[KeyInstr] Enable -gkey-instructions by default if optimisations are enabled

[OCHyams]

That's enabling Clang's -gkey-instructions, cc1's -gkey-instructions remains off by default.

Key Instructions improves the optimized-code debug-stepping experience in debuggers that use DWARF's is_stmt line table register to determine stepping behaviour.

The feature can be disabled with -gno-key-instructions.

RFC: https://discourse.llvm.org/t/rfc-improving-is-stmt-placement-for-better-interactive-debugging/82668

---

Questions:

LLDB - I believe LLDB ignores Clang's is_stmts for stepping, but I don't know whether moving is_stmts around in ways LLDB hasn't seen before (at least as output from Clang) is going to have any unexpected consequences. cc @adrian-prantl @JDevlieghere for opinions - what testing and checking would be needed / possible here, to be confident that this doesn't break optimized code debugging for LLDB?

Clang languages/extensions - I've implemented support for C/C++, but I've not tested or specifically implemented anything for other languages (?) or extensions (?) like CUDA. Is there a good way to filter for this in the driver code in this patch?

[llvmbot]

@llvm/pr-subscribers-clang-driver

@llvm/pr-subscribers-clang

Author: Orlando Cazalet-Hyams (OCHyams)

Changes

That's enabling Clang's -gkey-instructions, cc1's -gkey-instructions remains off by default.

Key Instructions improves the optimized-code debug-stepping experience in debuggers that use DWARF's is_stmt line table register to determine stepping behaviour.

The feature can be disabled with -gno-key-instructions.

RFC: https://discourse.llvm.org/t/rfc-improving-is-stmt-placement-for-better-interactive-debugging/82668

---

Questions:

LLDB - I believe LLDB ignores Clang's is_stmts for stepping, but I don't know whether moving is_stmts around in ways LLDB hasn't seen before (at least as output from Clang) is going to have any unexpected consequences. cc @adrian-prantl @JDevlieghere for opinions - what testing and checking would be needed / possible here, to be confident that this doesn't break optimized code debugging for LLDB?

Clang languages/extensions - I've implemented support for C/C++, but I've not tested or specifically implemented anything for other languages (?) or extensions (?) like CUDA. Is there a good way to filter for this in the driver code in this patch?

---

Full diff: https://github.com/llvm/llvm-project/pull/149509.diff

2 Files Affected:

• (modified) clang/lib/Driver/ToolChains/Clang.cpp (+7-1)
• (modified) clang/test/DebugInfo/KeyInstructions/flag.cpp (+31-2)

diff --git a/clang/lib/Driver/ToolChains/Clang.cpp b/clang/lib/Driver/ToolChains/Clang.cpp
index 8880c9375143f..38eebbd6f9b5f 100644
--- a/clang/lib/Driver/ToolChains/Clang.cpp
+++ b/clang/lib/Driver/ToolChains/Clang.cpp
@@ -4611,8 +4611,14 @@ renderDebugOptions(const ToolChain &TC, const Driver &D, const llvm::Triple &T,
       CmdArgs.push_back("-gembed-source");
   }
 
+  // Enable Key Instructions by default if optimisations are enabled and
+  // we're emitting DWARF.
+  Arg *OptLevel = Args.getLastArg(options::OPT_O_Group);
+  bool KeyInstructionsOnByDefault =
+      EmitDwarf && OptLevel && !OptLevel->getOption().matches(options::OPT_O0);
   if (Args.hasFlag(options::OPT_gkey_instructions,
-                   options::OPT_gno_key_instructions, false))
+                   options::OPT_gno_key_instructions,
+                   KeyInstructionsOnByDefault))
     CmdArgs.push_back("-gkey-instructions");
 
   if (EmitCodeView) {
diff --git a/clang/test/DebugInfo/KeyInstructions/flag.cpp b/clang/test/DebugInfo/KeyInstructions/flag.cpp
index e34faa6cbb347..05a34ac670feb 100644
--- a/clang/test/DebugInfo/KeyInstructions/flag.cpp
+++ b/clang/test/DebugInfo/KeyInstructions/flag.cpp
@@ -1,7 +1,5 @@
 // RUN: %clang -### -target x86_64 -c -gdwarf -gkey-instructions %s 2>&1 | FileCheck %s --check-prefixes=KEY-INSTRUCTIONS
 // RUN: %clang -### -target x86_64 -c -gdwarf -gno-key-instructions %s 2>&1 | FileCheck %s --check-prefixes=NO-KEY-INSTRUCTIONS
-//// Default: Off.
-// RUN: %clang -### -target x86_64 -c -gdwarf %s 2>&1 | FileCheck %s --check-prefixes=NO-KEY-INSTRUCTIONS
 
 //// Help.
 // RUN %clang --help | FileCheck %s --check-prefix=HELP
@@ -23,3 +21,34 @@ void f() {}
 // RUN: %clang_cc1 %s -triple x86_64-linux-gnu -gkey-instructions -debug-info-kind=line-tables-only -emit-llvm -o - | FileCheck %s --check-prefix=SMOKETEST-ON
 // SMOKETEST-ON: keyInstructions: true
 // SMOKETEST-ON: atomGroup: 1
+
+//// Enable Key Instructions by default if optimisations are enabled and we're
+//// emitting DWARF.
+////
+//// | opt level | -gkey-instructions | feature |
+//// |         0 |                 no |     off |
+//// |         0 |                yes |      on |
+//// |       >=1 |                 no |      on |
+//// |       >=1 |                yes |      on |
+//// |       >=1 |   no & no -g flags |     off |
+//// |       >=1 | no & emit codeview |     off |
+//
+// RUN: %clang %s     -target x86_64 -gdwarf    -gmlt                    -### 2>&1 | FileCheck %s --check-prefix=NO-KEY-INSTRUCTIONS
+// RUN: %clang %s     -target x86_64 -gdwarf    -gmlt -gkey-instructions -### 2>&1 | FileCheck %s --check-prefix=KEY-INSTRUCTIONS
+// RUN: %clang %s -O0 -target x86_64 -gdwarf    -gmlt                    -### 2>&1 | FileCheck %s --check-prefix=NO-KEY-INSTRUCTIONS
+// RUN: %clang %s -O0 -target x86_64 -gdwarf    -gmlt -gkey-instructions -### 2>&1 | FileCheck %s --check-prefix=KEY-INSTRUCTIONS
+// RUN: %clang %s -O1 -target x86_64 -gdwarf    -gmlt                    -### 2>&1 | FileCheck %s --check-prefix=KEY-INSTRUCTIONS
+// RUN: %clang %s -O1 -target x86_64 -gdwarf    -gmlt -gkey-instructions -### 2>&1 | FileCheck %s --check-prefix=KEY-INSTRUCTIONS
+// RUN: %clang %s -O1 -target x86_64                                     -### 2>&1 | FileCheck %s --check-prefixes=NO-KEY-INSTRUCTIONS
+// RUN: %clang %s -O1 -target x86_64 -gcodeview -gmlt                    -### 2>&1 | FileCheck %s --check-prefixes=NO-KEY-INSTRUCTIONS
+//
+// RUN: %clang %s     -target x86_64 -gdwarf    -gmlt                    -S -emit-llvm -o - | FileCheck %s --check-prefix=SMOKETEST-OFF
+// RUN: %clang %s     -target x86_64 -gdwarf    -gmlt -gkey-instructions -S -emit-llvm -o - | FileCheck %s --check-prefix=SMOKETEST-ON
+// RUN: %clang %s -O0 -target x86_64 -gdwarf    -gmlt                    -S -emit-llvm -o - | FileCheck %s --check-prefix=SMOKETEST-OFF
+// RUN: %clang %s -O0 -target x86_64 -gdwarf    -gmlt -gkey-instructions -S -emit-llvm -o - | FileCheck %s --check-prefix=SMOKETEST-ON
+// RUN: %clang %s -O1 -target x86_64 -gdwarf    -gmlt                    -S -emit-llvm -o - | FileCheck %s --check-prefix=SMOKETEST-ON
+// RUN: %clang %s -O1 -target x86_64 -gdwarf    -gmlt -gkey-instructions -S -emit-llvm -o - | FileCheck %s --check-prefix=SMOKETEST-ON
+// RUN: %clang %s -O1 -target x86_64                                     -S -emit-llvm -o - | FileCheck %s --check-prefixes=SMOKETEST-OFF,SMOKETEST-NO-DEBUG
+// RUN: %clang %s -O1 -target x86_64 -gcodeview -gmlt                    -S -emit-llvm -o - | FileCheck %s --check-prefixes=SMOKETEST-OFF
+// SMOKETEST-NO-DEBUG: llvm.module.flags
+// SMOKETEST-NO-DEBUG-NOT: DICompileUnit