[gn build] Add symbol_level to adjust debug info level

[aeubanks]

Summary

is_debug by default makes symbol_level = 2 and !is_debug means by
default symbol_level = 0.

[aeubanks]

don't pass both /Zi and /Zd

[aeubanks]

typo

[thakis]

llvm/utils/gn/build/BUILD.gn

75

I think this kind of suggests that what we have here is kind of a linear scale with a few steps, and controlling that via a collection of bools is a bit unwieldy.

Maybe we should get the route of having a symbol_level thing that can be 0, 1, or 2 (with 0 being the default for !is_debug builds, and 2 being the default for is_debug builds)? That'd match other GN-based builds, and it maps very clearly to a -g flag (at least on non-win clang).

And then my CL to turn on gdb indices would turn on indices for symbol level 2 (which would be the default for debug builds), and you could set symbol_level=1 and everyone would be happy, maybe.

[aeubanks]

So is_debug = true would just be an alias for symbol_level = 2 and is_debug = false would be symbol_level = 0? Or should we just remove is_debug and have people only use symbol_level? It seems weird that you could specify is_debug = true and symbol_level = 0 (and likewise is_debug = false and symbol_level = 2).

[thakis]

In D92958#2458734, @aeubanks wrote:

So is_debug = true would just be an alias for symbol_level = 2 and is_debug = false would be symbol_level = 0? Or should we just remove is_debug and have people only use symbol_level? It seems weird that you could specify is_debug = true and symbol_level = 0 (and likewise is_debug = false and symbol_level = 2).

is_debug = true would be an alias for symbol_level = 2; is_optimized = false, right.

[aeubanks]

add a new symbol_level and use that instead

[thakis]

lg with the Zd thing addressed. Thanks!

llvm/utils/gn/build/BUILD.gn

101

I think you don't need this. As far as I know, Zd is a cl.exe-level option that causes it to pass /DEBUG to link.exe and does nothing else. But we don't invoke cl.exe for links, we invoke link.exe directly. link.exe /debug will produce enough symbol information to create symbolized backtraces, but not enough to have line numbers. So you can either do nothing for symbol_level=1 with cflags and just pass /debug to ldflags like we already do, or you could do if (symbol_level == 1 && is_clang) cflags += -gline-tables-only (or -g1, same thing) so that we get the same behavior on linux and windows at least with clang as host compiler.

[aeubanks]

llvm/utils/gn/build/BUILD.gn

101

using clang, /Zd (without /Zi) does give line info.

In clang/test/Driver/cl-options.c I see

// RUN: %clang_cl /Zi /c -### -- %s 2>&1 | FileCheck -check-prefix=Zi %s
// Zi: "-gcodeview"
// Zi: "-debug-info-kind=limited"

// RUN: %clang_cl /Zd /c -### -- %s 2>&1 | FileCheck -check-prefix=Z7GMLT %s
// Z7GMLT: "-gcodeview"
// Z7GMLT: "-debug-info-kind=line-tables-only"

[thakis]

llvm/utils/gn/build/BUILD.gn

101

Huh, weird. https://reviews.llvm.org/rL274991 / https://reviews.llvm.org/rL275826 suggest a) some interesting backstory that I either never knew or since forgot b) that this used to be a cl.exe flag but is no longer. Does current cl.exe still accept /Zd?

[aeubanks]

use /Zi and -gline-tables-only

llvm/utils/gn/build/BUILD.gn

101

Thanks for the backgroun, looks like cl.exe doesn't recognize /Zd.

Now we always pass /Zi and add -gline-tables-only for is_clang && symbol_level == 1.

[thakis]

lgtm++

llvm/utils/gn/build/BUILD.gn

if (current_os == "ios") {
     ldflags += [ "-miphoneos-version-min=8.0" ]
   }
   if (current_os == "mac") {
     asmflags += [ "-mmacosx-version-min=10.10" ]
     cflags += [ "-mmacosx-version-min=10.10" ]
     ldflags += [ "-mmacosx-version-min=10.10" ]
   }
 
+  assert(!(is_debug && is_debug_lines), "Cannot specify both is_debug and is_debug_lines")
   if (host_os != "win") {
     if (is_debug) {
       cflags += [ "-g" ]
+    } else if (is_debug_lines) {
+      cflags += [ "-g1" ]
     }
     if (is_optimized) {
       cflags += [ "-O3" ]
     }
     cflags += [ "-fdiagnostics-color" ]
     if (use_lld) {
       ldflags += [ "-Wl,--color-diagnostics" ]
     }
     cflags_cc = [
       "-std=c++14",
       "-fvisibility-inlines-hidden",
     ]
   } else {
-    if (is_debug) {
+    if (is_debug || is_debug_lines) {
       cflags += [
-        "/Zi",
         "/FS",
       ]
+      if (is_debug) {
+        cflags += [ "/Zi" ]
+      } else if (is_debug_lines) {
+        cflags += [ "/Zd" ]
+      }
       ldflags += [ "/DEBUG" ]
 
       # Speed up links with ghash on windows.
       if (use_lld && is_clang) {
         cflags += [ "-gcodeview-ghash" ]
         ldflags += [ "/DEBUG:GHASH" ]
       }
     }

llvm/utils/gn/build/buildflags.gni

 declare_args() {
   # Whether to build with debug information.
   is_debug = false
 
+  # Whether to build with debug information only containing line info. Useful for stack traces.
+  is_debug_lines = false
+
   # Whether to build with tsan.
   use_tsan = false
 
   # Whether to build with ubsan.
   use_ubsan = false
 
   # Whether to build with asan.
   use_asan = false