[PATCH] D50152: [CodeGen] Merge equivalent block copy/helper functions

[John McCall via Phabricator via cfe-commits]

rjmccall added inline comments.


================
Comment at: lib/CodeGen/CGBlocks.cpp:1682
+          if (IsCopyHelper && Ctx.getBlockVarCopyInits(Var).CanThrow)
+            Name += "c";
+        }
----------------
ahatanak wrote:
> rjmccall wrote:
> > I don't think you need to add `d` to the name of a copy helper.  It's a bit weird, but while copying a `__block` variable can cause its copy helper to run, destroying it immediately afterwards can never cause its destroy helper to run.  That's because a newly-copied `__block` variable always has a reference count of 2: the new reference in the copy and the forwarding reference from the original.
> > 
> > I think that means you can just add a single letter which specifies whether the corresponding `__block` variable operation is known to be able to throw.
> I added 'd' to the name of the copy helper functions only because IRGen generates different code depending on whether the destructor can throw or not.
> 
> For example, if I compile the following code with -DTHROWS, IRGen uses 'invoke' (which jumps to the terminate block) for the calls to `_Block_object_dispose` on the EH path whereas it uses 'call' if the destructor doesn't throw.
> 
> ```
> struct S {
>   S();
> #ifdef THROWS
>   ~S() noexcept(false);
> #else
>   ~S() noexcept(true);
> #endif
>   S(const S &);
>   int a;
> };
> 
> void test() {
>   __block S s0, s1, s2;
>   ^{ (void)s0, (void)s1; (void)s2; };
> }
> ```
> 
> It seems like IRGen doesn't have to use 'invoke' when emitting a call to  `_Block_object_dispose` even when the class has a destructor that can throw, if I understood your explanation correctly?
Right.  It's specifically only true when unwinding after a copy, which is very atypical for C++ code, but nonetheless it's true.  We should make the call `nounwind` in these situations and leave a comment explaining why.  Did my explanation make any sense?


================
Comment at: lib/CodeGen/CGBlocks.cpp:1688
+        if (F == BLOCK_FIELD_IS_BLOCK)
+          Name += "b";
+      }
----------------
ahatanak wrote:
> rjmccall wrote:
> > Why `rb` for a captured block instead of some single-letter thing?  You don't need to emulate the structure of the flags here.
> I can use a single letter here, but I'm already using 'b' for byref captures. Perhaps I can use 'o' for non-arc objects, instead of 'r', and use 'r' for byref?
That seems reasonable.


Repository:
  rC Clang

https://reviews.llvm.org/D50152