(original) (raw)

I guess that somehow turns this inline declaration into the strong definition (despite the extern declaration coming after the inline definition). eg, that definition is emitted even without the call:

inline int foo() { }

extern inline int foo();

int bar() { }

But yeah, some way of specifying an extern definition (either separately, or by promoting the prior inline definition to an extern inline definition) is necessary. C inline is quirky (well, compared to C++ - they're probably all a bit quirky, just a matter of what you're used to).

On Mon, Aug 2, 2021 at 12:38 PM <paul.robinson@sony.com> wrote:

Clarification: According to the link provided earlier,

https://www.iar.com/knowledge/support/technical-notes/compiler/linker-error-undefined-external-for-inline-functions/
you need (exactly one) extern \_declaration\_ of the inline function, to keep it from vanishing.

inline int foo() { stuff; }

extern inline int foo(); // not a definition

int bar() { return foo(); }

--paulr

From: llvm-dev <llvm-dev-bounces@lists.llvm.org> On Behalf Of David Blaikie via llvm-dev
Sent: Monday, August 2, 2021 3:18 PM
To: Mariusz Sikora <msikora87@gmail.com>
Cc: llvm-dev <llvm-dev@lists.llvm.org>
Subject: Re: \[llvm-dev\] Inline function not eventually inlined is removed

Looks like both Clang and GCC discard the inline function definition even if the function is not inlined and the definition is needed (because C requires there be a separate non-inline definition for correctness): https://godbolt.org/z/hPjv1d1db

This code is probably (in C++ standard terminology, I'm not sure what terminology the C standard uses) "invalid, no diagnostic required" - that's usually the language for stuff that can cause linker errors like this.

On Mon, Aug 2, 2021 at 10:05 AM Mariusz Sikora <msikora87@gmail.com> 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.

On Fri, Jul 30, 2021 at 7:16 PM Fangrui Song <maskray@google.com> wrote:

On 2021-07-30, David Blaikie via llvm-dev wrote:
\>You're probably looking for some documentation about C inline semantics:
\>https://www.iar.com/knowledge/support/technical-notes/compiler/linker-error-undefined-external-for-inline-functions/

Additional notes: I think the -fgnu89-inline & C99 inline semantics were
designed explicitly the way so that vague linkage
(https://itanium-cxx-abi.github.io/cxx-abi/abi/prop-72-comdat.html) can
be avoided. The C inline behaviors are like always explicit
instantiation in C++.

(Seems that GNU has extensions for weak symbols on the a.out binary
format. Otherwise, if a binary format has neither weak symbol nor
COMDAT, vague linkage is not representable.)

\>On Fri, Jul 30, 2021 at 9:16 AM Mariusz Sikora via llvm-dev <
\>llvm-dev@lists.llvm.org> wrote:
\>
\>> Hello,
\>>
\>> I'm trying to understand why LLVM-12 is removing function which is marked
\>> inline despite the fact it was not inlined inside caller. Caller function
\>> still has a call to inline function and compilation is failing because of a
\>> lack of the symbol.
\>>
\>> Looking at debug logs I see:
\>>
\>> Inliner visiting SCC: sort: 1 call sites.
\>> Analyzing call of calculate... (caller:sort)
\>> .
\>> Cost: 960
\>> Threshold: 487
\>> NOT Inlining (cost=960, threshold=487), Call: call void
\>> @calculate(i32\* %a, i32\* %b)
\>>
\>> Code:
\>> int global = 0;
\>> void inline calculate(int a\[100\], int b\[100\]) {
\>> int i;
\>> #pragma unroll
\>> for (i = 0; i < 50; i++) {
\>> a\[i\] = b\[i\] + a\[i\];
\>> }
\>> }
\>>
\>> int sort(int a\[100\], int b\[100\]) {
\>> calculate(a, b);
\>> return a\[20\] + b\[30\] + global;
\>> }
\>>
\>> cli: clang -O3 -c inline1.c -o inline1\_clang.o
\>>
\>> ll file:
\>> ; Function Attrs: nounwind uwtable
\>> define dso\_local i32 @sort(i32\* %a, i32\* %b) local\_unnamed\_addr #0 {
\>> entry:
\>> tail call void @calculate(i32\* %a, i32\* %b)
\>> %arrayidx = getelementptr inbounds i32, i32\* %a, i64 20
\>> %0 = load i32, i32\* %arrayidx, align 4, !tbaa !2
\>> %arrayidx1 = getelementptr inbounds i32, i32\* %b, i64 30
\>> %1 = load i32, i32\* %arrayidx1, align 4, !tbaa !2
\>> %add = add nsw i32 %1, %0
\>> %2 = load i32, i32\* @global, align 4, !tbaa !2
\>> %add2 = add nsw i32 %add, %2
\>> ret i32 %add2
\>> }
\>>
\>> ; Function Attrs: inlinehint nounwind uwtable
\>> declare dso\_local void @calculate(i32\*, i32\*) local\_unnamed\_addr #1
\>>
\>> Thanks
\>> Mariusz Sikora
\>> \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_
\>> LLVM Developers mailing list
\>> llvm-dev@lists.llvm.org
\>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
\>>

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--

Pozdrawiam

Mariusz Sikora