[llvm-dev] Inline function not eventually inlined is removed (original) (raw)

David Blaikie via llvm-dev llvm-dev at lists.llvm.org
Mon Aug 2 12🔞06 PDT 2021

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 inline1clang.o >> >> ll file: >> ; Function Attrs: nounwind uwtable >> define dsolocal i32 @sort(i32* %a, i32* %b) localunnamedaddr #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 dsolocal void @calculate(i32*, i32*) localunnamedaddr #1 >> >> Thanks >> Mariusz Sikora >> ________________________ >> LLVM Developers mailing list >> llvm-dev at lists.llvm.org >> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev >> >______________________ >LLVM Developers mailing list >llvm-dev at lists.llvm.org >https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

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