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<!--LLVM PR SUMMARY COMMENT--> @llvm/pr-subscribers-clang Author: Prabhuk (Prabhuk) <details> <summary>Changes</summary> `CallGraphSection` CodeGen option emits the callgraph section. This patch documents the design, implementation and usage of this option and the underlying machinery. Inferring indirect call targets from a binary is challenging without source-level information. Hence, the reconstruction of a fine-grained call graph from the binary is unfeasible for indirect/virtual calls. To address this, CallGraphSection option collects the necessary information to construct the call graph while the source information is present, and stores it in a non-code section of the binary. Original RFC: https://lists.llvm.org/pipermail/llvm-dev/2021-June/151044.html Updated RFC: https://lists.llvm.org/pipermail/llvm-dev/2021-July/151739.html --- Full diff: https://github.com/llvm/llvm-project/pull/119468.diff 1 Files Affected: - (added) clang/docs/CallGraphSection.rst (+251) ``````````diff diff --git a/clang/docs/CallGraphSection.rst b/clang/docs/CallGraphSection.rst new file mode 100644 index 00000000000000..d5db507f9ca9a6 --- /dev/null +++ b/clang/docs/CallGraphSection.rst @@ -0,0 +1,251 @@ +================== +Call Graph Section +================== + +Introduction +============ + +With ``-fcall-graph-section``, the compiler will create a call graph section +in the object file. It will include type identifiers for indirect calls and +targets. This information can be used to map indirect calls to their receivers +with matching types. A complete and high-precision call graph can be +reconstructed by complementing this information with disassembly +(see ``llvm-objdump --call-graph-info``). + +Semantics +========= + +A coarse-grained, type-agnostic call graph may allow indirect calls to target +any function in the program. This approach ensures completeness since no +indirect call edge is missing. However, it is generally poor in precision +due to having unneeded edges. + +A call graph section provides type identifiers for indirect calls and targets. +This information can be used to restrict the receivers of an indirect target to +indirect calls with matching type. Consequently, the precision for indirect +call edges are improved while maintaining the completeness. + +The ``llvm-objdump`` utility provides a ``--call-graph-info`` option to extract +full call graph information by parsing the content of the call graph section +and disassembling the program for complementary information, e.g., direct +calls. + +Section layout +============== + +A call graph section consists of zero or more call graph entries. +Each entry contains information on a function and its indirect calls. + +An entry of a call graph section has the following layout in the binary: + ++---------------------+-----------------------------------------------------------------------+ +| Element | Content | ++=====================+=======================================================================+ +| FormatVersionNumber | Format version number. | ++---------------------+-----------------------------------------------------------------------+ +| FunctionEntryPc | Function entry address. | ++---------------------+-----------------------------------+-----------------------------------+ +| | A flag whether the function is an | - 0: not an indirect target | +| FunctionKind | indirect target, and if so, | - 1: indirect target, unknown id | +| | whether its type id is known. | - 2: indirect target, known id | ++---------------------+-----------------------------------+-----------------------------------+ +| FunctionTypeId | Type id for the indirect target. Present only when FunctionKind is 2. | ++---------------------+-----------------------------------------------------------------------+ +| CallSiteCount | Number of type id to indirect call site mappings that follow. | ++---------------------+-----------------------------------------------------------------------+ +| CallSiteList | List of type id and indirect call site pc pairs. | ++---------------------+-----------------------------------------------------------------------+ + +Each element in an entry (including each element of the contained lists and +pairs) occupies 64-bit space. + +The format version number is repeated per entry to support concatenation of +call graph sections with different format versions by the linker. + +As of now, the only supported format version is described above and has version +number 0. + +Type identifiers +================ + +The type for an indirect call or target is the function signature. +The mapping from a type to an identifier is an ABI detail. +In the current experimental implementation, an identifier of type T is +computed as follows: + + - Obtain the generalized mangled name for “typeinfo name for T”. + - Compute MD5 hash of the name as a string. + - Reinterpret the first 8 bytes of the hash as a little-endian 64-bit integer. + +To avoid mismatched pointer types, generalizations are applied. +Pointers in return and argument types are treated as equivalent as long as the +qualifiers for the type they point to match. +For example, ``char*``, ``char**``, and ``int*`` are considered equivalent +types. However, ``char*`` and ``const char*`` are considered separate types. + +Missing type identifiers +======================== + +For functions, two cases need to be considered. First, if the compiler cannot +deduce a type id for an indirect target, it will be listed as an indirect target +without a type id. Second, if an object without a call graph section gets +linked, the final call graph section will lack information on functions from +the object. For completeness, these functions need to be taken as receiver to +any indirect call regardless of their type id. +``llvm-objdump --call-graph-info`` lists these functions as indirect targets +with `UNKNOWN` type id. + +For indirect calls, current implementation guarantees a type id for each +compiled call. However, if an object without a call graph section gets linked, +no type id will be present for its indirect calls. For completeness, these calls +need to be taken to target any indirect target regardless of their type id. For +indirect calls, ``llvm-objdump --call-graph-info`` prints 1) a complete list of +indirect calls, 2) type id to indirect call mappings. The difference of these +lists allow to deduce the indirect calls with missing type ids. + +TODO: measure and report the ratio of missed type ids + +Performance +=========== + +A call graph section does not affect the executable code and does not occupy +memory during process execution. Therefore, there is no performance overhead. + +The scheme has not yet been optimized for binary size. + +TODO: measure and report the increase in the binary size + +Example +======= + +For example, consider the following C++ code: + +.. code-block:: cpp + + namespace { + // Not an indirect target + void foo() {} + } + + // Indirect target 1 + void bar() {} + + // Indirect target 2 + int baz(char a, float *b) { + return 0; + } + + // Indirect target 3 + int main() { + char a; + float b; + void (*fp_bar)() = bar; + int (*fp_baz1)(char, float*) = baz; + int (*fp_baz2)(char, float*) = baz; + + // Indirect call site 1 + fp_bar(); + + // Indirect call site 2 + fp_baz1(a, &b); + + // Indirect call site 3: shares the type id with indirect call site 2 + fp_baz2(a, &b); + + // Direct call sites + foo(); + bar(); + baz(a, &b); + + return 0; + } + +Following will compile it with a call graph section created in the binary: + +.. code-block:: bash + + $ clang -fcall-graph-section example.cpp + +During the construction of the call graph section, the type identifiers are +computed as follows: + ++---------------+-----------------------+----------------------------+----------------------------+ +| Function name | Generalized signature | Mangled name (itanium ABI) | Numeric type id (md5 hash) | ++===============+=======================+============================+============================+ +| bar | void () | _ZTSFvvE.generalized | f85c699bb8ef20a2 | ++---------------+-----------------------+----------------------------+----------------------------+ +| baz | int (char, void*) | _ZTSFicPvE.generalized | e3804d2a7f2b03fe | ++---------------+-----------------------+----------------------------+----------------------------+ +| main | int () | _ZTSFivE.generalized | a9494def81a01dc | ++---------------+-----------------------+----------------------------+----------------------------+ + +The call graph section will have the following content: + ++---------------+-----------------+--------------+----------------+---------------+--------------------------------------+ +| FormatVersion | FunctionEntryPc | FunctionKind | FunctionTypeId | CallSiteCount | CallSiteList | ++===============+=================+==============+================+===============+======================================+ +| 0 | EntryPc(foo) | 0 | (empty) | 0 | (empty) | ++---------------+-----------------+--------------+----------------+---------------+--------------------------------------+ +| 0 | EntryPc(bar) | 2 | TypeId(bar) | 0 | (empty) | ++---------------+-----------------+--------------+----------------+---------------+--------------------------------------+ +| 0 | EntryPc(baz) | 2 | TypeId(baz) | 0 | (empty) | ++---------------+-----------------+--------------+----------------+---------------+--------------------------------------+ +| 0 | EntryPc(main) | 2 | TypeId(main) | 3 | * TypeId(bar), CallSitePc(fp_bar()) | +| | | | | | * TypeId(baz), CallSitePc(fp_baz1()) | +| | | | | | * TypeId(baz), CallSitePc(fp_baz2()) | ++---------------+-----------------+--------------+----------------+---------------+--------------------------------------+ + + +The ``llvm-objdump`` utility can parse the call graph section and disassemble +the program to provide complete call graph information. This includes any +additional call sites from the binary: + +.. code-block:: bash + + $ llvm-objdump --call-graph-info a.out + + # Comments are not a part of the llvm-objdump's output but inserted for clarifications. + + a.out: file format elf64-x86-64 + # These warnings are due to the functions and the indirect calls coming from linked objects. + llvm-objdump: warning: 'a.out': callgraph section does not have type ids for 3 indirect calls + llvm-objdump: warning: 'a.out': callgraph section does not have information for 10 functions + + # Unknown targets are the 10 functions the warnings mention. + INDIRECT TARGET TYPES (TYPEID [FUNC_ADDR,]) + UNKNOWN 401000 401100 401234 401050 401090 4010d0 4011d0 401020 401060 401230 + a9494def81a01dc 401150 # main() + f85c699bb8ef20a2 401120 # bar() + e3804d2a7f2b03fe 401130 # baz() + + # Notice that the call sites share the same type id as target functions + INDIRECT CALL TYPES (TYPEID [CALL_SITE_ADDR,]) + f85c699bb8ef20a2 401181 # Indirect call site 1 (fp_bar()) + e3804d2a7f2b03fe 401191 4011a1 # Indirect call site 2 and 3 (fp_baz1() and fp_baz2()) + + INDIRECT CALL SITES (CALLER_ADDR [CALL_SITE_ADDR,]) + 401000 401012 # _init + 401150 401181 401191 4011a1 # main calls fp_bar(), fp_baz1(), fp_baz2() + 4011d0 401215 # __libc_csu_init + 401020 40104a # _start + + DIRECT CALL SITES (CALLER_ADDR [(CALL_SITE_ADDR, TARGET_ADDR),]) + 4010d0 4010e2 401060 # __do_global_dtors_aux + 401150 4011a6 401110 4011ab 401120 4011ba 401130 # main calls foo(), bar(), baz() + 4011d0 4011fd 401000 # __libc_csu_init + + FUNCTIONS (FUNC_ENTRY_ADDR, SYM_NAME) + 401000 _init + 401100 frame_dummy + 401234 _fini + 401050 _dl_relocate_static_pie + 401090 register_tm_clones + 4010d0 __do_global_dtors_aux + 401110 _ZN12_GLOBAL__N_13fooEv # (anonymous namespace)::foo() + 401150 main # main + 4011d0 __libc_csu_init + 401020 _start + 401060 deregister_tm_clones + 401120 _Z3barv # bar() + 401130 _Z3bazcPf # baz(char, float*) + 401230 __libc_csu_fini `````````` </details> https://github.com/llvm/llvm-project/pull/119468 _______________________________________________ llvm-branch-commits mailing list llvm-branch-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-branch-commits
