[llvm-dev] Inline function not eventually inlined is removed (original) (raw)
David Chisnall via llvm-dev llvm-dev at lists.llvm.org
Wed Aug 4 04:46:30 PDT 2021
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On 02/08/2021 18:05, Mariusz Sikora via llvm-dev wrote:
I'm just trying to understand is this Code undefined behavior or this is a bug in LLVM? Because why LLVM is removing functions without inlining it? For example GCC is not removing function event after inlining it.
C++ inline means 'the definition is provided in line with the
declaration, the compiler and linker are responsible for ensuring that
exactly one definition exists in the final binary'
C inline means 'this definition is provided in line with a declaration
and may be used by the compiler in preference to one that a linker finds'
C inline extern means 'a definition of this may appear in line with
the declaration but please provide a canonical definition here for when
the compiler decides not to emit it'
C inline static means 'a definition exists here inline and it is not
an error if this is not used. If it is, then it is private to this
compilation unit and it is not an error for the same static function to
exist in multiple compilation units'.
None of these say anything about whether the compiler is required to inline the function, but they all specify what must happen to the original definition:
- C++
inline: Must exist in at least one compilation unit and the linker must discard duplicates. - C
inline: Must be eliminated - C
inline extern: must be emitted, the linker should error if two definitions of the same inline extern function exist in different compilation units. - C
inline static: It must be emitted if references to it exist in the object code but it may be eliminated if it is unused (including if all uses of it are inlined).
Clang is generating IR that makes LLVM do exactly what the language semantics require: eliminate the definition.
The inline keyword is probably the most confusingly named keyword in
C/C++, though static comes close. The general rule of thumb for C is:
- If you think you mean
inlineyou probably meaninline static. - If you're really sure you mean
inline, you almost certainly mean__attribute__((always_inline))or__forceinline(depending on whether you're writing GNU or Microsoft-flavoured C) - If you're not sure but think you might mean
inline, you really mean to be writing C++ and not C.
David
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