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0 pointsaidenn04y ago0 comments

Over just the time I've been aware of things, there's been a constant positive feedback loop of "checked overflow isn't used by software, so CPU designers make it less performant" followed by "Checked overflow is less performant so software uses it less."

I wish there was a way out.

Language features are also often implemented at least partly because they can be done efficiently on the premiere hardware for the language. Then new hardware can make such features hard to implement.

WASM implemented return values in a way that was different from register hardware, and it makes efficient codegen of Common Lisp more challenging. This was brought to the attention of the committee while WASM was still in flux, and they (perhaps rightfully) decided CL was insufficiently important to change things.

I'm sure that people brought up the overflow situation to the RISC-V designers, and it was similarly dismissed. It's just unfortunate that legacy software is such a big driver of CPU features as that's a race towards lowest-common-denominator hardware.

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R0b0t14y ago

There's a blog page somewhere that's a rant for implementing saturating and other arithmetic modes. Would be a really good idea.

Main one is interrupt on overflow.

imtringued4y ago

I agree. A lot of software only wants protection against overflows but does not depend on them for functionality. If something wants to read out the carry bit, it should be explicit and although it is unfortunate, indicating that requires a full instruction.

titzer4y ago

> WASM implemented return values in a way that was different from register hardware, and it makes efficient codegen of Common Lisp more challenging. This was brought to the attention of the committee while WASM was still in flux, and they (perhaps rightfully) decided CL was insufficiently important to change things.

Can you refresh my memory here? What exactly is different about Wasm return values than any other function-oriented language?

aidenn0OP4y ago

Common Lisp will often have auxiliary return values that are often not needed (e.g. the mathematical floor function returns the remainder as a second value). Unused extra values are silently discarded.

So you can do, for example (+ (floor x y) z) without worrying about the second value to floor.

A lisp compiler for a register machine will usually return the first N values in registers (for some small value of N), so the common case of using only the primary return value generates exactly the same code regardless of how many values are actually returned.

I don't remember the details, but however wasm happens to implement multiple return values, you can't just pretend that a function that returns 2 values only returns one.

titzer4y ago

Return values are pushed onto the Wasm operand stack in left-to-right order. Engines use registers for params/returns up to a point in optimized code and then spill to the stack. So at the call site you can just drop the return values you don't want and should end up with machine code that does exactly what you want.

2 more replies

throwaway815234y ago

Common Lisp supports multiple value returns, which C doesn't. Go sort of does, though.

titzer4y ago

Wasm supports multiple value returns.

j / k navigate · click thread line to collapse

0 pointsaidenn04y ago0 comments

I wish there was a way out.

0 comments

R0b0t14y ago

There's a blog page somewhere that's a rant for implementing saturating and other arithmetic modes. Would be a really good idea.

Main one is interrupt on overflow.

imtringued4y ago

titzer4y ago

Can you refresh my memory here? What exactly is different about Wasm return values than any other function-oriented language?

aidenn0OP4y ago

So you can do, for example (+ (floor x y) z) without worrying about the second value to floor.

I don't remember the details, but however wasm happens to implement multiple return values, you can't just pretend that a function that returns 2 values only returns one.

titzer4y ago

2 more replies

throwaway815234y ago

Common Lisp supports multiple value returns, which C doesn't. Go sort of does, though.

titzer4y ago

Wasm supports multiple value returns.

j / k navigate · click thread line to collapse