[llvm] [libc] Include CPU model in overlay CI sccache key (PR #196477)

[Jeff Bailey via llvm-commits]

https://github.com/kaladron updated https://github.com/llvm/llvm-project/pull/196477

>From 9ef656e1b612793e33c3c63d2bb18f946e872e4e Mon Sep 17 00:00:00 2001
From: Jeff Bailey <jbailey at raspberryginger.com>
Date: Fri, 8 May 2026 07:46:17 +0100
Subject: [PATCH 1/2] [libc] Include CPU model in overlay CI sccache key

The overlay CI compiles opt_host memory tests with -march=native,
which generates object files specific to the runner CPU model.
sccache treats -march=native as a literal string in its hash key,
so cached .o files compiled on one CPU model are served to runners
with a different CPU. When the cached binary uses instructions
the current CPU lacks, the test crashes with SIGILL.

Added a Detect CPU Model step that captures the CPU model string
and includes it in the sccache cache key. Runners with different
CPUs now get separate cache buckets, preventing cross-CPU cache
poisoning.

Assisted-by: Automated tooling, human reviewed.
---
 .github/workflows/libc-overlay-tests.yml | 21 ++++++++++++++++++++-
 1 file changed, 20 insertions(+), 1 deletion(-)

diff --git a/.github/workflows/libc-overlay-tests.yml b/.github/workflows/libc-overlay-tests.yml
index 07059e0271af5..96c3e23bc4896 100644
--- a/.github/workflows/libc-overlay-tests.yml
+++ b/.github/workflows/libc-overlay-tests.yml
@@ -44,6 +44,25 @@ jobs:
     - uses: actions/checkout at de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
       with:
         persist-credentials: false
+
+    # The libc build uses -march=native for opt_host tests, which means
+    # the generated object files are specific to the runner's CPU model.
+    # sccache treats -march=native as a literal string in its cache key,
+    # so without per-CPU cache keys, object files compiled on one CPU
+    # model are silently served to runners with a different CPU, causing
+    # illegal instruction crashes at runtime.
+    - name: Detect CPU model
+      id: cpu-info
+      shell: bash
+      run: |
+        if [ "$RUNNER_OS" = "Linux" ]; then
+          cpu_model=$(cat /proc/cpuinfo | grep 'model name' | head -1 | cut -d: -f2 | xargs | tr ' ' '-')
+        elif [ "$RUNNER_OS" = "macOS" ]; then
+          cpu_model=$(sysctl -n machdep.cpu.brand_string | tr ' ' '-')
+        else
+          cpu_model="generic"
+        fi
+        echo "cpu-model=${cpu_model}" >> "$GITHUB_OUTPUT"
     
     # Libc's build is relatively small comparing with other components of LLVM.
     # A fresh linux overlay takes about 180MiB of uncompressed disk space, which can
@@ -56,7 +75,7 @@ jobs:
       uses: hendrikmuhs/ccache-action at 33522472633dbd32578e909b315f5ee43ba878ce # v1.2.22
       with:
         max-size: 1G
-        key: libc_overlay_build_${{ matrix.os }}_${{ matrix.compiler.c_compiler }}
+        key: libc_overlay_build_${{ matrix.os }}_${{ matrix.compiler.c_compiler }}_${{ steps.cpu-info.outputs.cpu-model }}
         variant: sccache
     
     # MPFR is required by some of the mathlib tests.

>From eea69c08e8f8c506ac2394428aac7f61f1af1c49 Mon Sep 17 00:00:00 2001
From: Jeff Bailey <jbailey at raspberryginger.com>
Date: Fri, 8 May 2026 07:49:57 +0100
Subject: [PATCH 2/2] Fix ARM Linux CPU detection

ARM /proc/cpuinfo lacks a model name field. Fall back to CPU
implementer + CPU part for ARM. Guard the grep with || true to
prevent pipefail from aborting on a missing field.

Assisted-by: Automated tooling, human reviewed.
---
 .github/workflows/libc-overlay-tests.yml | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/.github/workflows/libc-overlay-tests.yml b/.github/workflows/libc-overlay-tests.yml
index 96c3e23bc4896..a020f0bfd5cd3 100644
--- a/.github/workflows/libc-overlay-tests.yml
+++ b/.github/workflows/libc-overlay-tests.yml
@@ -56,13 +56,19 @@ jobs:
       shell: bash
       run: |
         if [ "$RUNNER_OS" = "Linux" ]; then
-          cpu_model=$(cat /proc/cpuinfo | grep 'model name' | head -1 | cut -d: -f2 | xargs | tr ' ' '-')
+          # x86 has 'model name', ARM has 'CPU implementer' + 'CPU part'.
+          cpu_model=$(grep -m1 'model name' /proc/cpuinfo | cut -d: -f2 | xargs | tr ' ' '-' || true)
+          if [ -z "$cpu_model" ]; then
+            impl=$(grep -m1 'CPU implementer' /proc/cpuinfo | cut -d: -f2 | xargs)
+            part=$(grep -m1 'CPU part' /proc/cpuinfo | cut -d: -f2 | xargs)
+            cpu_model="arm-${impl:-unknown}-${part:-unknown}"
+          fi
         elif [ "$RUNNER_OS" = "macOS" ]; then
           cpu_model=$(sysctl -n machdep.cpu.brand_string | tr ' ' '-')
         else
           cpu_model="generic"
         fi
-        echo "cpu-model=${cpu_model}" >> "$GITHUB_OUTPUT"
+        echo "cpu-model=${cpu_model:-unknown}" >> "$GITHUB_OUTPUT"
     
     # Libc's build is relatively small comparing with other components of LLVM.
     # A fresh linux overlay takes about 180MiB of uncompressed disk space, which can