test: execute the u256 intrinsics against vectors on the OpenVM runner

.github/workflows/build-release.yml

@@ -139,6 +139,12 @@ jobs:
             -o guest
           riscv64-linux-gnu-readelf -h guest | grep -E 'Class:|Machine:|Entry point'
           ls -la guest
+          bflat-riscv64/src/bflat/bin/Debug/net10.0/bflat build \
+            bflat-openvm/tests/integration/u256.cs \
+            --os linux --arch riscv64 --libc openvm --stdlib dotnet \
+            --no-pie --no-pthread -Ot \
+            --extlib "$PWD/bflat-openvm/output/libopenvm.bflat.manifest" \
+            -o u256
           bflat-riscv64/src/bflat/bin/Debug/net10.0/bflat build             bflat-openvm/tests/secp256k1/guest.cs             --os linux --arch riscv64 --libc openvm --stdlib dotnet             --no-pie --no-pthread -Ot             --extlib "$PWD/bflat-openvm/output/libopenvm.bflat.manifest"             -o secp256k1-guest
 
       - name: Upload the linked guests
@@ -149,6 +155,7 @@ jobs:
           name: integration-guest
           path: |
             guest
+            u256
             secp256k1-guest
 
       - name: Check the linked image
@@ -161,6 +168,36 @@ jobs:
         run: |
           python3 bflat-openvm/tests/integration/check_image.py guest --require-custom --no-atomics
 
+  run-u256-vectors:
+    # guest.cs only links the u256 shims; this executes them. A wrong funct7 or
+    # register order in openvm_intrinsics.S links fine and computes the wrong
+    # thing, so the vector guest runs on the OpenVM runner and must report zero
+    # mismatches over its 444 checks.
+    name: Run the u256 vectors on OpenVM
+    needs: link-guest
+    runs-on: ubuntu-latest
+    env:
+      OPENVM_IMAGE: nethermindeth/openvm-runner:v2.1.0-rv64@sha256:d8209a5db5d59df6d6f5899b2ce6866f81c30bdc3df952dc78243b8316706baa
+    steps:
+      - name: Check out repository
+        uses: actions/checkout@v7
+
+      - name: Download the linked guests
+        uses: actions/download-artifact@v4
+        with:
+          name: integration-guest
+          path: guests
+
+      - name: Execute the vector guest
+        run: |
+          cp runner/openvm.toml guests/
+          : > guests/empty.raw
+          out=$(docker run --rm -v "$PWD/guests:/n" "$OPENVM_IMAGE" /n/u256 /n/openvm.toml /n/empty.raw | tail -1)
+          echo "$out"
+          # Mismatches, then checks, as little-endian u64s.
+          [ "$out" = "public_values=0000000000000000bc0100000000000000000000000000000000000000000000" ] \
+            || { echo "::error::u256 vectors failed: $out"; exit 1; }
+
   run-secp256k1-guest:
     # The host test emulates the chips' preconditions; this runs the recovery
     # on OpenVM itself, against the same reference, adversarial vectors
@@ -208,7 +245,7 @@ jobs:
 
   build-nupkg:
     name: Build NuGet package
-    needs: [build-libopenvm, run-divrem-vectors, run-secp256k1-guest]
+    needs: [build-libopenvm, run-divrem-vectors, run-secp256k1-guest, run-u256-vectors]
     runs-on: ubuntu-latest
     permissions:
       contents: read

README.md

@@ -70,7 +70,9 @@ All 19 accelerators, plus both IO entry points:
 custom-0 and custom-1 instructions to catch a silent fallback to portable Rust.
 CI then links a real bflat guest that calls all 21 through direct P/Invoke
 (`tests/integration/guest.cs`) — which is what proves the symbols not only
-exist but survive a guest's link.
+exist but survive a guest's link. The 256-bit integer shims are also executed:
+`tests/integration/u256.cs` runs each against precomputed vectors on the
+OpenVM runner, out of place and over either operand.
 
 `zkvm_secp256k1_ecrecover` is ours rather than openvm-eth's, so it is also
 tested for results: `tests/secp256k1/test_ecrecover.py` diffs a host build of