fix(bench): three CI-only failures — PIE, the pre-project payload, and an unpacked dependency

**我先前说「CI 通过」是错的。** 当时还有 10 个 job 在跑,我看到「20 绿 0 红」就
下了结论,而那 10 个里就有这些格子。拿部分数据当结论,在这个整场都在讲「失败看
起来像成功」的分支上尤其不该发生。

三个都只在 CI 上出现,本地一直绿 —— 原因各不相同:

**1. fixture 的 xmake 臂:PIE 没有说出口。**

    relocation R_X86_64_32 against `.rodata.str1.1' can not be used
    when making a PIE object; recompile with -fPIE
    ld: failed to set dynamic section sizes: bad value

载荷 gcc 在 CI runner 上默认 PIE 链接、在开发机上不是,所以编译不带 `-fPIE` 的
对象只在 CI 被拒。12 个格子红。mcpp 与 cmake 在这里都产出 PIE(bazel 适配器为此
传 `--force_pic`),所以把它显式写出来,四条臂产出同一种可执行文件。

**2. cmake 的载荷 flags 到得太晚。** `bench_hermetic_payload()` 依赖
`CMAKE_CXX_COMPILER_ID`,而它要 `project()` 之后才有 —— 但失败的正是 `project()`
里的那次探测:

    /usr/bin/ld: cannot find crt1.o: No such file or directory

CMake 把它报成「编译器无法编译一个简单程序」,既不提 sysroot 也不提载荷。开发机
上因为宿主有 crt1.o 而通过。新增 `bench_hermetic_payload_preproject()`,按编译器
**路径**判断(调用方本来就用 `-DCMAKE_CXX_COMPILER` 给了它),在 project() 之前
补上 `-B`/`--sysroot`。

**3. 依赖根本没被解包。** mcpp 从全局构建缓存取 `mcpplibs.cmdline`,**缓存命中
不解包源码**,于是新 runner 上两条外部臂都拿不到那三个单元。xmake 臂的守卫如实
报了出来;cmake 臂当时只 warning、然后静默地少编三个单元 —— 已改成 FATAL_ERROR,
和 xmake 那条对齐。bench.yml 在矩阵开始前丢掉该包的缓存,让下一次 mcpp 构建从源码
编一次(三个翻译单元,发生在任何测量之前)。

Author: speak-agent

.github/workflows/bench.yml

@@ -299,6 +299,20 @@ jobs:
           # and say nothing.
           BIN=$(bash .github/tools/newest_artifact.sh target 'mcpp')
           echo "MCPP_UNDER_TEST=$BIN" >> "$GITHUB_ENV"
+
+          # THE FOREIGN ARMS NEED THE DEPENDENCY'S SOURCES, and a cache hit never
+          # unpacks them. mcpp serves `mcpplibs.cmdline` from its global build
+          # cache, so on a fresh runner nothing is unpacked under
+          # registry/data/xpkgs — and the cmake and xmake arms, which compile
+          # those units from source to match what mcpp linked, then fail:
+          #     bench: mcpplibs.cmdline 0.0.1 is not unpacked under .../xpkgs
+          # (that message is the xmake arm's guard doing its job — the cmake arm
+          # used to warn and silently build without the units instead).
+          #
+          # Dropping the cached package makes the next mcpp build compile it from
+          # source, which unpacks it. Cheap — three translation units — and it
+          # happens before any measurement starts.
+          rm -rf "$HOME"/.mcpp/build-cache/v1/pkg/mcpplibs/*cmdline* || true
           echo "under test : $("$BIN" --version)"
           echo "reference  : $(command -v mcpp && mcpp --version || echo 'not installed')"
 

bench/projects/common/cmake/hermetic_payload.cmake

@@ -220,3 +220,53 @@ function(bench_add_source_dep target name version)
     target_sources(${target} PRIVATE ${impls})
   endif()
 endfunction()
+
+# ── The half that must run BEFORE project() ─────────────────────────────────
+#
+# `bench_hermetic_payload()` above keys off CMAKE_CXX_COMPILER_ID, which does
+# not exist until project() has probed the compiler — and that probe is exactly
+# what fails without these flags:
+#
+#     [2/2] .../xim-x-gcc/16.1.0/bin/g++ ... -o cmTC_c87ee
+#     /usr/bin/ld: cannot find crt1.o: No such file or directory
+#     /usr/bin/ld: cannot find crti.o: No such file or directory
+#
+# CMake reports that as "The C++ compiler is not able to compile a simple test
+# program", naming neither the sysroot nor the payload. It passed on developer
+# boxes because a host crt1.o was findable there and did not on CI.
+#
+# So this one keys off the compiler PATH, which the caller already has from
+# -DCMAKE_CXX_COMPILER. Same flags, same reasoning as the GNU branch above;
+# a compiler outside the registry is left alone, exactly as there.
+function(bench_hermetic_payload_preproject)
+  if(NOT CMAKE_CXX_COMPILER OR WIN32)
+    return()
+  endif()
+  get_filename_component(_real "${CMAKE_CXX_COMPILER}" REALPATH)
+  string(FIND "${_real}" "xpkgs" _pos)
+  if(_pos EQUAL -1)
+    return()
+  endif()
+  # clang carries its own include chain (see the Clang branch above) and does
+  # not need -B/--sysroot to link a test program; only the gcc payload does.
+  if(NOT _real MATCHES "g\\+\\+$" AND NOT _real MATCHES "gcc$")
+    return()
+  endif()
+  bench_registry_xpkgs(_xpkgs)
+  bench_newest_package("${_xpkgs}" "xim-x-binutils" _binutils)
+  set(_sysroot "${_xpkgs}/../subos/default")
+  set(_add "")
+  if(_binutils)
+    string(APPEND _add " -B${_binutils}/bin")
+  endif()
+  if(IS_DIRECTORY "${_sysroot}")
+    string(APPEND _add " --sysroot=${_sysroot}")
+  endif()
+  if(_add STREQUAL "")
+    return()
+  endif()
+  foreach(_v CMAKE_CXX_FLAGS CMAKE_C_FLAGS CMAKE_EXE_LINKER_FLAGS)
+    set(${_v} "${${_v}}${_add}" PARENT_SCOPE)
+  endforeach()
+  message(STATUS "bench: hermetic payload applied before project()${_add}")
+endfunction()

bench/projects/mcpp/CMakeLists.txt

@@ -51,6 +51,9 @@ set(CMAKE_CXX_STANDARD 23)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
 set(CMAKE_CXX_EXTENSIONS OFF)
 
+include(${CMAKE_CURRENT_LIST_DIR}/../common/cmake/hermetic_payload.cmake)
+bench_hermetic_payload_preproject()
+
 project(mcpp CXX)
 # Every mcpp module says `import std;`. This asks CMake to build the standard
 # library module from the compiler's own libstdc++.modules.json, which the
@@ -140,9 +143,16 @@ set(MCPP_CMDLINE_SRC
 if(IS_DIRECTORY "${MCPP_CMDLINE_SRC}")
   file(GLOB MCPP_CMDLINE_MODULES CONFIGURE_DEPENDS "${MCPP_CMDLINE_SRC}/*.cppm")
 else()
-  message(WARNING "mcpplibs.cmdline ${MCPP_CMDLINE_VERSION} not unpacked at "
-                  "${MCPP_CMDLINE_SRC}; this build will not match mcpp's own")
-  set(MCPP_CMDLINE_MODULES "")
+  # FATAL, not a warning. Warning here builds mcpp WITHOUT three of its units
+  # and the failure lands at the link as `undefined reference to ...cmdline...`,
+  # naming a consumer rather than the missing package — and on CI it did exactly
+  # that. A description that cannot name the same sources mcpp compiled is not a
+  # comparison arm. (The xmake arm beside this one raises for the same reason.)
+  message(FATAL_ERROR
+          "mcpplibs.cmdline ${MCPP_CMDLINE_VERSION} is not unpacked at "
+          "${MCPP_CMDLINE_SRC} — build the tree with mcpp once first, so both "
+          "arms compile the same dependency sources. A cache hit does NOT unpack "
+          "them; bench.yml drops the cached package before the matrix for this.")
 endif()
 
 add_executable(mcpp ${MCPP_SOURCES})

bench/projects/xlings/CMakeLists.txt

@@ -84,6 +84,9 @@ if(NOT CMAKE_C_COMPILER AND CMAKE_CXX_COMPILER)
   endif()
 endif()
 
+include(${CMAKE_CURRENT_LIST_DIR}/../common/cmake/hermetic_payload.cmake)
+bench_hermetic_payload_preproject()
+
 project(xlings C CXX)
 set(CMAKE_CXX_MODULE_STD 1)
 

bench/src/fixture/buildfiles.cppm

@@ -173,6 +173,19 @@ inline void emit_xmake(const std::filesystem::path& root, Variant variant, const
         // cost none of the others do.
         lua += "    set_policy(\"build.c++.modules.std\", false)\n";
     }
+    // ⚠️ PIE MUST BE EXPLICIT ON BOTH SIDES. The payload gcc defaults to a PIE
+    // link on the CI runners and to a non-PIE link on some developer boxes, so
+    // compiling without `-fPIE` produced objects the linker then refused:
+    //     relocation R_X86_64_32 against `.rodata.str1.1' can not be used
+    //     when making a PIE object; recompile with -fPIE
+    //     ld: failed to set dynamic section sizes: bad value
+    // Twelve fixture cells red on CI, green locally — the difference was the
+    // driver's default, not the engine. mcpp and cmake produce PIE here (the
+    // bazel adapter passes --force_pic for the same reason), so saying it out
+    // loud keeps all four arms producing the same kind of executable.
+    lua += "    add_cxflags(\"-fPIE\", {force = true})\n";
+    lua += "    add_ldflags(\"-pie\", {force = true})\n";
+
     // Same payload flags as the cmake arm: xmake is handed the driver through
     // CXX, and a registry gcc without -B/--sysroot cannot link.
     if (!pf.compile.empty())