No sane compiler would optimize atomics (2015)

Re:No sane compiler would optimize atomics (2015) by stormbrew on Sunday July 11, 2021 @11:58PM (#27806889)

I'm not sure where you got that idea. C was never ever a macro assembler, and its antecedent language (B[1]) is actually even more abstracted away. C was a slight lowering, not a raising, of hardware abstraction.

If C were a macro assembler the following things would be true of it, most likely:

  - types would have fixed bit sizes (ie. uint32_t would be the norm)
  - you'd be able to do various 'normal' things for assembly like rotate left and right.
  - inline assembly would probably be.. you know.. standard instead of something that compiler vendors only started putting in in like the 80s.

You might be thinking of the fact that C++ was originally sort of a Macro C (aka Cfront)?

[1] Ever wondered why `int` is so fuzzily defined in C? It's because in B there was only one type and it was always word sized. That's also why various things are typed `int` by default if you don't specify.

[ Parent | on HN ]

Re:No sane compiler would optimize atomics (2015) by Someone on Monday July 12, 2021 @03:48AM (#27808046)

Another extremely normal thing assembly allows you to do, but C doesn’t: access carry and overflow flags.

[ Parent | on HN ]

Re:No sane compiler would optimize atomics (2015) by spc476 on Monday July 12, 2021 @04:35AM (#27808268)

Not all CPUs have carry and overflow flags. For example, MIPS, or SPARC, or the new RISC-V.

[ Parent | on HN ]

Re:No sane compiler would optimize atomics (2015) by stormbrew on Monday July 12, 2021 @02:21PM (#27813365)

really relevant to this discussion, though, the pdp11 did.

[ Parent | on HN ]

Re:No sane compiler would optimize atomics (2015) by umanwizard on Monday July 12, 2021 @01:00PM (#27812426)

Another (modern) example: SIMD instructions.

[ Parent | on HN ]

Re:No sane compiler would optimize atomics (2015) by overgard on Monday July 12, 2021 @10:05AM (#27810397)

> I'm not sure where you got that idea

History: C was essentially PDP11 macro assembler. There are so many assumptions in C that basically came from the PDP11. C was originally a very utilitarian language to make writing unix easier. That it became massively popular was more of a happy accident.

[ Parent | on HN ]

Re:No sane compiler would optimize atomics (2015) by stormbrew on Monday July 12, 2021 @02:20PM (#27813357)

I'm sorry, but this is just a gross mischaracterization of what a "macro assembler" is. I agree it was utilitarian and made to make unix easier and that many of its core assumptions were based on the platform of the pdp11, but it never did in any way resemble a macro assembler as far as I know.

And in fact, as gizmo86 pointed out, one of the explicit goals of C in terms of "making unix easier to write" was, afaik, that it would allow the code to be more portable to other architectures, because as the 11 in the name implies, the history that led to the creation of unix was not short on new architectures, and one of the key weaknesses of Multics was, afaik, the fact that it was rigidly written specifically for one specific architecture.

By this logic nearly every programming language is just a macro assembler at a certain level of (non-)complexity. All compilers do is translate syntax (macros?) into machine code (assembly?). So, is pascal also a macro assembler?

Anyways here is a list of instructions on the pdp11 that you can't directly represent even in the earliest versions of C to my knowledge:

  ROR
  ROL
  SWAB
  SXT
  ADC
  SBC
  BVC
  BVS
  BCS
  MARK
  HALT
  WAIT
  RESET
  MTPD
  MTPI
  MFPD
  MFPI
  MTPS
  MFPS
  MFPT
  CLC
  CLV
  CLZ
  CLN
  CCC
  SEC
  SEV
  SEZ
  SEN
  SCC

That's the distinguishing factor of a "compiler" vs. a "macro assembler". A macro assembler is a layer on top of an assembler that still allows you full access to the instruction set and allows control flow that most compilers would consider unsafe. C was never that. It wasn't derived from a language that was that and it didn't, to my knowledge, allow that even on pdp-11 unix.

I'd be really happy to see some code from one of the code archives of unix' history that proves me wrong there, though, if you have it.

[ Parent | on HN ]

Re:No sane compiler would optimize atomics (2015) by pjmlp on Tuesday July 13, 2021 @04:04AM (#27818597)

UNIX was originally written in Assembly, C was used to rewrite the 4th version.

[ Parent | on HN ]