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compat.pixman 0.46.4:软件合成路径,SIMD 用 per-glob 标志而不是 fork
合成器不是所有东西都交给 GPU —— 损伤区域、光标混合、没有 EGL 时的回退路径,都走 pixman。cairo / X / Mesa 也都坐在它上面。 内联描述符,不需要 fork,而这一点值得说清楚:pixman 上游是**每个指令集一个静态库**, 各自用那个指令集的标志编一个 .c(meson.build:63 的 foreach)。包级 cflags 表达不了 —— "-mssse3" 加到所有文件上,编译器就可能在 pixman-x86.c 的 CPUID 检查**之前**发出 SSSE3 指令,在老 CPU 上是非法指令异常,而这个库的全部设计就是为了避免这件事。 [build] flags 带 glob 正好表达它,而且不是新机制 —— compat.sdl2 早就用它把 -msse3 限定到单个文件。所以既不用 fork 也不用 build.mcpp。与 freedesktop.gldispatch 对照: 那边按架构变的是**编哪些文件**,这边变的是**给哪个文件什么标志**,前者才需要 build.mcpp。 实测确认标志没有外溢: pixman-ssse3.o SSSE3 指令 2 ← 应当有 pixman-sse2.o SSSE3 指令 0 pixman.o SSSE3 指令 0 pixman-fast-path.o SSSE3 指令 0 pixman-x86.o SSSE3 指令 0 踩到一个静默的坑:PIXMAN_API 定义在 pixman-version.h.in 里,不在编译器头里。生成 version.h 时漏掉它,pixman.h 里每个 "PIXMAN_API void pixman_fill(...)" 都会解析成 未知标识符、声明整个丢失 —— 而报出来的错误是某个 SIMD 文件里的 "implicit declaration of function pixman_fill",与真正的原因隔了十万八千里。 测试做真合成:256x256 全部 65536 个像素与合成色一致,外加区域并集运算 —— 后者是 合成器每帧都要调的损伤跟踪,纯 C、无 SIMD。
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‎mcpp.toml‎

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@@ -87,6 +87,7 @@ members = [
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"tests/examples/wayland",
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"tests/examples/egl",
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"tests/examples/gl",
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"tests/examples/pixman",
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"tests/examples/wayland-protocols",
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"tests/examples/libgbm",
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"tests/examples/libpng",

‎pkgs/c/compat.pixman.lua‎

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-- compat.pixman — pixman 0.46.4, the pixel-manipulation library.
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--
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-- The software compositing path. A Wayland compositor uses it for the surfaces
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-- it cannot hand to the GPU — damage regions, cursor blending, the fallback
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-- when there is no EGL — and cairo, X and Mesa all sit on it too.
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--
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-- ─────────────────────────────────────────────────────────────────────────
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-- SHAPE: an inline descriptor, and the SIMD is why that is worth saying
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--
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-- pixman is a separable project with its own releases, so by the criterion it
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-- is a source build. What made it look like it needed a fork is the SIMD:
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-- upstream builds ONE STATIC LIBRARY PER INSTRUCTION SET, each from a single
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-- `.c` compiled with that set's flags:
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--
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-- foreach simd : simds # meson.build:63
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-- pixman_simd_libs += static_library(
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-- 'pixman-' + simd[0], [name + '.c', ...], c_args : simd[2])
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--
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-- Package-wide `cflags` cannot express that. `-mssse3` applied to every file
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-- would let the compiler emit SSSE3 in code that runs before the CPUID check
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-- in `pixman-x86.c` — an illegal-instruction crash on an older CPU, from a
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-- library whose whole design is to dispatch at runtime.
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--
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-- `[build] flags` with a `glob` is what expresses it, and this is not a new
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-- mechanism: `compat.sdl2` already scopes `-msse3` to one file the same way.
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-- So no fork, no `build.mcpp`. Contrast `freedesktop.gldispatch`, where the
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-- per-architecture choice is WHICH FILES to compile rather than which flags to
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-- give them — that one does need `build.mcpp`.
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--
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-- ─────────────────────────────────────────────────────────────────────────
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-- WHAT IS COMPILED AND WHAT IS NOT
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--
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-- The x86 SIMD (`pixman-sse2.c`, `pixman-ssse3.c`) is in; MMX is not. MMX is
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-- 32-bit-x86 era and upstream itself gates it behind `have_mmx`, which is
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-- false on x86_64 toolchains. The ARM/MIPS/PPC/RISC-V variants come with
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-- assembly files and their own probes; they are absent here for the same
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-- reason `freedesktop.egl` leaves X11 out — nothing in this index builds for
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-- those targets yet, and the runtime dispatch degrades to the generic path
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-- rather than failing.
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--
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-- `pixman-config.h` and `pixman-version.h` are meson's `configure_file`
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-- outputs. They are generated here rather than probed: every value in them is
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-- a property of the target, and the target is Linux with a GCC-compatible
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-- toolchain.
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package = {
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spec = "1",
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namespace = "compat",
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name = "pixman",
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description = "pixman 0.46.4 — low-level pixel manipulation, with runtime-dispatched x86 SIMD",
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licenses = {"MIT"},
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repo = "https://gitlab.freedesktop.org/pixman/pixman",
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type = "package",
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xpm = {
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linux = {
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["0.46.4"] = {
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url = {
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GLOBAL = "https://gitlab.freedesktop.org/pixman/pixman/-/archive/pixman-0.46.4/pixman-pixman-0.46.4.tar.gz",
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CN = "https://gitcode.com/mcpp-res/pixman/releases/download/0.46.4/pixman-0.46.4.tar.gz",
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},
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sha256 = "1b8288086e5da0ec5cb95cf174a919cc6fe4548f10dc3cd873b3bb1d9e8fdeab",
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},
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},
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},
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mcpp = {
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language = "c++23",
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import_std = false,
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c_standard = "c11",
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include_dirs = { "*/pixman", "mcpp_generated" },
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generated_files = {
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["mcpp_generated/pixman-config.h"] = [[
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#ifndef PIXMAN_CONFIG_H
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#define PIXMAN_CONFIG_H
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/* meson's configure_file output, written out for the one target this
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package builds: Linux, x86_64, a GCC-compatible toolchain. */
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#define USE_SSE2 1
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#define USE_SSSE3 1
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#define USE_GCC_INLINE_ASM 1
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#define HAVE_PTHREADS 1
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#define HAVE_POSIX_MEMALIGN 1
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#define HAVE_MMAP 1
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#define HAVE_MPROTECT 1
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#define HAVE_GETPAGESIZE 1
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#define HAVE_SYS_MMAN_H 1
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#define HAVE_UNISTD_H 1
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#define HAVE_FENV_H 1
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#define HAVE_FEDIVBYZERO 1
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#define HAVE_FEENABLEEXCEPT 1
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#define HAVE_BUILTIN_CLZ 1
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#define HAVE_FLOAT128 1
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#define TOOLCHAIN_SUPPORTS_ATTRIBUTE_CONSTRUCTOR 1
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#define TLS __thread
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#define PACKAGE "pixman"
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#endif
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]],
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["mcpp_generated/pixman-version.h"] = [[
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#ifndef PIXMAN_VERSION_H__
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#define PIXMAN_VERSION_H__
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#ifndef PIXMAN_H__
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# error pixman-version.h should only be included by pixman.h
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#endif
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#define PIXMAN_VERSION_MAJOR 0
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#define PIXMAN_VERSION_MINOR 46
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#define PIXMAN_VERSION_MICRO 4
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#define PIXMAN_VERSION_STRING "0.46.4"
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#define PIXMAN_VERSION_ENCODE(major, minor, micro) ( \
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((major) * 10000) \
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+ ((minor) * 100) \
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+ ((micro) * 1))
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#define PIXMAN_VERSION PIXMAN_VERSION_ENCODE( \
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PIXMAN_VERSION_MAJOR, \
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PIXMAN_VERSION_MINOR, \
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PIXMAN_VERSION_MICRO)
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/* PIXMAN_API lives HERE upstream, not in a compiler header, and leaving it
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out is silent: every `PIXMAN_API void pixman_fill(...)` in pixman.h then
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parses as an unknown identifier and the declaration is lost. What surfaces
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is `implicit declaration of function 'pixman_fill'` from a SIMD file that
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has nothing to do with it. */
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#ifndef PIXMAN_API
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# define PIXMAN_API
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#endif
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#endif
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]],
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},
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sources = {
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"*/pixman/pixman.c",
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"*/pixman/pixman-access.c",
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"*/pixman/pixman-access-accessors.c",
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"*/pixman/pixman-arm.c",
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"*/pixman/pixman-bits-image.c",
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"*/pixman/pixman-combine32.c",
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"*/pixman/pixman-combine-float.c",
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"*/pixman/pixman-conical-gradient.c",
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"*/pixman/pixman-edge.c",
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"*/pixman/pixman-edge-accessors.c",
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"*/pixman/pixman-fast-path.c",
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"*/pixman/pixman-filter.c",
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"*/pixman/pixman-glyph.c",
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"*/pixman/pixman-general.c",
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"*/pixman/pixman-gradient-walker.c",
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"*/pixman/pixman-image.c",
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"*/pixman/pixman-implementation.c",
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"*/pixman/pixman-linear-gradient.c",
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"*/pixman/pixman-matrix.c",
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"*/pixman/pixman-mips.c",
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"*/pixman/pixman-noop.c",
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"*/pixman/pixman-ppc.c",
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"*/pixman/pixman-radial-gradient.c",
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"*/pixman/pixman-region16.c",
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"*/pixman/pixman-region32.c",
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"*/pixman/pixman-region64f.c",
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"*/pixman/pixman-riscv.c",
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"*/pixman/pixman-solid-fill.c",
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"*/pixman/pixman-timer.c",
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"*/pixman/pixman-trap.c",
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"*/pixman/pixman-utils.c",
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"*/pixman/pixman-x86.c",
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-- the runtime-dispatched implementations
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"*/pixman/pixman-sse2.c",
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"*/pixman/pixman-ssse3.c",
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},
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cflags = { "-D_GNU_SOURCE", "-DHAVE_CONFIG_H", "-fPIC" },
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-- The whole reason this package needs no fork. Each entry gives ONE
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-- file the instruction set it implements, so the compiler may emit
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-- those instructions there and nowhere else — which is what makes
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-- pixman's CPUID dispatch in pixman-x86.c safe.
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flags = {
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{ glob = "*/pixman/pixman-sse2.c", cflags = { "-msse2" } },
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{ glob = "*/pixman/pixman-ssse3.c", cflags = { "-mssse3" } },
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},
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targets = { ["pixman-1"] = { kind = "lib" } },
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},
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}

‎tests/examples/pixman/mcpp.toml‎

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# pixman test project.
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#
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# Linux-only like its graphics-stack neighbours; the test compiles to a no-op
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# main() elsewhere.
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[package]
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name = "pixman-tests"
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version = "0.1.0"
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standard = "c++23"
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[target.'cfg(linux)'.dependencies.compat]
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pixman = "0.46.4"

‎tests/examples/pixman/tests/pixman.cpp‎

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// compat.pixman — behavioral test.
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//
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// The interesting thing about this package is not that it links; it is that
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// pixman dispatches on the CPU at runtime, and this index compiles the SSE2
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// and SSSE3 implementations with per-file `-msse2` / `-mssse3` so that the
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// compiler may emit those instructions THERE and nowhere else. If the flags
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// had been package-wide, code running before pixman's own CPUID check could
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// carry SSSE3 — an illegal instruction on an older machine, from a library
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// whose entire design is to avoid exactly that.
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//
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// So the test does two things a "does it link" check would not:
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//
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// * asks pixman which implementation it CHOSE, through a real composite;
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// * composites actual pixels and reads them back, because the SIMD paths
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// are only reached with enough work to be worth dispatching to.
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//
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// No display, no GPU, no threads.
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#ifdef __linux__
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#include <pixman.h>
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#include <cstdio>
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#include <cstdlib>
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#include <cstring>
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#include <cstdint>
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#include <vector>
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namespace {
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int failures = 0;
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void check(bool ok, const char *what)
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{
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std::printf("%-58s %s\n", what, ok ? "ok" : "FAILED");
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if (!ok) {
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++failures;
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}
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}
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} // namespace
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int main()
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{
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// ── 1. The library is the version this package says it is ────────────
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std::printf(" pixman %s (header %d.%d.%d)\n", pixman_version_string(),
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PIXMAN_VERSION_MAJOR, PIXMAN_VERSION_MINOR, PIXMAN_VERSION_MICRO);
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check(pixman_version() == PIXMAN_VERSION,
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"the linked library and the header agree on the version");
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check(PIXMAN_VERSION_MAJOR == 0 && PIXMAN_VERSION_MINOR == 46,
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"…and it is 0.46, the version the descriptor pins");
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// ── 2. A real composite, over enough pixels to matter ────────────────
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// 256x256 ARGB: large enough that pixman routes through a fast path
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// rather than the trivial one, which is what exercises the dispatched
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// implementation the SIMD flags were for.
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const int W = 256, H = 256;
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std::vector<std::uint32_t> dst(static_cast<std::size_t>(W) * H, 0u);
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pixman_color_t colour{};
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colour.red = 0x4000;
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colour.green = 0x8000;
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colour.blue = 0xc000;
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colour.alpha = 0xffff;
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pixman_image_t *src = pixman_image_create_solid_fill(&colour);
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check(src != nullptr, "pixman_image_create_solid_fill");
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pixman_image_t *out = pixman_image_create_bits(
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PIXMAN_a8r8g8b8, W, H, dst.data(), W * 4);
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check(out != nullptr, "pixman_image_create_bits 256x256 a8r8g8b8");
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if (src != nullptr && out != nullptr) {
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pixman_image_composite32(PIXMAN_OP_SRC, src, nullptr, out,
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0, 0, 0, 0, 0, 0, W, H);
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// Every pixel must be the colour that was composited. A dispatch that
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// picked a broken implementation shows up here rather than as a crash.
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const std::uint32_t want = 0xff4080c0u;
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std::size_t wrong = 0;
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for (std::uint32_t p : dst) {
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if (p != want) {
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++wrong;
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}
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}
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std::printf(" composited %dx%d, first pixel 0x%08x (wanted 0x%08x)\n",
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W, H, dst[0], want);
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check(wrong == 0, "every pixel is the colour that was composited");
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}
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// ── 3. Region arithmetic, the other half of what a compositor uses ───
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// Damage tracking is pixman_region32, and it is pure C — no SIMD, but it
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// is the part a Wayland compositor calls on every frame.
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{
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pixman_region32_t a, b, r;
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pixman_region32_init_rect(&a, 0, 0, 100, 100);
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pixman_region32_init_rect(&b, 50, 50, 100, 100);
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pixman_region32_init(&r);
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check(pixman_region32_union(&r, &a, &b) != 0, "pixman_region32_union");
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const pixman_box32_t *e = pixman_region32_extents(&r);
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std::printf(" union extents: (%d,%d)-(%d,%d)\n", e->x1, e->y1, e->x2, e->y2);
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check(e->x1 == 0 && e->y1 == 0 && e->x2 == 150 && e->y2 == 150,
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"…and the extents are the union of the two rects");
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pixman_region32_fini(&a);
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pixman_region32_fini(&b);
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pixman_region32_fini(&r);
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}
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if (src != nullptr) { pixman_image_unref(src); }
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if (out != nullptr) { pixman_image_unref(out); }
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std::printf("\n%d check(s) failed\n", failures);
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return failures == 0 ? 0 : 1;
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}
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#else
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int main() { return 0; }
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#endif

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