[Driver] Default Generic_GCC aarch64 to use -fasynchronous-unwind-tables

[MaskRay]

Summary

In GCC, aarch64-*-linux and aarch64-*-freebsd made the switch in 2018
(https://gcc.gnu.org/pipermail/gcc-patches/2018-March/495549.html).
In Clang, FreeBSD/Fuchsia/NetBSD/MinGW aarch64 default to -fasynchronous-unwind-tables.

This patch defaults Generic_GCC aarch64 (which affects Linux) to use -fasynchronous-unwind-tables.

[MaskRay]

Drop an unneeded change

[psmith]

Personally I'm in favour of clang and gcc behaving the same way unless there is a good reason not to. I've shared the review internally to see if anyone has any concerns. May be worth informing the clang built linux community to see if they will need to make any alterations as a result.

[nickdesaulniers]

Thanks for the heads up, I don't see this being a problem for us, especially since GCC already behaves this way;

arch/arm64/Makefile:
55 # Avoid generating .eh_frame* sections.
56 KBUILD_CFLAGS += -fno-asynchronous-unwind-tables -fno-unwind-tables
57 KBUILD_AFLAGS += -fno-asynchronous-unwind-tables -fno-unwind-tables

[nickdesaulniers]

Also, built and boot tested an aarch64 defconfig just to be safe.

[joerg]

I have no problem with the change, but please adjust the description to take about -funwind-tables. I don't think we do async by default, at least not by design.

[MaskRay]

In D91760#2406650, @joerg wrote:

I have no problem with the change, but please adjust the description to take about -funwind-tables. I don't think we do async by default, at least not by design.

AFAICT we don't make distinction of -funwind-tables and -fasynchronous-unwind-tables. I think the thing llvm implements is -fasynchronous-unwind-tables. What is the difference of -funwind-tables?

If you care, you may want to change GCC aarch64-*-netbsd to default to -fasynchronous-unwind-tables. I think it does not do it currently..

[joerg]

The difference is whether we promise to be instruction precise or not. I'm not sure we do or want to promise that as default.

[MaskRay]

In D91760#2406747, @joerg wrote:

The difference is whether we promise to be instruction precise or not. I'm not sure we do or want to promise that as default.

gcc/common/config/{aarch64,rs6000}/*-common.c enables -fasynchronous-unwind-tables so I think the title is right about the eventual state.
I think the current state for x86-64 is that the produced .cfi instructions satisfy the asynchronous requirement.
Other targets (e.g. aarch64) mostly satisfy the requirement, but their epilogues may miss some instructions due to missing hooks required by CFIInstrInserter. This is just a QoI issue (and if I'll probably work on this on my spare time).
(GCC has its problems, too. It does not analyze inline asm and cannot produce 100% asynchronous unwind tables)

I think it is right that in this patch we say we that we default to -fasynchronous-unwind-tables.

[MaskRay]

@psmith Do they have concerns? :)

[psmith]

Radio silence so far; I think no news is good news applies in this case. I'm happy to say no objections.

[MaskRay]

In D91760#2413294, @psmith wrote:

Radio silence so far; I think no news is good news applies in this case. I'm happy to say no objections.

Thanks. I'll commit then.

clang/lib/Driver/ToolChains/Gnu.cpp

 void Generic_GCC::printVerboseInfo(raw_ostream &OS) const {
   // Print the information about how we detected the GCC installation.
   GCCInstallation.print(OS);
   CudaInstallation.print(OS);
   RocmInstallation.print(OS);
 }
 
 bool Generic_GCC::IsUnwindTablesDefault(const ArgList &Args) const {
-  return getArch() == llvm::Triple::x86_64;
+  switch (getArch()) {
+  case llvm::Triple::aarch64:
+  case llvm::Triple::x86_64:
+    return true;
+  default:
+    return false;
+  }
 }
 
 bool Generic_GCC::isPICDefault() const {
   switch (getArch()) {
   case llvm::Triple::x86_64:
     return getTriple().isOSWindows();
   case llvm::Triple::mips64:
   case llvm::Triple::mips64el:

clang/test/Driver/aarch64-features.c

 // RUN: %clang -target aarch64-none-linux-gnu -### %s -fsyntax-only 2>&1 | FileCheck %s
 // RUN: %clang -target arm64-none-linux-gnu -### %s -fsyntax-only 2>&1 | FileCheck %s
 
+// CHECK: "-munwind-tables"
+
 // The AArch64 PCS states that chars should be unsigned.
 // CHECK: fno-signed-char