Reapply "[Clang] Enable -fextend-variable-liveness at -Og (#118026)"

Relands this feature after several fixes:

* Force fake uses to be emitted before musttail calls (#136867)
* Added soften-float legalization for fake uses (#142714)
* Treat fake uses as size-less instructions in a SystemZ assert (#144390)

If further issues with fake uses are found then this may be reverted again,
but all currently-known issues are resolved.

This reverts commit 2dc6e98.

Author: SLTozer

clang/docs/CommandGuide/clang.rst

@@ -460,8 +460,11 @@ Code Generation Options
     :option:`-Oz` Like :option:`-Os` (and thus :option:`-O2`), but reduces code
     size further.
 
-    :option:`-Og` Like :option:`-O1`. In future versions, this option might
-    disable different optimizations in order to improve debuggability.
+    :option:`-Og` Similar to :option:`-O1`, but with slightly reduced
+    optimization and better variable visibility. The same optimizations are run
+    as at :option:`-O1`, but the ``-fextend-variable-liveness`` flag is
+    also set, which tries to prevent optimizations from reducing the liveness of
+    user variables, improving their availability when debugging.
 
     :option:`-O` Equivalent to :option:`-O1`.
 

clang/docs/ReleaseNotes.rst

@@ -366,6 +366,10 @@ Modified Compiler Flags
 
 - The ``-fveclib=libmvec`` option now supports AArch64 targets (requires GLIBC 2.40 or newer).
 
+- The ``-Og`` optimization flag now sets ``-fextend-variable-liveness``,
+  reducing performance slightly while reducing the number of optimized-out
+  variables. (#GH118026)
+
 Removed Compiler Flags
 -------------------------
 

clang/lib/Driver/ToolChains/Clang.cpp

@@ -7555,7 +7555,13 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
   if (Args.hasArg(options::OPT_fretain_comments_from_system_headers))
     CmdArgs.push_back("-fretain-comments-from-system-headers");
 
-  Args.AddLastArg(CmdArgs, options::OPT_fextend_variable_liveness_EQ);
+  if (Arg *A = Args.getLastArg(options::OPT_fextend_variable_liveness_EQ)) {
+    A->render(Args, CmdArgs);
+  } else if (Arg *A = Args.getLastArg(options::OPT_O_Group);
+             A && A->containsValue("g")) {
+    // Set -fextend-variable-liveness=all by default at -Og.
+    CmdArgs.push_back("-fextend-variable-liveness=all");
+  }
 
   // Forward -fcomment-block-commands to -cc1.
   Args.AddAllArgs(CmdArgs, options::OPT_fcomment_block_commands);

clang/test/Driver/extend-variable-liveness.c

@@ -1,7 +1,8 @@
 // Tests that -fextend-variable-liveness and its aliases are correctly passed
-// by the driver.
+// by the driver, and are set by default at -Og.
 
 // RUN: %clang -### -c %s 2>&1 | FileCheck %s --check-prefixes=CHECK,DEFAULT
+// RUN: %clang -### -Og -c %s 2>&1 | FileCheck %s --check-prefixes=CHECK,ALL
 // RUN: %clang -fextend-variable-liveness=none -### -c %s 2>&1 | FileCheck %s --check-prefixes=CHECK,NONE
 // RUN: %clang -fextend-variable-liveness=this -### -c %s 2>&1 | FileCheck %s --check-prefixes=CHECK,THIS
 // RUN: %clang -fextend-variable-liveness=all -### -c %s 2>&1 | FileCheck %s --check-prefixes=CHECK,ALL