ARM: extraneous unwinder reference with -fprofile-arcs

[arndb]

Bugzilla Link	43234
Version	8.0
OS	Linux
Blocks	#4440
CC	@calixteman,@kbeyls,@nickdesaulniers,@zygoloid,@smithp35,@vedantk

Extended Description

I got a link error on a 32-bit ARM kernel build with CONFIG_GCOV_KERNEL and CONFIG_UNWIND_FRAME_POINTER set:

kernel/trace/trace_clock.o:(.ARM.exidx+0x0): undefined reference to __aeabi_unwind_cpp_pr0' kernel/trace/trace_clock.o:(.ARM.exidx+0x8): undefined reference to __aeabi_unwind_cpp_pr0'
kernel/trace/trace_clock.o:(.ARM.exidx+0x10): undefined reference to `__aeabi_unwind_cpp_pr0'

I created a reduced test case at https://godbolt.org/z/gBQtAP

Basically, clang inserts __llvm_gcov_writeout/__llvm_gcov_flush/__llvm_gcov_init functions into any file it compiles, even for an empty C file input. These functions contain .fnstart/.fnend statements even when building with "-fno-omit-frame-pointer -fno-unwind-tables", which then results in unresolved references to __aeabi_unwind_cpp_pr0.

[arndb]

assigned to @nickdesaulniers

[vedantk]

It looks like the gcov pass has a few too many ways to create functions. Maybe there should just be a single helper? Like

+ static FunctionCallee                                                                                                                                                           
+ getOrInsertGCOVHelperFunc(Module *M, StringRef Name, FunctionType *FTy,                                                                                                         
+                           const GCOVOptions &Opts,                                                                                                                              
+                           AttributeList AL = AttributeList()) {                                                                                                                 
+   FunctionCallee Callee = M->getOrInsertFunction(Name, FTy, AL);                                                                                                                
+   Function *F = cast<Function>(Callee.getCallee());                                                                                                                             
+   F->setUnnamedAddr(GlobalValue::UnnamedAddr::Global);                                                                                                                          
+   F->setLinkage(GlobalValue::InternalLinkage);                                                                                                                                  
+   F->addFnAttr(Attribute::NoInline);                                                                                                                                            
+   F->setDoesNotThrow();                                                                                                                                                         
+   if (Opts.NoRedZone)                                                                                                                                                           
+     F->addFnAttr(Attribute::NoRedZone);                                                                                                                                         
+   return Callee;                                                                                                                                                                
+ }