[DLCov 1/5] Add CMake option for enhanced debug line coverage tracking (#107278)

This is part of a series of patches that tries to improve DILocation bug
detection in Debugify. This first patch adds the necessary CMake flag to
LLVM and a variable defined by that flag to LLVM's config header, allowing
the next patch to track information without affecting normal builds.

This series of patches adds a "DebugLoc coverage tracking" feature, that
inserts conditionally-compiled tracking information into DebugLocs (and
by extension, to Instructions), which is used by Debugify to provide
more accurate and detailed coverage reports. When enabled, this features
tracks whether and why we have intentionally dropped a DebugLoc,
allowing Debugify to ignore false positives. An optional additional
feature allows also storing a stack trace of the point where a DebugLoc
was unintentionally dropped/not generated, which is used to make fixing
detected errors significantly easier. The goal of these features is to
provide useful tools for developers to fix existing DebugLoc errors and
allow reliable detection of regressions by either manual inspection or
an automated script.

Author: SLTozer

llvm/CMakeLists.txt

@@ -536,6 +536,10 @@ endif()
 
 option(LLVM_ENABLE_CRASH_DUMPS "Turn on memory dumps on crashes. Currently only implemented on Windows." OFF)
 
+set(LLVM_ENABLE_DEBUGLOC_COVERAGE_TRACKING "DISABLED" CACHE STRING
+  "Enhance Debugify's line number coverage tracking; enabling this is ABI-breaking. Can be DISABLED, or COVERAGE.")
+set_property(CACHE LLVM_ENABLE_DEBUGLOC_COVERAGE_TRACKING PROPERTY STRINGS DISABLED COVERAGE)
+
 set(WINDOWS_PREFER_FORWARD_SLASH_DEFAULT OFF)
 if (MINGW)
   # Cygwin doesn't identify itself as Windows, and thus gets path::Style::posix

llvm/cmake/modules/HandleLLVMOptions.cmake

@@ -196,6 +196,16 @@ else()
   message(FATAL_ERROR "Unknown value for LLVM_ABI_BREAKING_CHECKS: \"${LLVM_ABI_BREAKING_CHECKS}\"!")
 endif()
 
+string(TOUPPER "${LLVM_ENABLE_DEBUGLOC_COVERAGE_TRACKING}" uppercase_LLVM_ENABLE_DEBUGLOC_COVERAGE_TRACKING)
+
+if( uppercase_LLVM_ENABLE_DEBUGLOC_COVERAGE_TRACKING STREQUAL "COVERAGE" )
+  set( ENABLE_DEBUGLOC_COVERAGE_TRACKING 1 )
+elseif( uppercase_LLVM_ENABLE_DEBUGLOC_COVERAGE_TRACKING STREQUAL "DISABLED" OR NOT DEFINED LLVM_ENABLE_DEBUGLOC_COVERAGE_TRACKING )
+  # The DISABLED setting is default and requires no additional defines.
+else()
+  message(FATAL_ERROR "Unknown value for LLVM_ENABLE_DEBUGLOC_COVERAGE_TRACKING: \"${LLVM_ENABLE_DEBUGLOC_COVERAGE_TRACKING}\"!")
+endif()
+
 if( LLVM_REVERSE_ITERATION )
   set( LLVM_ENABLE_REVERSE_ITERATION 1 )
 endif()

llvm/docs/CMake.rst

@@ -482,6 +482,15 @@ enabled sub-projects. Nearly all of these variable names begin with
 **LLVM_ENABLE_BINDINGS**:BOOL
   If disabled, do not try to build the OCaml bindings.
 
+**LLVM_ENABLE_DEBUGLOC_COVERAGE_TRACKING**:STRING
+  Enhances Debugify's ability to detect line number errors by storing extra
+  information inside Instructions, removing false positives from Debugify's
+  results at the cost of performance. Allowed values are `DISABLED` (default)
+  and `COVERAGE`. `COVERAGE` tracks whether and why a line number was
+  intentionally dropped or not generated for an instruction, allowing Debugify
+  to avoid reporting these as errors; this comes with a small performance cost
+  of ~0.1%. `COVERAGE` is an ABI-breaking option.
+
 **LLVM_ENABLE_DIA_SDK**:BOOL
   Enable building with MSVC DIA SDK for PDB debugging support. Available
   only with MSVC. Defaults to ON.

llvm/include/llvm/Config/config.h.cmake

@@ -19,6 +19,10 @@
 /* Define to 1 to enable crash memory dumps, and to 0 otherwise. */
 #cmakedefine01 LLVM_ENABLE_CRASH_DUMPS
 
+/* Define to 1 to enable expensive checks for debug location coverage checking,
+   and to 0 otherwise. */
+#cmakedefine01 ENABLE_DEBUGLOC_COVERAGE_TRACKING
+
 /* Define to 1 to prefer forward slashes on Windows, and to 0 prefer
    backslashes. */
 #cmakedefine01 LLVM_WINDOWS_PREFER_FORWARD_SLASH