[llvm-dev] False Negatives Wunused-function in clang found by gcc

Tue Mar 2 10:06:26 PST 2021

Yep, +1 to that.

On Tue, Mar 2, 2021 at 10:00 AM Aaron Ballman <aaron at aaronballman.com>
wrote:

> On Tue, Mar 2, 2021 at 12:03 PM David Blaikie <dblaikie at gmail.com> wrote:
> >
> > Any reason it shouldn't be a subgroup of -Wunused-function? I guess
> maybe because people already had codebases that were -Wunused-function
> clean, and we wouldn't want to regress that/start warning on new
> things/force them to have to add -Wno-unused-member-function to unbreak
> their builds?
> >
> > (I guess there's some history here? Like perhaps -Wunused-function was
> initially implemented without member function support, and it was added
> later & so added under a separate flag to make rollout easier? But I think
> it'd probably still be OK for it to be under a subgroup - people could opt
> out of it initially without losing all their existing
> -Wunused-function-cleanliness, and opt in if they ever decided to do the
> extra cleanup)
>
> Here is the commit that introduced the feature (under a different flag
> that was later renamed to -Wunused-member-function):
>
> https://github.com/llvm/llvm-project/commit/cad715fb9bfd50370fa22ca62634048dcc85598b
> but I was not able to locate the discussion about the patch from the
> list archives.
>
> It looks like the big reason it's not under -Wunused is because the
> LLVM codebase isn't ready to handle it, but given that 11 years have
> passed, I feel like it would be reasonable to revisit the warning
> groups. Logically, I would expect -Wunused-function to include
> -Wunused-member-function and I would expect -Wunused to include them
> both. If LLVM is not able to handle that change, we can alter our
> CMake files.
>
> ~Aaron
>
> >
> > On Tue, Mar 2, 2021 at 6:05 AM Aaron Ballman <aaron at aaronballman.com>
> wrote:
> >>
> >> On Mon, Mar 1, 2021 at 8:22 PM David Blaikie <dblaikie at gmail.com>
> wrote:
> >> >
> >> > Not sure where the regression might've leaked in. But does seem to be
> something to do with anonymous namespace support and member functions:
> https://godbolt.org/z/zz8WP1
> >>
> >> I don't think there's a regression here, unless I'm missing something.
> >> -Wunused-member-function is what diagnoses this
> >> (https://godbolt.org/z/MxP4h6) but it is not a subgroup of either
> >> -Wunused-function or -Wunused
> >> (
> https://github.com/llvm/llvm-project/blob/main/clang/include/clang/Basic/DiagnosticGroups.td#L839
> ).
> >> Perhaps we should explicitly enable it?
> >>
> >> ~Aaron
> >>
> >> >
> >> >
> >> >
> >> > On Mon, Mar 1, 2021 at 5:12 PM Luke Benes via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> >> >>
> >> >>
> >> >> By building llvm-project with gcc 7 and 10, I found multiple
> Wunused-function warnings that clang 11/12 missed.
> >> >>
> >> >> All of these warnings were recently introduced, and the authors
> fixed many of them.   Any ideas why clang misses so many of this category?
> And why the ones missed are recent changes to the llvm codebase?
> >> >>
> >> >> -Luke
> >> >>
> >> >>
> >> >> ==========
> >> >>
> >> >>  After dab953c8e44a2 with gcc 10.2.1, I see the following new
> warnings:
> >> >>
> >> >> [ 65%] Building CXX object
> lib/Linker/CMakeFiles/LLVMLinker.dir/IRMover.cpp.o
> >> >> /llvm/lib/Transforms/Instrumentation/DataFlowSanitizer.cpp:1600:1:
> warning: ‘std::pair<llvm::Value*, llvm::Value*>
> {anonymous}::DataFlowSanitizer::getShadowOriginAddress(llvm::Value*,
> llvm::Align, llvm::Instruction*)’ defined but not used [-Wunused-function]
> >> >>  1600 | DataFlowSanitizer::getShadowOriginAddress(Value *Addr, Align
> InstAlignment,
> >> >>       | ^~~~~~~~~~~~~~~~~
> >> >> /llvm/lib/Transforms/Instrumentation/DataFlowSanitizer.cpp:1510:6:
> warning: ‘void {anonymous}::DFSanFunction::setOrigin(llvm::Instruction*,
> llvm::Value*)’ defined but not used [-Wunused-function]
> >> >>  1510 | void DFSanFunction::setOrigin(Instruction *I, Value *Origin)
> {
> >> >>       |      ^~~~~~~~~~~~~
> >> >> /llvm/lib/Transforms/Instrumentation/DataFlowSanitizer.cpp:1476:8:
> warning: ‘llvm::Value* {anonymous}::DFSanFunction::getOrigin(llvm::Value*)’
> defined but not used [-Wunused-function]
> >> >>  1476 | Value *DFSanFunction::getOrigin(Value *V) {
> >> >>       |        ^~~~~~~~~~~~~
> >> >> /llvm/lib/Transforms/Instrumentation/DataFlowSanitizer.cpp:1469:8:
> warning: ‘llvm::Value* {anonymous}::DFSanFunction::getRetvalOriginTLS()’
> defined but not used [-Wunused-function]
> >> >>  1469 | Value *DFSanFunction::getRetvalOriginTLS() { return
> DFS.RetvalOriginTLS; }
> >> >>       |        ^~~~~~~~~~~~~
> >> >>
> >> >> FIX:
> https://github.com/llvm/llvm-project/commit/9524632fa2bf8c894e4aac27f03631a1915d84d3
> >> >>
> >> >> ==========
> >> >>
> >> >> After bc8e262afe83, with gcc 10.1 but not clang 11, I'm seeing:
> >> >>
> >> >> [ 21%] Building CXX object
> lib/MC/MCParser/CMakeFiles/LLVMMCParser.dir/MasmParser.cpp.o
> >> >> /llvm/lib/MC/MCParser/MasmParser.cpp:3836:6: warning: ‘bool
> {anonymous}::MasmParser::emitStructValue(const {anonymous}::StructInfo&)’
> defined but not used [-Wunused-function]
> >> >>  3836 | bool MasmParser::emitStructValue(const StructInfo
> &Structure) {
> >> >>       |      ^~~~~~~~~~
> >> >>
> >> >> FIX: https://reviews.llvm.org/D83898
> >> >>
> >> >> ==========
> >> >>
> >> >> In 931a68f26b9a3, I'm seeing the following warning with gcc7 and
> gcc10:
> >> >> [ 70%] Building CXX object
> lib/CodeGen/CMakeFiles/LLVMCodeGen.dir/RegAllocPBQP.cpp.o
> >> >> /llvm/lib/CodeGen/RegAllocFast.cpp:384:6: warning: ‘bool
> {anonymous}::RegAllocFast::verifyRegStateMapping(const
> {anonymous}::RegAllocFast::LiveReg&) const’ defined but not used
> [-Wunused-function]
> >> >>  bool RegAllocFast::verifyRegStateMapping(const LiveReg &LR) const {
> >> >>       ^~~~~~~~~~~~
> >> >>
> >> >> FIX: 0671a4c5087d40450603d9d26cf239f1a8b1367e
> >> >>
> >> >> _______________________________________________
> >> >> LLVM Developers mailing list
> >> >> llvm-dev at lists.llvm.org
> >> >> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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