker has no reasonable workaround. This has a trivial workaround:
gdc -simd=AFX foo.d
becomes:
gdc -simd=AFX -version=AFX foo.d
It's even simpler if you use a makefile variable:
FPU=AFX
gdc -simd=$(FPU) -version=$(FPU)
You also mentioned being blocked (i.e. demotivated) for *years* by this, and I
assume that may be because we don't care about SIMD support. That would be
wrong, as I care a lot about it. But I had no idea you were having a problem
with this, as you did not file any bug reports. Suffering in silence is never
going to work :-)
2. I'm not sure these global settings are the best approach, especially if
one is writing applications that dynamically adjusts based on the CPU the
user is running on.
They are necessary to provide a baseline. It is typical when building
code that you specify a min-spec. This is what's used by default
throughout the application.
It is not necessary to do it that way. Call std.cpuid to determine what is
available at runtime, and issue an error message if not. There is no runtime
cost to that. In fact, it has to be done ANYWAY, as it isn't user friendly to
seg fault trying to execute instructions that do not exist.
Runtime selection is not practical in a broad sense. Emitting small
fragments of SIMD here and there will probably take a loss if they are
all surrounded by a runtime selector. SIMD is all about pipelining,
and runtime branches on SIMD version are antithesis to good SIMD
usage; they can't be applied for small-scale deployment.
In my experience, runtime selection is desirable for large scale
instantiations at an outer level of the work loop. I've tried to
design this intent in my library, by making each simd API capable of
receiving SIMD version information via template arg, and within the
library, the version is always passed through to dependent calls.
The Idea is, if you follow this pattern; propagating a SIMD version
template arg through to your outer function, then you can instantiate
your higher-level work function for any number of SIMD feature
combinations you feel is appropriate.
Doing it at a high level is what I meant, not for each SIMD code fragment.
Naturally, this process requires a default, otherwise this usage
baggage will cloud the API everywhere (rather than in the few cases
where a developer specifically wants to make use of it), and many
developers in 2015 feel SSE2 is a weak default. I would choose SSE4.1
in my applications, xbox developers would choose AVX1, it's very
application/target-audience specific, but SSE2 is the only reasonable
selection if we are not to accept a hint from the command line.
I still don't see how it is a problem to do the switch at a high level. Heck,
you could put the ENTIRE ENGINE inside a template, have a template parameter be
the instruction set, and instantiate the template for each supported instruction
set.
Then,
void app(int simd)() { ... my fabulous app ... }
int main() {
auto fpu = core.cpuid.getfpu();
switch (fpu) {
case SIMD: app!(SIMD)(); break;
case SIMD4: app!(SIMD4)(); break;
default: error("unsupported FPU"); exit(1);
}
}
I've done it with a template arg because it can be manually
propagated, and users can extrapolate the pattern into their outer
work functions, which can then easily have multiple versions
instantiated for runtime selection.
I think it's also important to mangle it into the symbol name for the
reasons I mention above.
Note that version identifiers are not usable directly as template parameters.
You'd have to set up a mapping.
And yes, if mangled in as part of the symbol, the linker won't pick the wrong
one.
