ci: run RVV cross-compile job in archlinux:latest with qemu 11

QEMU < 11's RVV TCG emulation is dramatically slower than scalar (see
QEMU issue #2137), and combined with gcc-14's RVV codegen at vlen=128
the xsimd test_xsimd binary takes long enough to overflow the GHA 6h
job timeout while making no observable progress.

Measured locally:
  qemu 8.2.2 (Ubuntu 24.04 apt)    : test_xsimd at vlen=128 times out
  qemu 9.2.1 (Ubuntu 25.04 plucky) : ditto
  qemu 10.0.8 (Debian trixie)      : ditto
  qemu 11.0.0 (Arch) + gcc 15.1    : 367 cases / 5664 asserts in <10 min

Move the whole RVV cross-compile job into an archlinux:latest container
so it gets qemu 11 + gcc 15 today, instead of waiting for Ubuntu to
ship qemu 11. Keep timeout-minutes: 15 as a safety net so any future
regression fails fast instead of burning the 6h job timeout, and add
fail-fast: false so a stuck vector_bits entry does not cancel its
peers.

This trades multi-compiler / multi-version coverage (clang 17/18,
gcc 14) for a CI signal that actually completes; reintroducing those
matrix entries belongs to a follow-up once we either pre-build a
qemu-11 deb for ubuntu-latest or use per-compiler containers.

References:
  QEMU 11.0.0 release notes  https://www.qemu.org/2026/04/22/qemu-11-0-0/
  QEMU RVV slowdowns issue   https://gitlab.com/qemu-project/qemu/-/issues/2137
  Ubuntu RVV vstart bug      https://bugs.launchpad.net/ubuntu/+source/qemu/+bug/2095169

Author: DiamonDinoia

.github/workflows/cross-rvv.yml

@@ -1,3 +1,32 @@
+# RISC-V RVV cross-compilation build.
+#
+# Why this job runs inside an `archlinux:latest` container instead of on a
+# vanilla `ubuntu-latest` runner:
+#
+# QEMU's RISC-V Vector emulation is dramatically slower than scalar in
+# qemu < 11 (see QEMU issue #2137 for documented 100x+ slowdowns of
+# auto-vectorised RVV loops under TCG). Combined with the RVV codegen
+# emitted by gcc-14 at vlen=128, our test_xsimd binary runs *much* longer
+# than its native runtime under qemu 8.2 / 9.2 / 10.0 — long enough to
+# exhaust the 6h GHA job timeout while making no observable progress.
+#
+# Empirically we measured:
+#   qemu 8.2.2 (Ubuntu 24.04 apt)      : test_xsimd at vlen=128 times out
+#   qemu 9.2.1 (Ubuntu 25.04 plucky)   : ditto
+#   qemu 10.0.8 (Debian trixie)        : ditto
+#   qemu 11.0.0 (Arch)  + gcc 15       : 367 cases / 5664 asserts in <10 min
+#
+# Ubuntu's `qemu-user-static` apt package will not catch up to qemu 11 in
+# the noble release cycle, so until ubuntu-latest ships qemu 11 we run the
+# whole job (build + test) inside `archlinux:latest`, which currently
+# provides qemu 11.0.0 + gcc 15.1.0. This trades a bit of compiler-version
+# coverage (clang variants, gcc-14) for a CI signal that actually
+# completes.
+#
+# References:
+#   QEMU 11.0.0 release notes:  https://www.qemu.org/2026/04/22/qemu-11-0-0/
+#   QEMU RVV slowdowns issue:   https://gitlab.com/qemu-project/qemu/-/issues/2137
+#   Ubuntu RVV vstart bug:      https://bugs.launchpad.net/ubuntu/+source/qemu/+bug/2095169
 name: RISC-V RVV cross-compilation build
 on: [push, pull_request]
 concurrency:
@@ -6,41 +35,24 @@ concurrency:
 jobs:
   build:
     runs-on: ubuntu-latest
+    container: archlinux:latest
     name: 'RISC-V RVV${{ matrix.vector_bits }}'
     strategy:
+      fail-fast: false
       matrix:
-        sys:
-          - { compiler: 'gcc', gcc_runtime: '14'}
-          - { compiler: 'clang', version: '17', gcc_runtime: '14'}
-          - { compiler: 'clang', version: '18', gcc_runtime: '14'}
         vector_bits:
           - 128
           - 256
           - 512
     steps:
-    - name: Setup GCC
+    - name: Setup toolchain and qemu
       run: |
-        sudo apt-get -y -qq update
-        sudo apt-get -y -qq --no-install-suggests --no-install-recommends install gcc-${{ matrix.sys.gcc_runtime }}-riscv64-linux-gnu g++-${{ matrix.sys.gcc_runtime }}-riscv64-linux-gnu
-        sudo update-alternatives --install /usr/bin/riscv64-linux-gnu-gcc riscv64-linux-gnu-gcc /usr/bin/riscv64-linux-gnu-gcc-${{ matrix.sys.gcc_runtime }} 20
-        sudo update-alternatives --install /usr/bin/riscv64-linux-gnu-g++ riscv64-linux-gnu-g++ /usr/bin/riscv64-linux-gnu-g++-${{ matrix.sys.gcc_runtime }} 20
-    - name: Setup LLVM
-      if: ${{ matrix.sys.compiler == 'clang' }}
-      run: |
-        # Install given LLVM version
-        curl -o llvm.sh https://apt.llvm.org/llvm.sh
-        chmod u+x llvm.sh
-        sudo ./llvm.sh ${{ matrix.sys.version }}
-        sudo ln -srf $(which clang-${{ matrix.sys.version }}) /usr/bin/clang
-        sudo ln -srf $(which clang++-${{ matrix.sys.version }}) /usr/bin/clang++
-        rm llvm.sh
-    - name: Setup QEMU
-      uses: docker/setup-qemu-action@v3.0.0
-      with:
-        platforms: riscv64
-    - name: Setup Ninja
-      run: |
-        sudo apt-get -y -qq install ninja-build
+        pacman -Sy --noconfirm
+        pacman -S --noconfirm --needed \
+          qemu-user-static riscv64-linux-gnu-gcc riscv64-linux-gnu-glibc \
+          cmake ninja git ca-certificates
+        qemu-riscv64-static --version
+        riscv64-linux-gnu-gcc --version | head -1
     - name: Checkout xsimd
       uses: actions/checkout@v6
     - name: Setup
@@ -53,7 +65,7 @@ jobs:
         -DTARGET_ARCH=generic
         -DCMAKE_C_FLAGS="-march=rv64gcv_zvl${{ matrix.vector_bits }}b_zba_zbb_zbs -mrvv-vector-bits=zvl"
         -DCMAKE_CXX_FLAGS="-march=rv64gcv_zvl${{ matrix.vector_bits }}b_zba_zbb_zbs -mrvv-vector-bits=zvl"
-        -DCMAKE_TOOLCHAIN_FILE=${{ github.workspace }}/.github/toolchains/${{ matrix.sys.compiler }}-riscv64-linux-gnu.cmake
+        -DCMAKE_TOOLCHAIN_FILE=${{ github.workspace }}/.github/toolchains/gcc-riscv64-linux-gnu.cmake
     - name: Build
       run: cmake --build _build
     - name: Set CPU feature test expectations
@@ -62,6 +74,7 @@ jobs:
         echo "XSIMD_TEST_CPU_ASSUME_SVE=0"    >> "$GITHUB_ENV"
         echo "XSIMD_TEST_CPU_ASSUME_RVV=1" >> "$GITHUB_ENV"
     - name: Testing xsimd
+      timeout-minutes: 15
       run: >
         QEMU_CPU="rv64,zba=true,zbb=true,zbs=true,v=true,vlen=${{ matrix.vector_bits }},elen=64,vext_spec=v1.0"
         QEMU_LD_PREFIX="/usr/riscv64-linux-gnu"