[Driver] Add option to manually control discarding value names in LLVM IR.

Summary:
Currently, assertion-disabled Clang builds emit value names when generating LLVM IR. This is controlled by the `NDEBUG` macro, and is not easily overridable. In order to get IR output containing names from a release build of Clang, the user must manually construct the CC1 invocation w/o the `-discard-value-names` option. This is less than ideal.

For example, Godbolt uses a release build of Clang, and so when asked to emit LLVM IR the result lacks names, making it harder to read. Manually invoking CC1 on Compiler Explorer is not feasible.

This patch adds the driver options `-fdiscard-value-names` and `-fno-discard-value-names` which allow the user to override the default behavior. If neither is specified, the old behavior remains.

Reviewers: erichkeane, aaron.ballman, lebedev.ri

Reviewed By: aaron.ballman

Subscribers: bogner, cfe-commits

Differential Revision: https://reviews.llvm.org/D42887

llvm-svn: 324498

clang/docs/UsersManual.rst

@@ -1855,6 +1855,27 @@ features. You can "tune" the debug info for one of several different debuggers.
   must come first.)
 
 
+Controlling LLVM IR Output
+--------------------------
+
+Controlling Value Names in LLVM IR
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Emitting value names in LLVM IR increases the size and verbosity of the IR.
+By default, value names are only emitted in assertion-enabled builds of Clang.
+However, when reading IR it can be useful to re-enable the emission of value
+names to improve readability.
+
+.. option:: -fdiscard-value-names
+
+  Discard value names when generating LLVM IR.
+
+.. option:: -fno-discard-value-names
+
+  Do not discard value names when generating LLVM IR. This option can be used
+  to re-enable names for release builds of Clang.
+
+
 Comment Parsing Options
 -----------------------
 

clang/include/clang/Driver/Options.td

@@ -790,6 +790,10 @@ def fdiagnostics_show_template_tree : Flag<["-"], "fdiagnostics-show-template-tr
     HelpText<"Print a template comparison tree for differing templates">;
 def fdeclspec : Flag<["-"], "fdeclspec">, Group<f_clang_Group>,
   HelpText<"Allow __declspec as a keyword">, Flags<[CC1Option]>;
+def fdiscard_value_names : Flag<["-"], "fdiscard-value-names">, Group<f_clang_Group>,
+  HelpText<"Discard value names in LLVM IR">, Flags<[DriverOption]>;
+def fno_discard_value_names : Flag<["-"], "fno-discard-value-names">, Group<f_clang_Group>,
+  HelpText<"Do not discard value names in LLVM IR">, Flags<[DriverOption]>;
 def fdollars_in_identifiers : Flag<["-"], "fdollars-in-identifiers">, Group<f_Group>,
   HelpText<"Allow '$' in identifiers">, Flags<[CC1Option]>;
 def fdwarf2_cfi_asm : Flag<["-"], "fdwarf2-cfi-asm">, Group<clang_ignored_f_Group>;

clang/lib/Driver/ToolChains/Clang.cpp

@@ -3266,13 +3266,24 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
   if (!C.isForDiagnostics())
     CmdArgs.push_back("-disable-free");
 
-// Disable the verification pass in -asserts builds.
 #ifdef NDEBUG
-  CmdArgs.push_back("-disable-llvm-verifier");
-  // Discard LLVM value names in -asserts builds.
-  CmdArgs.push_back("-discard-value-names");
+  const bool IsAssertBuild = false;
+#else
+  const bool IsAssertBuild = true;
 #endif
 
+  // Disable the verification pass in -asserts builds.
+  if (!IsAssertBuild)
+    CmdArgs.push_back("disable-llvm-verifier");
+
+  // Discard value names in assert builds unless otherwise specified.
+  if (const Arg *A = Args.getLastArg(options::OPT_fdiscard_value_names,
+                                     options::OPT_fno_discard_value_names)) {
+    if (A->getOption().matches(options::OPT_fdiscard_value_names))
+      CmdArgs.push_back("-discard-value-names");
+  } else if (!IsAssertBuild)
+    CmdArgs.push_back("-discard-value-names");
+
   // Set the main file name, so that debug info works even with
   // -save-temps.
   CmdArgs.push_back("-main-file-name");

clang/test/Driver/clang_f_opts.c

@@ -517,3 +517,8 @@
 // RUN: %clang -### -S %s 2>&1 | FileCheck -check-prefix=CHECK-NO-CF-PROTECTION-BRANCH %s
 // CHECK-CF-PROTECTION-BRANCH: -fcf-protection=branch
 // CHECK-NO-CF-PROTECTION-BRANCH-NOT: -fcf-protection=branch
+
+// RUN: %clang -### -S -fdiscard-value-names %s 2>&1 | FileCheck -check-prefix=CHECK-DISCARD-NAMES %s
+// RUN: %clang -### -S -fno-discard-value-names %s 2>&1 | FileCheck -check-prefix=CHECK-NO-DISCARD-NAMES %s
+// CHECK-DISCARD-NAMES: "-discard-value-names"
+// CHECK-NO-DISCARD-NAMES-NOT: "-discard-value-names"