[llvm-dev] Inline function not eventually inlined is removed

[David Blaikie via llvm-dev]

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 at 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 at 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 at 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 at lists.llvm.org
>> >> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>> >>
>>
>> >_______________________________________________
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>>
>>
>
> --
> Pozdrawiam
> Mariusz Sikora
>
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