[PATCH] D56567: [ADT] Force attribute used on functions marked as always_inline.

[Davide Italiano via Phabricator via llvm-commits]

davide marked 2 inline comments as done.
davide added inline comments.


================
Comment at: llvm/include/llvm/ADT/SmallVector.h:128
   // forward iterator creation methods.
-  LLVM_ATTRIBUTE_ALWAYS_INLINE
+  LLVM_ATTRIBUTE_ALWAYS_INLINE LLVM_ATTRIBUTE_USED
   iterator begin() { return (iterator)this->BeginX; }
----------------
dexonsmith wrote:
> I don't think we want this for Release or RelWithDebInfo builds since it will bloat the binaries.  How is `LLVM_ATTRIBUTE_USED` conditionalized in those cases?
Thanks, I agree this should be emitted only at `-O0`. 
Is there an easy way of conveying that information?


================
Comment at: llvm/include/llvm/ADT/StringRef.h:127
     LLVM_NODISCARD
-    LLVM_ATTRIBUTE_ALWAYS_INLINE
+    LLVM_ATTRIBUTE_ALWAYS_INLINE LLVM_ATTRIBUTE_USED
     const char *data() const { return Data; }
----------------
dexonsmith wrote:
> This already has `LLVM_NODISCARD`.  How does that differ from `LLVM_ATTRIBUTE_USED`?
They're completely different attributes, to the best of my knowledge.

1) __attribute__((used)) forces the symbol to be emitted even if unreferenced.
http://gcc.gnu.org/onlinedocs/gcc-4.7.2/gcc/Variable-Attributes.html

2) [[nodiscard]] causes the compiler to emit a warning if the return value is discarded.
https://en.cppreference.com/w/cpp/language/attributes

Example:


```
* thread #1, queue = 'com.apple.main-thread', stop reason = breakpoint 1.1
    frame #0: 0x0000000100000fa3 a.out`main at pat.cpp:12:3
   9   	
   10  	int main(void) {
   11  	  Baciotto blondie;
-> 12  	  return blondie.patatino();
   13  	}
(lldb) p blondie
(Baciotto) $0 = {}
(lldb) p blondie.patatino()
(int) $1 = 23
(lldb) ^D
Davides-Mac-Pro:bin davide$ cat pat.cpp 
class Baciotto {
public:
  __attribute__((used))
  __attribute__((always_inline))
  int patatino() {
    return 23;
  }
};

int main(void) {
  Baciotto blondie;
  return blondie.patatino();
}
```

If you use [[nodiscard]], the symbol won't be emitted.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D56567/new/

https://reviews.llvm.org/D56567