diff --git a/.github/workflows/issue-write.yml b/.github/workflows/issue-write.yml
index 1e0d0649a6a8a..e1201c726e549 100644
--- a/.github/workflows/issue-write.yml
+++ b/.github/workflows/issue-write.yml
@@ -10,6 +10,7 @@ on:
       - "CI Checks"
       - "Test Issue Write"
       - "Check LLVM ABI annotations"
+      - "Diff test-suite codegen"
     types:
       - completed
 
diff --git a/.github/workflows/test-suite.yml b/.github/workflows/test-suite.yml
new file mode 100644
index 0000000000000..74e99e66f55ba
--- /dev/null
+++ b/.github/workflows/test-suite.yml
@@ -0,0 +1,156 @@
+# When /test-suite is commented on a PR, checks out the PR, builds clang and
+# then the test-suite in several configurations. It then checks out the base of
+# the PR, builds clang and the test-suite again, and then uploads the diff of
+# the codegen.
+
+# TODO: React to the comment to let the user know the workflow has kicked
+# off. We'll need to do this in a separate workflow like issue-write.yml with
+# elevated permissions, since this workflow can't have write permissions on
+# account for the arbitrary code execution.
+
+name: Diff test-suite codegen
+
+on:
+  issue_comment:
+    types:
+      - created
+
+permissions:
+  contents: read
+
+jobs:
+  test-suite:
+    name: Build test-suite and diff
+    runs-on: ubuntu-24.04
+    container:
+      image: ghcr.io/llvm/ci-ubuntu-24.04:35e958d7f0b7
+    if: github.event.issue.pull_request && startswith(github.event.comment.body, '/test-suite')
+    steps:
+      - name: Check permissions
+        uses: actions/github-script@3a2844b7e9c422d3c10d287c895573f7108da1b3 # v9.0.0
+        with:
+          script: |
+            await github.rest.teams.getMembershipForUserInOrg({
+               org: 'llvm',
+               team_slug: 'llvm-committers',
+               username: context.actor
+            })
+      - name: Get pull request
+        id: get-pr
+        uses: actions/github-script@3a2844b7e9c422d3c10d287c895573f7108da1b3 # v9.0.0
+        with:
+          script: |
+            const { data: pr } = await github.rest.pulls.get({
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+              pull_number: context.payload.issue.number
+            })
+            return pr
+      - name: Check pull request is mergeable
+        if: ${{ !fromJSON(steps.get-pr.outputs.result).mergeable }}
+        run: |
+          cat << EOF > comments
+          [{"body": "Unable to diff test-suite with PR, PR isn't mergeable"}]
+          EOF
+          exit 1
+      - name: Checkout pull request
+        uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
+        with:
+          ref: ${{ fromJSON(steps.get-pr.outputs.result).merge_commit_sha }}
+          repository: ${{ fromJSON(steps.get-pr.outputs.result).head.repo.full_name }}
+          fetch-depth: 2
+          path: llvm-project
+          persist-credentials: false
+      - name: Checkout llvm/llvm-test-suite
+        uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
+        with:
+          repository: llvm/llvm-test-suite
+          path: llvm-test-suite
+          persist-credentials: false
+      - name: Setup environment variables
+        run: |
+          echo "HEAD_SHA=$(git rev-parse HEAD)" >> $GITHUB_ENV
+          echo "BASE_SHA=$(git rev-parse HEAD^)" >> $GITHUB_ENV
+          SCRIPTS_DIR="$GITHUB_WORKSPACE/llvm-project/.github/workflows/test-suite"
+          echo "SCRIPTS_DIR=$SCRIPTS_DIR" >> $GITHUB_ENV
+          echo "$SCRIPTS_DIR" >> $GITHUB_PATH
+          echo "$GITHUB_WORKSPACE/llvm-project/build/bin" >> $GITHUB_PATH
+        working-directory: llvm-project
+      - name: Install system dependencies
+        run: |
+          # Install newer version of CMake (llvm/ci-ubuntu-24.04 image has CMake 3.28)
+          # TODO(boomanaiden154): Remove once upgraded to Ubuntu 26.04
+          sudo apt-get update
+          sudo apt-get install -y wget
+          wget -O - https://apt.kitware.com/keys/kitware-archive-latest.asc 2>/dev/null | gpg --dearmor - | sudo tee /usr/share/keyrings/kitware-archive-keyring.gpg >/dev/null
+          echo 'deb [signed-by=/usr/share/keyrings/kitware-archive-keyring.gpg] https://apt.kitware.com/ubuntu/ noble main' | sudo tee /etc/apt/sources.list.d/kitware.list >/dev/null
+
+          sudo apt-get update
+          sudo apt-get install -y cmake tcl libc6-dev-arm64-cross libc6-dev-riscv64-cross libgcc-14-dev-arm64-cross libgcc-14-dev-riscv64-cross libstdc++-14-dev-arm64-cross libstdc++-14-dev-riscv64-cross
+      - name: Configure Clang
+        run: cmake -B build -C $SCRIPTS_DIR/llvm.cmake llvm -GNinja
+        working-directory: llvm-project
+      - name: Build Clang @ head
+        run: ninja -C build
+        working-directory: llvm-project
+      - name: Configure and build test-suite @ head
+        run: |
+          configure-and-build.sh rva23u64-O3-head $SCRIPTS_DIR/riscv64.cmake
+          configure-and-build.sh armv9-a-O3-head $SCRIPTS_DIR/aarch64.cmake
+          configure-and-build.sh x86_64-O3-head $SCRIPTS_DIR/x86_64.cmake
+        working-directory: llvm-test-suite
+      - name: Build Clang @ base
+        run: git checkout $BASE_SHA && ninja -C build
+        working-directory: llvm-project
+      - name: Configure and build test-suite @ base
+        run: |
+          configure-and-build.sh rva23u64-O3-base $SCRIPTS_DIR/riscv64.cmake
+          configure-and-build.sh armv9-a-O3-base $SCRIPTS_DIR/aarch64.cmake
+          configure-and-build.sh x86_64-O3-base $SCRIPTS_DIR/x86_64.cmake
+        working-directory: llvm-test-suite
+      - name: Compute diffs
+        run: |
+          mkdir diffs
+          ./utils/tdiff.py -a build.rva23u64-O3-base -b build.rva23u64-O3-head -s all > diffs/rva23u64-O3.diff || true
+          ./utils/tdiff.py -a build.armv9-a-O3-base -b build.armv9-a-O3-head -s all > diffs/armv9-a-O3.diff || true
+          ./utils/tdiff.py -a build.x86_64-O3-base -b build.x86_64-O3-head -s all > diffs/x86_64-O3.diff || true
+        working-directory: llvm-test-suite
+      - name: Upload diffs
+        uses: actions/upload-artifact@bbbca2ddaa5d8feaa63e36b76fdaad77386f024f #v7.0.0
+        id: upload-diffs
+        with:
+          name: diffs
+          path: llvm-test-suite/diffs
+      - name: Upload results
+        uses: actions/upload-artifact@bbbca2ddaa5d8feaa63e36b76fdaad77386f024f #v7.0.0
+        with:
+          name: results
+          path: llvm-test-suite/results*.json
+      - name: Create comment
+        env:
+	  ARTIFACT_URL: ${{ steps.upload-diffs.outputs.artifact-url }}
+        run: |
+          cat << EOF > comments
+          [{"body" : "test-suite diff from $BASE_SHA...$HEAD_SHA: $ARTIFACT_URL"}]
+          EOF
+      - name: Create comment on failure
+        if: failure()
+        env:
+          RUN_URL: ${{ github.server_url }}/${{ github.repository }}/actions/runs/${{ github.run_id }}
+        run: |
+          cat << EOF > comments
+          [{"body" : "Failed to get test-suite diff from $BASE_SHA...$HEAD_SHA: $RUN_URL"}]
+          EOF
+      - name: Save PR number
+        if: always()
+        env:
+          PR_NUMBER: ${{ fromJSON(steps.get-pr.outputs.result).number }}
+        run: echo $PR_NUMBER > pr_number
+      - name: Upload comment and PR number
+        uses: actions/upload-artifact@bbbca2ddaa5d8feaa63e36b76fdaad77386f024f # v7.0.0
+        if: always()
+        with:
+          name: workflow-args
+          path: |
+            comments
+            pr_number
diff --git a/.github/workflows/test-suite/aarch64.cmake b/.github/workflows/test-suite/aarch64.cmake
new file mode 100644
index 0000000000000..ecc69bb5e0f3c
--- /dev/null
+++ b/.github/workflows/test-suite/aarch64.cmake
@@ -0,0 +1,9 @@
+set(CMAKE_SYSTEM_NAME Linux)
+set(CMAKE_C_COMPILER $ENV{GITHUB_WORKSPACE}/llvm-project/build/bin/clang)
+set(CMAKE_CXX_COMPILER $ENV{GITHUB_WORKSPACE}/llvm-project/build/bin/clang++)
+set(CMAKE_C_COMPILER_TARGET aarch64-linux-gnu)
+set(CMAKE_CXX_COMPILER_TARGET aarch64-linux-gnu)
+set(CMAKE_C_FLAGS_INIT "-march=armv9-a -save-temps=obj")
+set(CMAKE_CXX_FLAGS_INIT "-march=armv9-a -save-temps=obj")
+set(CMAKE_SYSTEM_PROCESSOR arm64)
+set(CMAKE_LINKER_TYPE LLD)
diff --git a/.github/workflows/test-suite/configure-and-build.sh b/.github/workflows/test-suite/configure-and-build.sh
new file mode 100755
index 0000000000000..3b78f88f9c6ca
--- /dev/null
+++ b/.github/workflows/test-suite/configure-and-build.sh
@@ -0,0 +1,12 @@
+#!/bin/sh
+
+set -eux
+
+cmake -B build.$1 \
+      --toolchain $2 \
+      -C cmake/caches/O3.cmake \
+      -GNinja \
+      -DTEST_SUITE_BENCHMARKING_ONLY=ON \
+      -DTEST_SUITE_RUN_BENCHMARKS=OFF
+ninja -C build.$1
+llvm-lit build.$1 -o results.$1.json
diff --git a/.github/workflows/test-suite/llvm.cmake b/.github/workflows/test-suite/llvm.cmake
new file mode 100644
index 0000000000000..ae7ef254eb524
--- /dev/null
+++ b/.github/workflows/test-suite/llvm.cmake
@@ -0,0 +1,6 @@
+set(CMAKE_BUILD_TYPE Release CACHE STRING "")
+set(LLVM_TARGETS_TO_BUILD "AArch64;RISCV;X86" CACHE STRING "")
+set(LLVM_ENABLE_PROJECTS "clang;lld" CACHE STRING "")
+set(LLVM_APPEND_VC_REV OFF CACHE BOOL "")
+set(CMAKE_C_COMPILER_LAUNCHER sccache CACHE STRING "")
+set(CMAKE_CXX_COMPILER_LAUNCHER sccache CACHE STRING "")
diff --git a/.github/workflows/test-suite/riscv64.cmake b/.github/workflows/test-suite/riscv64.cmake
new file mode 100644
index 0000000000000..47499a6e14ec7
--- /dev/null
+++ b/.github/workflows/test-suite/riscv64.cmake
@@ -0,0 +1,9 @@
+set(CMAKE_SYSTEM_NAME Linux)
+set(CMAKE_C_COMPILER $ENV{GITHUB_WORKSPACE}/llvm-project/build/bin/clang)
+set(CMAKE_CXX_COMPILER $ENV{GITHUB_WORKSPACE}/llvm-project/build/bin/clang++)
+set(CMAKE_C_COMPILER_TARGET riscv64-linux-gnu)
+set(CMAKE_CXX_COMPILER_TARGET riscv64-linux-gnu)
+set(CMAKE_C_FLAGS_INIT "-march=rva23u64 -save-temps=obj")
+set(CMAKE_CXX_FLAGS_INIT "-march=rva23u64 -save-temps=obj")
+set(CMAKE_SYSTEM_PROCESSOR riscv64)
+set(CMAKE_LINKER_TYPE LLD)
diff --git a/.github/workflows/test-suite/x86_64.cmake b/.github/workflows/test-suite/x86_64.cmake
new file mode 100644
index 0000000000000..943630267dfcc
--- /dev/null
+++ b/.github/workflows/test-suite/x86_64.cmake
@@ -0,0 +1,5 @@
+set(CMAKE_C_COMPILER $ENV{GITHUB_WORKSPACE}/llvm-project/build/bin/clang)
+set(CMAKE_CXX_COMPILER $ENV{GITHUB_WORKSPACE}/llvm-project/build/bin/clang++)
+set(CMAKE_C_FLAGS_INIT "-save-temps=obj")
+set(CMAKE_CXX_FLAGS_INIT "-save-temps=obj")
+set(CMAKE_LINKER_TYPE LLD)