[LLVM] Emit dwarf data for changed-signature and new functions

Add a new pass EmitChangedFuncDebugInfo which will add dwarf for
additional functions including functions with signature change
and new functions.

The previous approach in [1] tries to add debuginfo for those
optimization passes which cause signature changes. Based on
discussion in [1], it is preferred to have a specific pass to
add debuginfo and later on dwarf generation can include those
new debuginfo.

The following is an example:
Source:
  $ cat test.c
  struct t { int a; };
  char *tar(struct t *a, struct t *d);
  __attribute__((noinline)) static char * foo(struct t *a, struct t *d, int b)
  {
    return tar(a, d);
  }
  char *bar(struct t *a, struct t *d)
  {
    return foo(a, d, 1);
  }
Compiled and dump dwarf with:
  clang -O2 -c -g test.c
  llvm-dwarfdump test.o

0x0000005c:   DW_TAG_subprogram
                DW_AT_low_pc    (0x0000000000000010)
                DW_AT_high_pc   (0x0000000000000015)
                DW_AT_frame_base        (DW_OP_reg7 RSP)
                DW_AT_linkage_name      ("foo")
                DW_AT_name      ("foo")
                DW_AT_decl_file ("/home/yhs/tests/sig-change/deadarg/test.c")
                DW_AT_decl_line (3)
                DW_AT_type      (0x000000bb "char *")
                DW_AT_artificial        (true)
                DW_AT_external  (true)

0x0000006c:     DW_TAG_formal_parameter
                  DW_AT_location        (DW_OP_reg5 RDI)
                  DW_AT_decl_file       ("/home/yhs/tests/sig-change/deadarg/test.c")
                  DW_AT_decl_line       (3)
                  DW_AT_type    (0x000000c4 "t *")

0x00000075:     DW_TAG_formal_parameter
                  DW_AT_location        (DW_OP_reg4 RSI)
                  DW_AT_decl_file       ("/home/yhs/tests/sig-change/deadarg/test.c")
                  DW_AT_decl_line       (3)
                  DW_AT_type    (0x000000c4 "t *")

0x0000007e:     DW_TAG_inlined_subroutine
                  DW_AT_abstract_origin (0x0000009a "foo")
                  DW_AT_low_pc  (0x0000000000000010)
                  DW_AT_high_pc (0x0000000000000015)
                  DW_AT_call_file       ("/home/yhs/tests/sig-change/deadarg/test.c")
                  DW_AT_call_line       (0)

0x0000008a:       DW_TAG_formal_parameter
                    DW_AT_location      (DW_OP_reg5 RDI)
                    DW_AT_abstract_origin       (0x000000a2 "a")

0x00000091:       DW_TAG_formal_parameter
                    DW_AT_location      (DW_OP_reg4 RSI)
                    DW_AT_abstract_origin       (0x000000aa "d")

0x00000098:       NULL

0x00000099:     NULL

0x0000009a:   DW_TAG_subprogram
                DW_AT_name      ("foo")
                DW_AT_decl_file ("/home/yhs/tests/sig-change/deadarg/test.c")
                DW_AT_decl_line (3)
                DW_AT_prototyped        (true)
                DW_AT_type      (0x000000bb "char *")
                DW_AT_inline    (DW_INL_inlined)

0x000000a2:     DW_TAG_formal_parameter
                  DW_AT_name    ("a")
                  DW_AT_decl_file       ("/home/yhs/tests/sig-change/deadarg/test.c")
                  DW_AT_decl_line       (3)
                  DW_AT_type    (0x000000c4 "t *")

0x000000aa:     DW_TAG_formal_parameter
                  DW_AT_name    ("d")
                  DW_AT_decl_file       ("/home/yhs/tests/sig-change/deadarg/test.c")
                  DW_AT_decl_line       (3)
                  DW_AT_type    (0x000000c4 "t *")

0x000000b2:     DW_TAG_formal_parameter
                  DW_AT_name    ("b")
                  DW_AT_decl_file       ("/home/yhs/tests/sig-change/deadarg/test.c")
                  DW_AT_decl_line       (3)
                  DW_AT_type    (0x000000d8 "int")

0x000000ba:     NULL

There are some restrictions in the current implementation:
  - Only C language is supported
  - BPF target is excluded as one of main goals for this pull request
    is to generate proper vmlinux BTF for arch's like x86_64/arm64 etc.
  - Function must not be a intrinsic, decl only, return value size more
    than arch register size and func with variable arguments.

I have tested this patch set by building latest bpf-next linux kernel.
For no-lto case:
  65341 original number of functions
  1085  new functions with this patch
For thin-lto case:
  65595 original number of functions
  2492  new functions with this patch

  [1] #127855

Author: Yonghong Song

llvm/include/llvm/Transforms/Utils/EmitChangedFuncDebugInfo.h

@@ -0,0 +1,33 @@
+//===- EmitChangedFuncDebugInfo.h - Emit Additional Debug Info -*- C++ --*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+/// \file
+/// Emit debug info for changed or new funcs.
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_TRANSFORMS_UTILS_EMITCHANGEDFUNCDEBUGINFO_H
+#define LLVM_TRANSFORMS_UTILS_EMITCHANGEDFUNCDEBUGINFO_H
+
+#include "llvm/IR/PassManager.h"
+
+namespace llvm {
+
+class Module;
+
+// Pass that emits late dwarf.
+class EmitChangedFuncDebugInfoPass
+    : public PassInfoMixin<EmitChangedFuncDebugInfoPass> {
+public:
+  EmitChangedFuncDebugInfoPass() = default;
+
+  PreservedAnalyses run(Module &M, ModuleAnalysisManager &AM);
+};
+
+} // end namespace llvm
+
+#endif // LLVM_TRANSFORMS_UTILS_EMITCHANGEDFUNCDEBUGINFO_H

llvm/lib/Passes/PassBuilder.cpp

@@ -350,6 +350,7 @@
 #include "llvm/Transforms/Utils/DXILUpgrade.h"
 #include "llvm/Transforms/Utils/Debugify.h"
 #include "llvm/Transforms/Utils/DeclareRuntimeLibcalls.h"
+#include "llvm/Transforms/Utils/EmitChangedFuncDebugInfo.h"
 #include "llvm/Transforms/Utils/EntryExitInstrumenter.h"
 #include "llvm/Transforms/Utils/FixIrreducible.h"
 #include "llvm/Transforms/Utils/HelloWorld.h"

llvm/lib/Passes/PassBuilderPipelines.cpp

@@ -135,6 +135,7 @@
 #include "llvm/Transforms/Utils/AssumeBundleBuilder.h"
 #include "llvm/Transforms/Utils/CanonicalizeAliases.h"
 #include "llvm/Transforms/Utils/CountVisits.h"
+#include "llvm/Transforms/Utils/EmitChangedFuncDebugInfo.h"
 #include "llvm/Transforms/Utils/EntryExitInstrumenter.h"
 #include "llvm/Transforms/Utils/ExtraPassManager.h"
 #include "llvm/Transforms/Utils/InjectTLIMappings.h"
@@ -1640,9 +1641,12 @@ PassBuilder::buildModuleOptimizationPipeline(OptimizationLevel Level,
   if (PTO.CallGraphProfile && !LTOPreLink)
     MPM.addPass(CGProfilePass(isLTOPostLink(LTOPhase)));
 
-  // RelLookupTableConverterPass runs later in LTO post-link pipeline.
-  if (!LTOPreLink)
+  // RelLookupTableConverterPass and EmitChangedFuncDebugInfoPass run later in
+  // LTO post-link pipeline.
+  if (!LTOPreLink) {
     MPM.addPass(RelLookupTableConverterPass());
+    MPM.addPass(EmitChangedFuncDebugInfoPass());
+  }
 
   return MPM;
 }

llvm/lib/Passes/PassRegistry.def

@@ -75,6 +75,7 @@ MODULE_PASS("dfsan", DataFlowSanitizerPass())
 MODULE_PASS("dot-callgraph", CallGraphDOTPrinterPass())
 MODULE_PASS("dxil-upgrade", DXILUpgradePass())
 MODULE_PASS("elim-avail-extern", EliminateAvailableExternallyPass())
+MODULE_PASS("emit-changed-func-debuginfo", EmitChangedFuncDebugInfoPass())
 MODULE_PASS("extract-blocks", BlockExtractorPass({}, false))
 MODULE_PASS("expand-variadics",
             ExpandVariadicsPass(ExpandVariadicsMode::Disable))

llvm/lib/Transforms/Utils/CMakeLists.txt

@@ -23,6 +23,7 @@ add_llvm_component_library(LLVMTransformUtils
   DebugSSAUpdater.cpp
   DeclareRuntimeLibcalls.cpp
   DemoteRegToStack.cpp
+  EmitChangedFuncDebugInfo.cpp
   DXILUpgrade.cpp
   EntryExitInstrumenter.cpp
   EscapeEnumerator.cpp