yonghong-song added a comment.
In D100567#2695827 <https://reviews.llvm.org/D100567#2695827>, @dblaikie wrote:
> Generally looks OK, but could you explain more about the problems
> with/motivation for the declaration/definition issue? (probably worth noting
> in the patch description maybe comment in code and test)
First about testing Fn->getSubprogram(), the reason is to avoid repeatedly
attempt to generate debuginfo, e.g.,
extern int foo(int);
long test() { ... &foo ... &foo ... &foo ... }
Without checking it will try to generate the debuginfo three times although
later it is deduplicated, but still a waste.
But Fn->getSubprogram() alone may cause issues if the debuginfo has been
generated for a definition,
extern int do_work2(int);
long bpf_helper2(void *callback_fn);
int do_work2(int arg) {
return arg;
}
long prog2() {
return bpf_helper2(&do_work2);
}
We will have a segfault
#3 0x0000000001bf2138 CrashRecoverySignalHandler(int)
CrashRecoveryContext.cpp:0:0
#4 0x00007f93e4fdfb20 __restore_rt sigaction.c:0:0
#5 0x0000000001506f70 llvm::Value::getMetadata(unsigned int) const
(/home/yhs/work/llvm-project/llvm/build.cur/install/bin/clang-13+0x1506f70)
#6 0x00000000015074db llvm::Function::getSubprogram() const
(/home/yhs/work/llvm-project/llvm/build.cur/install/bin/clang-13+0x15074db)
#7 0x00000000021ff5cf
clang::CodeGen::CodeGenFunction::EmitDeclRefLValue(clang::DeclRefExpr const*)
(/home/yhs/work/llvm-project/llvm/build.cur/install/bin
/clang-13+0x21ff5cf)
#8 0x00000000021fd866 clang::CodeGen::CodeGenFunction::EmitLValue(clang::Expr
const*) (/home/yhs/work/llvm-project/llvm/build.cur/install/bin/clang-13+0x21
fd866)
So we need to stop calling Fn->getSubprogram() if FD->isDefined() is true.
Hence I added FD->isDefined().
In summary, the new code
if (!FD->isDefined() && !Fn->getSubprogram()) {
CGM.getModuleDebugInfo()->EmitFunctionDecl(FD, FD->getLocation(), T,
Fn);
}
is an optimization to avoid repeatedly generating the debuginfo. If
FD->isDefined(), stop,
otherwise, if Fn->getSubprogram() which means having generated, stop.
Otherwise, generate one.
Note that my previous code without " if (!FD->isDefined() &&
!Fn->getSubprogram())" totally
working fine, I just feel a little bit waste.
I didn't debug why we have error for this test and you may have a better idea
from the above
stack trace.
extern int do_work2(int);
long bpf_helper2(void *callback_fn);
int do_work2(int arg) {
return arg;
}
long prog2() {
return bpf_helper2(&do_work2);
}
Apparently, do_work2 already generated a subprogram debuginfo when inside
prog2() another attempt tried to check whether the subprogram has been
generated or not.
I have added a test for this.
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D100567/new/
https://reviews.llvm.org/D100567
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