[clang] fix generation of .debug_aranges with LTO (resubmit)

[azat]

Summary

Right now in case of LTO the section is not emited:

$ cat test.c
void __attribute__((optnone)) bar()
{
}
void __attribute__((optnone)) foo()
{
        bar();
}
int main()
{
        foo();
}

$ clang -flto=thin -gdwarf-aranges -g -O3 test.c
$ eu-readelf -waranges a.out  | fgrep -c -e foo -e bar
0

$ clang -gdwarf-aranges -g -O3 test.c
$ eu-readelf -waranges a.out  | fgrep -c -e foo -e bar
2

Fix this by passing explicitly --plugin-opt=-generate-arange-section.

But as mentioned by @dblaikie:

the better solution would be to encode this in IR metadata
(DICompileUnit) if it's reasonable to respect this option on a per-CU
basis (which it probably is, though that'd be a bit of backend work) -
or to set it as IR global metadata (like how the DWARF version is
encoded - probably using the same IR linker merging strategy, of
choosing the highest value (so if any module has aranges, then a linked
module has aranges too - even for the CUs that had it turned off) if
it's really not feasible to support on a per-CU basis) but probably
OK-enough, given that aranges is hopefully on the way out & not worth
all the work of the deeper fix

This is the resubmit, previous submission [1], got reverted, due to test
failures, that had been addressed in [2] and [3].

[1]: https://reviews.llvm.org/D133092
[3]: https://reviews.llvm.org/D133841
[4]: https://reviews.llvm.org/D133847

Signed-off-by: Azat Khuzhin <[e-mail]>
Suggested-by: OCHyams <[e-mail]>

[dblaikie]

I think @MaskRay was saying this doesn't actually work & different flags are needed to pass to lld - have you tested this end-to-end/with a real compilation, does that work? (are there other cases that already use -mllvm to pass flags down to lld?)

[azat]

I think @MaskRay was saying this doesn't actually work & different flags are needed to pass to lld

I thought that @MaskRay was talking about more generic approach that will not require if for the linker name, and indeed --plugin-opt=-generate-arange-section is better, and both gold and bfd support it!

have you tested this end-to-end/with a real compilation, does that work? (are there other cases that already use -mllvm to pass flags down to lld?)

Sure:

$ cat test.c
void __attribute__((optnone)) bar()
{
}
void __attribute__((optnone)) foo()
{
        bar();
}
int main()
{
        foo();
}
$ clang -c -flto=thin -fuse-ld=lld -gdwarf-aranges -g -O3 test.c
$ clang -flto=thin -fuse-ld=lld -gdwarf-aranges -g -O3 test.c -### # to get full command
$ /usr/bin/ld.lld ... -mllvm -generate-arange-section
$ eu-readelf -waranges a.out  | grep -F -c -e foo -e bar
2

[azat]

Use --plugin-opt=-generate-arange-section instead.

[MaskRay]

clang/test/Driver/debug-options-aranges.c

2

The message is outdated now. This just checks how -gdwarf-aranges ias passed to the compiler and linker.

4

--target=x86_64

7

delete empty //

[azat]

Add a real check of .debug_aranges in the test

[azat]

Test cleanup

[azat]

The message is outdated now.

Actually even before it does not check that .debug_aranges had been emitted.

This just checks how -gdwarf-aranges ias passed to the compiler and linker.

I've improved the test (though maybe Address Range Header match requires regex?)

clang/test/Driver/debug-options-aranges.c

4

error: 'x86_64': unable to pass LLVM bit-code files to linker

[azat]

Fix clang-format and tiny test refactoring

[probinson]

clang/test/Driver/debug-options-aranges.c

8

I don't see any FileCheck commands that use prefix GARANGE.

[dblaikie]

clang/test/Driver/debug-options-aranges.c

16

Generally Clang tests don't test the LLVM behavior - this test should probably just check how -gdwarf-aranges is passed to the linker. Other linker-level tests will test that, given that flag, the linker does the right thing.

[azat]

Update the test

[azat]

clang/test/Driver/debug-options-aranges.c

8

Indeed, that was a leftover. Removed.

16

Okay, removed.

[azat]

Closing in favor of https://reviews.llvm.org/D133092

clang/lib/Driver/ToolChains/CommonArgs.cpp

#endif
     SmallString<1024> Plugin;
     llvm::sys::path::native(
         Twine(D.Dir) + "/../" CLANG_INSTALL_LIBDIR_BASENAME "/LLVMgold" +
             Suffix,
         Plugin);
     CmdArgs.push_back(Args.MakeArgString(Plugin));
   }
 
+  // Note, this solution is far from perfect, better to encode it into IR
+  // metadata, but this may not be worth it, since it looks like aranges is on
+  // the way out.
+  if (Args.hasArg(options::OPT_gdwarf_aranges)) {
+    CmdArgs.push_back(
+        Args.MakeArgString("--plugin-opt=-generate-arange-section"));
+  }
+
   // Try to pass driver level flags relevant to LTO code generation down to
   // the plugin.
 
   // Handle flags for selecting CPU variants.
   std::string CPU = getCPUName(D, Args, ToolChain.getTriple());
   if (!CPU.empty())
     CmdArgs.push_back(Args.MakeArgString(Twine("-plugin-opt=mcpu=") + CPU));
 

clang/test/Driver/debug-options-aranges.c

+// REQUIRES: lld
+
+// Check that the linker plugin will get -generate-arange-section
+//
+// RUN: %clang -### -g -target x86_64-linux -flto      -fuse-ld=lld -gdwarf-aranges %s 2>&1 | FileCheck %s
+// RUN: %clang -### -g -target x86_64-linux -flto=thin -fuse-ld=lld -gdwarf-aranges %s 2>&1 | FileCheck %s
+// CHECK: --plugin-opt=-generate-arange-section