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I don't think "you can implement this slightly modified version of the proposal in GCC macros" is a good reason to change the defer proposal. It doesn't work in

I don't think "you can implement this slightly modified version of the proposal in GCC macros" is a good reason to change the defer proposal. It doesn't work in clang¹ or any other non-GCC C implementation, a trailing semicolon after the close brace is an ugly wart, and saying "if you want to implement this all you have to do is implement [[gnu::cleanup]] and nested local functions" is presumably much more work for other C implementations than just implementing defer is. I also question whether this even works in GCC with executable stacks disabled (I know GCC can optimize away the trampoline since the function doesn't escape the local scope, but does GCC let you write nested functions at all without executable stacks?). ¹The author says it works in clang using Apple's Blocks feature, but Blocks should not be required for defer and the variable semantics are wrong so it's a non-starter.

kenferry 2y ago
I don't think they're advocating not doing defer in C? They're saying you can backport the functionality if needed, or if you want to start using it now.
- lilyball 2y ago
  They're recommending changes to the proposal though, such as requiring a trailing semicolon after the close brace. It also changes the syntactical category of the defer statement, though it's not clear to me what that actually affects.
eru 2y ago
Why do you think you need executable stacks for nested functions? I don't know what GCC is doing, but functional programming languages usually 'compile away' the nesting of functions fairly early, see https://en.wikipedia.org/wiki/Lambda_lifting https://en.wikipedia.org/wiki/Lambda_lifting Update: Oh, I see, it's because GCC doesn't want to change how function are passed around ('function pointers' in C speak), so they need to get creative. Functional languages use something closer to what C people would call a 'fat pointer' to fix this.
- uecker 2y ago
  GCC creates trampolines only when it needs the address of the nested function, which it does not in this case (formally it does but not really). Also new version can place the trampoline on the heap (-ftrampline-impl=heap) and then you also need no executable stack. I wide pointer would be better though.
  - eru 2y ago
    > Also new version can place the trampoline on the heap (-ftrampline-impl=heap) and then you also need no executable stack. I wide pointer would be better though. Wouldn't you need an executable heap, though? (That's not as outlandish, because eg a JIT would need that to. But most straightforward C programs don't need to create any executable memory contents at runtime.)
    - uecker 2y ago
      It does use executable heap space, but I think it uses specific pages which might also be pre-initialized, but I need to check how they implemented it.

kenferry 2y ago

I don't think they're advocating not doing defer in C? They're saying you can backport the functionality if needed, or if you want to start using it now.

lilyball 2y ago
They're recommending changes to the proposal though, such as requiring a trailing semicolon after the close brace. It also changes the syntactical category of the defer statement, though it's not clear to me what that actually affects.

eru 2y ago

Why do you think you need executable stacks for nested functions? I don't know what GCC is doing, but functional programming languages usually 'compile away' the nesting of functions fairly early, see https://en.wikipedia.org/wiki/Lambda_lifting https://en.wikipedia.org/wiki/Lambda_lifting Update: Oh, I see, it's because GCC doesn't want to change how function are passed around ('function pointers' in C speak), so they need to get creative. Functional languages use something closer to what C people would call a 'fat pointer' to fix this.

uecker 2y ago
GCC creates trampolines only when it needs the address of the nested function, which it does not in this case (formally it does but not really). Also new version can place the trampoline on the heap (-ftrampline-impl=heap) and then you also need no executable stack. I wide pointer would be better though.
- eru 2y ago
  > Also new version can place the trampoline on the heap (-ftrampline-impl=heap) and then you also need no executable stack. I wide pointer would be better though. Wouldn't you need an executable heap, though? (That's not as outlandish, because eg a JIT would need that to. But most straightforward C programs don't need to create any executable memory contents at runtime.)
  - uecker 2y ago
    It does use executable heap space, but I think it uses specific pages which might also be pre-initialized, but I need to check how they implemented it.