Hacker News new | ask | show | jobs
by mpweiher 683 days ago
But having a straightforward/predictable mapping to the underlying machine and its semantics is included in the C model of computation.

And that is actually not just compatible with the C "model of computation" being otherwise quite incomplete, these two properties are really just two sides of the same coin.

The whole idea of an "abstract C machine" that unambiguously and completely specifies behavior is a fiction.
2 comments
trealira 682 days ago
> But having a straightforward/predictable mapping to the underlying machine and its semantics is included in the C model of computation.

While you can often guess what the assembly will be from looking at C code given that you're familiar with the compiler, exactly how C is to be translated into assembly isn't well-specified.

For example, you can't expect that all uses of the multiplication operator "*" results in an actual x86 mul instruction. Many users expect constant propagation, so you can write something like "2 * SOME_CONSTANT" without computing that value at runtime; there is no guarantee of this behavior, though. Also, for unsigned integers, when optimizations are turned on, many expect compilers to emit left shift instructions when multiplying by a constant power of two, but again, there's no guarantee of this. That's not to say this behavior couldn't be part of a specification, but it's just an informal expectation right now.

What I think people might want is some readable, well-defined set of attribute grammars[0] for translation of C into assembly for varying optimization levels - then, you really would be able to know exactly how some piece of C code under some context would be translated into assembly. They've already been used for writing code generator generators in compilers, but what I'm thinking is something more abstract, not as concrete as a code generation tool.

[0]: https://en.wikipedia.org/wiki/Attribute_grammar

link
mpweiher 681 days ago
> exactly how C is to be translated into assembly isn't well-specified.

Exactly! It's not well-specified so the implementation is not prevented from doing a straightforward mapping to the machine by some part of the spec that doesn't map well to the actual machine.

link
dathinab 682 days ago
> But having a straightforward/predictable mapping to the underlying machine and its semantics is included in the C model of computation.

not rally, or at least not in a way which would count as "high level assembler". If it would the majority of optimizations compilers do today would not be standard conform.

Like there is a mapping to behavior but not a mapping to assembly.

Which is where the abstract C machine as a hypothetical machine formed from the rules of the standard comes in. Kinda as a mind model which runs the behavior mappings instead of running any specific assembly. But then it not being ambiguous and complete doesn't change anything about C not being high level assembly, actually it makes C even less high level assembly.

link
mpweiher 682 days ago
> If it would the majority of optimizations compilers do today would not be standard conform.

They aren't.

link
pjmlp 682 days ago
So you can easily tell, just by looking to the C source code, if plain Assembly instructions are being used from four books of ISA manual, if the compiler is able to automatically vectorize a code region including which flavour of vector instructions, or completely replace specific math code patterns for a single opcode.
link