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

[Fāng-ruì Sòng via llvm-dev]

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.

This is a user error. You may be looking for -fgnu89-inline & C99
inline documentation.
An extern inline definition is needed.

Inlining can paper over the user error. clang decides that inlining is
not good because the trip count is too large.
If you decrease the number of iterations, you may find that clang
inlines the function and the linker error goes away.

> 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