6 ms·

>Pragmas are processed by the pre-processor This is wrong a lot of pragmas, I would even say most pragmas are not handled by preprocessor. Some examples: pragm

>Pragmas are processed by the pre-processor This is wrong a lot of pragmas, I would even say most pragmas are not handled by preprocessor. Some examples: pragma pack, warning control, pragma GCC unroll, per function optimization setting changes, all the pragmas which 1:1 map to c++11 style attributes. None of that is handled by preprocessor, pragma once seems like rare one which is. Yes all of them are compiler specific, but handling compiler specific behavior is one of the main purpose of pragmas.

s28l 4y ago
I meant that they are processed before any syntactical analysis, so they're not particularly useful for this kind of thing. For example, these are all wrong usages of our hypothetical `#pragma unreachable`: void foo(); #pragma unreachable class bar { #pragma unreachable }; namespace foobar { #pragma unreachable } But pragmas can generally be used anywhere, barring tokenization issues. The end result is that `pragma unreachable` would likely end up turning into a magic function call inside the compiler, since it really only makes sense in a spot where you can invoke a function. Also, I think you are conflating pragmas with `__attribute__`, which is how you set per function optimization settings. If you do that with pragmas, then it isn't limited to a single function.
- addaon 4y ago
  This is the intent and purpose of _Pragma; it provides a way to use existing #pragmas that are tokenized and handled a bit later, so they can e.g. be included in macro expansions.
  - s28l 4y ago
    I think the only benefit of `_Pragma` is that it enables defining a macro that expands into a `#pragma` definition. I don't think there's any other use case beyond that.
- gpderetta 4y ago
  If pragma works for omp for it would work for unreachable. But it is just ugly and there really wouldn't be any reason to use it here.

s28l 4y ago

I meant that they are processed before any syntactical analysis, so they're not particularly useful for this kind of thing. For example, these are all wrong usages of our hypothetical `#pragma unreachable`: void foo(); #pragma unreachable class bar { #pragma unreachable }; namespace foobar { #pragma unreachable } But pragmas can generally be used anywhere, barring tokenization issues. The end result is that `pragma unreachable` would likely end up turning into a magic function call inside the compiler, since it really only makes sense in a spot where you can invoke a function. Also, I think you are conflating pragmas with `__attribute__`, which is how you set per function optimization settings. If you do that with pragmas, then it isn't limited to a single function.

addaon 4y ago
This is the intent and purpose of _Pragma; it provides a way to use existing #pragmas that are tokenized and handled a bit later, so they can e.g. be included in macro expansions.
- s28l 4y ago
  I think the only benefit of `_Pragma` is that it enables defining a macro that expands into a `#pragma` definition. I don't think there's any other use case beyond that.
gpderetta 4y ago
If pragma works for omp for it would work for unreachable. But it is just ugly and there really wouldn't be any reason to use it here.