[llvm-dev] Inline function not eventually inlined is removed
David Chisnall via llvm-dev
llvm-dev at lists.llvm.org
Wed Aug 4 04:46:30 PDT 2021
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
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
- 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
- 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 `inline` you probably mean `inline 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.
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