[clang-cl] Support the /JMC flag

The introduction and some examples are on this page:
https://devblogs.microsoft.com/cppblog/announcing-jmc-stepping-in-visual-studio/

The `/JMC` flag enables these instrumentations:
- Insert at the beginning of every function immediately after the prologue with
  a call to `void __fastcall __CheckForDebuggerJustMyCode(unsigned char *JMC_flag)`.
  The argument for `__CheckForDebuggerJustMyCode` is the address of a boolean
  global variable (the global variable is initialized to 1) with the name
  convention `__<hash>_<filename>`. All such global variables are placed in
  the `.msvcjmc` section.
- The `<hash>` part of `__<hash>_<filename>` has a one-to-one mapping
  with a directory path. MSVC uses some unknown hashing function. Here I
  used DJB.
- Add a dummy/empty COMDAT function `__JustMyCode_Default`.
- Add `/alternatename:__CheckForDebuggerJustMyCode=__JustMyCode_Default` link
  option via ".drectve" section. This is to prevent failure in
  case `__CheckForDebuggerJustMyCode` is not provided during linking.

Implementation:
All the instrumentations are implemented in an IR codegen pass. The pass is placed immediately before CodeGenPrepare pass. This is to not interfere with mid-end optimizations and make the instrumentation target-independent (I'm still working on an ELF port in a separate patch).

Reviewed By: hans

Differential Revision: https://reviews.llvm.org/D118428

1 files changed, 8 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/CommandFlags.cpp b/llvm/lib/CodeGen/CommandFlags.cpp
index cb77407..87758c4 100644
--- a/llvm/lib/CodeGen/CommandFlags.cpp
+++ b/llvm/lib/CodeGen/CommandFlags.cpp
@@ -95,6 +95,7 @@ CGOPT(bool, ForceDwarfFrameSection)
 CGOPT(bool, XRayOmitFunctionIndex)
 CGOPT(bool, DebugStrictDwarf)
 CGOPT(unsigned, AlignLoops)
+CGOPT(bool, JMCInstrument)
 
 codegen::RegisterCodeGenFlags::RegisterCodeGenFlags() {
 #define CGBINDOPT(NAME)                                                        \
@@ -464,6 +465,12 @@ codegen::RegisterCodeGenFlags::RegisterCodeGenFlags() {
                                       cl::desc("Default alignment for loops"));
   CGBINDOPT(AlignLoops);
 
+  static cl::opt<bool> JMCInstrument(
+      "enable-jmc-instrument",
+      cl::desc("Instrument functions with a call to __CheckForDebuggerJustMyCode"),
+      cl::init(false));
+  CGBINDOPT(JMCInstrument);
+
 #undef CGBINDOPT
 
   mc::RegisterMCTargetOptionsFlags();
@@ -539,6 +546,7 @@ codegen::InitTargetOptionsFromCodeGenFlags(const Triple &TheTriple) {
   Options.XRayOmitFunctionIndex = getXRayOmitFunctionIndex();
   Options.DebugStrictDwarf = getDebugStrictDwarf();
   Options.LoopAlignment = getAlignLoops();
+  Options.JMCInstrument = getJMCInstrument();
 
   Options.MCOptions = mc::InitMCTargetOptionsFromFlags();