picolibc 1.8.12 and compiler-rt-builtins 22.1.8 — the C library, as source (#336)

⭐⭐ SOURCE PACKAGES, NOT A PREBUILT SYSROOT, AND THE MEASUREMENTS SAY WHY.

A prebuilt C library ships one build per ABI a target table can name — seven for
Cortex-M alone — and every consumer then finds the right one through a `libdir`
convention that must match the payload byte for byte. That convention is what
#481 was filed about, and on ARM it cannot work at all: the key
`<march>/<mabi>` does not separate the float ABI, because `mabi` there names the
procedure call standard and is `aapcs` either way.

Measured while building the prebuilt first: the seven profiles collapsed into
FIVE directories and the soft-float row received a library carrying
`Tag_ABI_HardFP_use`. Nothing failed at build time.

⚠️ And the headers were never per-profile. Across those seven builds the whole
include tree — `picolibc.h` and `newlib.h`, which meson GENERATES, included — is
byte-identical. The prebuilt shipped seven copies of one directory.

Compiled with the same `compile_flags` as the program consuming it, ABI
agreement holds by construction, and the version lands in `mcpp.lock` rather
than being pinned inside a target table.

## What the source route cost instead

Four upstream partitions a flat library has to be told about, each found by a
link error rather than by reading:

* `libos/semihost` and `libos/dummyhost` define the same syscalls and upstream
  keeps them in separate ARCHIVES, where link order decides. A feature here.
* `semihost/fake` is the COMPLEMENT of `semihost/common` — 13 of 26 files — and
  the complement cannot be computed from filenames: `fake_io.c` collides with
  `common/iob.c` on `stdin`/`stdout`/`stderr`.
* the machine directory carries variants for every ARM profile upstream serves;
  `memcpy-armv7a.S` needs ARM mode and Thumb-2, which no M-profile core has.
* a negative glob in a base source set applies to the MERGED set, so
  `!…/machine/**` cancels the per-target block that adds the right one back.

And for the builtins, one more: upstream picks per CPU and the sets are nested —
25 files on armv6-m, 90 on armv8-m.main — so the list is the INTERSECTION, and
the routines it omits come from the generic C. An exclusion pattern was not
enough either: `!builtins/arm/*vfp.S` misses `save_vfp_d8_d15_regs.S`.

## Measured

Both build for all seven M-profile rows. A `printf("%.2f")` program built
against them plus `cortex-m-rt` boots on QEMU's `mps2-an385`, prints `13.00` and
exits 0 — and the same source without the feature fails at `'stdio.h' file not
found`, which is the zero-libc tier behaving as documented.

Both tarballs verified against their bytes and re-fetched from the CN mirror,
byte-identical. Index lints pass.

Author: Sunrisepeak

pkgs/c/compiler-rt-builtins.lua

@@ -0,0 +1,55 @@
+-- The compiler-rt builtins as a SOURCE package.
+--
+-- ⭐ TWO PACKAGES RATHER THAN ONE, AND THE EDGE IS THE REASON. picolibc's
+-- `printf` formats floats through ryu, which calls routines no C library
+-- defines — and on rv64 a 128-bit shift the instruction set has none for. A C
+-- library carrying its own copy would be wrong for anyone supplying their own
+-- builtins; the dependency edge says the same thing and can be overridden.
+--
+-- ⚠️ AND compiler-rt DOES NOT RECOGNISE A `thumb*` TRIPLE. Configuring its own
+-- CMake with `thumbv6m-none-eabi` produces a build tree with NO builtins target
+-- at all: cmake succeeds, ninja reports "no work to do", and the failure
+-- surfaces later as a missing file. A source package has no archive to name and
+-- no triple to translate.
+package = {
+    spec        = "1",
+    namespace   = "mcpplibs",
+    name        = "compiler-rt-builtins",
+    description = "The compiler-rt builtins as a source package: the routines a compiler emits calls to, compiled with the consuming program's own flags",
+    licenses    = {"Apache-2.0 WITH LLVM-exception"},
+    repo        = "https://github.com/mcpplibs/compiler-rt-builtins",
+    type        = "package",
+
+    xpm = {
+        linux = {
+            ["22.1.8"] = {
+                url    = {
+                    GLOBAL = "https://github.com/mcpplibs/compiler-rt-builtins/archive/refs/tags/22.1.8.tar.gz",
+                    CN     = "https://gitcode.com/mcpp-res/compiler-rt-builtins/releases/download/22.1.8/compiler-rt-builtins-22.1.8.tar.gz",
+                },
+                sha256 = "fe00cb58128c00a3a47483f30a828bed52f4a35529c1bb0d4cb8f0376c9c492d",
+            },
+        },
+        macosx = {
+            ["22.1.8"] = {
+                url    = {
+                    GLOBAL = "https://github.com/mcpplibs/compiler-rt-builtins/archive/refs/tags/22.1.8.tar.gz",
+                    CN     = "https://gitcode.com/mcpp-res/compiler-rt-builtins/releases/download/22.1.8/compiler-rt-builtins-22.1.8.tar.gz",
+                },
+                sha256 = "fe00cb58128c00a3a47483f30a828bed52f4a35529c1bb0d4cb8f0376c9c492d",
+            },
+        },
+        windows = {
+            ["22.1.8"] = {
+                url    = {
+                    GLOBAL = "https://github.com/mcpplibs/compiler-rt-builtins/archive/refs/tags/22.1.8.tar.gz",
+                    CN     = "https://gitcode.com/mcpp-res/compiler-rt-builtins/releases/download/22.1.8/compiler-rt-builtins-22.1.8.tar.gz",
+                },
+                sha256 = "fe00cb58128c00a3a47483f30a828bed52f4a35529c1bb0d4cb8f0376c9c492d",
+            },
+        },
+    },
+
+    -- The package's own manifest, inside the tarball's wrap directory.
+    mcpp = "*/mcpp.toml",
+}

pkgs/p/picolibc.lua

@@ -0,0 +1,72 @@
+-- picolibc as a SOURCE package: compiled with the consuming program's own flags.
+--
+-- ⭐⭐ THERE IS NO MULTILIB HERE, AND THAT IS WHY IT IS A SOURCE PACKAGE.
+--
+-- A prebuilt C library ships one build per ABI a target table can name — seven
+-- for Cortex-M alone — and every consumer then finds the right one through a
+-- `libdir` convention that must match the payload byte for byte. That
+-- convention is what #481 was filed about, and on ARM it cannot work at all:
+-- the key `<march>/<mabi>` does not separate the float ABI, because `mabi`
+-- there names the procedure call standard and is `aapcs` either way.
+--
+-- Measured while building the prebuilt (`xim:picolibc-arm`, which remains
+-- published and is NOT the route): the seven profiles collapsed into five
+-- directories and the soft-float row received a library carrying
+-- `Tag_ABI_HardFP_use`. Nothing failed at build time.
+--
+-- ⚠️ AND THE HEADERS WERE NEVER PER-PROFILE. Measured across those seven
+-- builds, the whole include tree — `picolibc.h` and `newlib.h`, which meson
+-- GENERATES, included — is byte-identical. The prebuilt ships seven copies of
+-- one directory.
+--
+-- ⭐ Compiled with the SAME `compile_flags` as the program consuming it, ABI
+-- agreement holds by construction rather than by a naming convention, and the
+-- version lands in `mcpp.lock` rather than being pinned inside a target table.
+--
+-- ⚠️ `picocrt` IS DELIBERATELY ABSENT. A startup object decides where execution
+-- begins and how it reaches the host — which board is running. Choosing among
+-- picolibc's nine variants is a board-support package's job; `cortex-m-rt`
+-- supplies its own, including the thread-pointer initialisation without which
+-- a `printf` program links cleanly, runs, prints nothing and hangs.
+package = {
+    spec        = "1",
+    namespace   = "mcpplibs",
+    name        = "picolibc",
+    description = "picolibc as a source package: a freestanding C library compiled with the consuming program's own flags, so there is no multilib and no ABI convention to match",
+    licenses    = {"BSD-3-Clause", "BSD-2-Clause"},
+    repo        = "https://github.com/mcpplibs/picolibc",
+    type        = "package",
+
+    xpm = {
+        linux = {
+            ["1.8.12"] = {
+                url    = {
+                    GLOBAL = "https://github.com/mcpplibs/picolibc/archive/refs/tags/1.8.12.tar.gz",
+                    CN     = "https://gitcode.com/mcpp-res/picolibc/releases/download/1.8.12/picolibc-1.8.12.tar.gz",
+                },
+                sha256 = "8f3d7a41d9981905d389b321edbfa2fac4946e5acc3a0677617b8184b21d4420",
+            },
+        },
+        macosx = {
+            ["1.8.12"] = {
+                url    = {
+                    GLOBAL = "https://github.com/mcpplibs/picolibc/archive/refs/tags/1.8.12.tar.gz",
+                    CN     = "https://gitcode.com/mcpp-res/picolibc/releases/download/1.8.12/picolibc-1.8.12.tar.gz",
+                },
+                sha256 = "8f3d7a41d9981905d389b321edbfa2fac4946e5acc3a0677617b8184b21d4420",
+            },
+        },
+        windows = {
+            ["1.8.12"] = {
+                url    = {
+                    GLOBAL = "https://github.com/mcpplibs/picolibc/archive/refs/tags/1.8.12.tar.gz",
+                    CN     = "https://gitcode.com/mcpp-res/picolibc/releases/download/1.8.12/picolibc-1.8.12.tar.gz",
+                },
+                sha256 = "8f3d7a41d9981905d389b321edbfa2fac4946e5acc3a0677617b8184b21d4420",
+            },
+        },
+    },
+
+    -- The package's own manifest, inside the tarball's wrap directory.
+    mcpp = "*/mcpp.toml",
+}