Upgrade Llvm

Upgrading the vendored LLVM in ponyc

ponyc vendors LLVM as a submodule at lib/llvm/src, pinned by hash in lib/CMakeLists.txt and patched from lib/llvm/patches/. Upgrading it means moving the pin, re-checking the patches, fixing whatever LLVM removed or changed under src/libponyc/codegen/, and rebuilding LLVM from source.

Run every command from the repo root.

Gotchas that will bite you (read first)

• The submodule is a shallow clone. lib/CMakeLists.txt clones it with --depth 1 and fetches no tags. A plain git fetch origin tag llvmorg-XX.Y.Z deepens it to the whole of llvm-project's history up to that tag. Pass --depth 1.
• A patch that fails the forward check has not necessarily been upstreamed. The forward check also fails when the patch is already applied to the tree, and when git apply cannot read the tree at all. Work through step 4 before you delete anything.
• Don't rm -rf build. That directory holds every configured build tree, not just LLVM's. The vendored LLVM builds in build/build_libs and installs to build/libs; remove only those two.
• Stage the submodule pointer before you build. cmake -P lib/build-libs.cmake runs git submodule update --init, which resets lib/llvm/src to the recorded pointer. A pointer change that exists only in the working tree is lost.
• ponyc still builds when an intrinsic's signature changes. The failure comes later, when ponyc compiles a Pony program, so run the tests and not just the build.

Strategy

Upgrade one major version at a time, and give each version its own commit. One PR can carry more than one version. Separate commits show which LLVM version each source change came from, and let you bisect the branch when something breaks. ponylang repos squash-merge, so those commits become one commit on main — you can only bisect them while the branch is open.

Steps

Steps 1, 8, and 9 run once for the PR. Steps 2 through 7 repeat for each major version you move through.

1. Branch

Branch off main.

2. Update the submodule

# Discard any patches applied to the tree
git -C lib/llvm/src checkout -- .
# Fetch and check out the new tag, keeping the clone shallow
git -C lib/llvm/src fetch --depth 1 origin tag llvmorg-XX.Y.Z
git -C lib/llvm/src checkout llvmorg-XX.Y.Z

git checkout -- . throws away every modification to a tracked file in the submodule, including a patch you rebased by hand. Save that work outside the tree first.

3. Update LLVM_DESIRED_HASH

Get the new commit hash:

git -C lib/llvm/src rev-parse HEAD

Set LLVM_DESIRED_HASH in lib/CMakeLists.txt to this value.

4. Check the patches

The build applies every .diff in lib/llvm/patches/, so check each one. Run the same check the build runs, with the same flags:

git -C lib/llvm/src apply --check -p 1 --ignore-whitespace --whitespace=nowarn \
  ../patches/<patch>.diff

If that fails, run the reverse check before concluding anything:

git -C lib/llvm/src apply --reverse --check -p 1 --ignore-whitespace \
  --whitespace=nowarn ../patches/<patch>.diff

• Forward check passes: the patch still applies. Keep it.
• Forward fails, reverse passes: the change is already in the tree. On a tree you reset in step 2, LLVM upstreamed it, so delete the patch file. On a tree you did not reset, an earlier build applied it — reset and check again.
• Both fail: either LLVM moved the code the patch touches, or git apply could not read the tree. In the second case git apply reports fatal: not a git repository, which comes from a dangling submodule gitlink — a .git file pointing at a .git/modules/... target that isn't there, left behind when the tree was copied or rsynced without the parent's .git/modules. Otherwise, rebase the patch onto the new source rather than deleting it.

5. Commit the submodule pointer

git add lib/llvm/src lib/CMakeLists.txt
git commit -m "Upgrade LLVM AA.B.C -> XX.Y.Z"

cmake -P lib/build-libs.cmake resets the submodule to the pointer recorded in the index, so stage this before step 6 or the new pointer is lost. Amend this commit as the next two steps produce changes.

6. Build, and fix what breaks

rm -rf build/libs build/build_libs         # Drop the old LLVM; the next command rebuilds it
cmake -DJOBS=<n> -P lib/build-libs.cmake   # Build LLVM; see BUILD.md for picking <n>
cmake --preset release                     # Configure the ponyc build
cmake --build --preset release             # Build ponyc

JOBS defaults to 4. BUILD.md says to pass your own core count and keep it modest, because LLVM is memory-hungry. On Windows use the Windows presets (BUILD.md).

Two kinds of failure come out of this step.

A patch-hash mismatch, if you added, removed, or edited a patch file in step 4. The lib configure step prints both hashes and stops:

Patch hash actual '<actual>' does not match desired '<desired>'

Copy <actual> into PATCHES_DESIRED_HASH in lib/CMakeLists.txt. The hash is a SHA256 over the seed string needed_if_no_patches and the SHA256 of each patch file's text with newlines squashed to spaces, which is awkward to reproduce in a shell — let CMake compute it. If no patches remain at all, the value is SHA256("needed_if_no_patches") = 3e16c097794cb669a8f6a0bd7600b440205ac5c29a6135750c2e83263eb16a95.

Compile errors, from LLVM APIs that were removed or changed. See "Common migration patterns" below. Fix them, amend the commit, and build again.

7. Run the tests

ctest --preset release -L ci-core

The ci-core label covers libponyc.tests, where the codegen assertions live, along with the full-program, stdlib, and examples tests. Running only full-programs-release skips the codegen gtests.

This is narrower than what the PR will run: CI runs ci-core in both debug and release on Linux, macOS, and Windows, and separately builds and runs pony-compiler-tests. A failure that only shows up in one config, or on one platform, will not appear here.

To reproduce a tier-3 BSD failure, the create-dragonfly-dev-env, create-freebsd-dev-env, and create-openbsd-dev-env skills stand up a local VM matching that CI job.

8. Add a release note

An LLVM upgrade is user-facing. Add .release-notes/update-llvm-to-XX.md:

## Update to LLVM XX.Y.Z

We've updated the LLVM version used to build Pony from AA.B.C to XX.Y.Z.

Do not edit next-release.md; CI aggregates the individual files.

9. Push and open the PR

Title the PR Update to LLVM XX.Y.Z, matching the release note. Before merging, a Pony team member adds a changelog - changed label, which puts a line built from the PR title into that CHANGELOG section. That is a separate mechanism from the release note file, which reaches next-release.md on its own.

Common migration patterns

LLVMConst* to LLVMBuild* (constant expression removal)

LLVM removed a number of constant expression functions. The LLVMBuild* equivalents constant-fold when given constant operands, so they compute the same thing; they take a builder and a name as well:

// Before:
return LLVMConstShl(l_value, r_value);
// After:
return LLVMBuildShl(c->builder, l_value, r_value, "");

LLVMConstICmp, LLVMConstFCmp, LLVMConstShl, and LLVMConstMul are gone from LLVM 22.1.6's Core.h. LLVMConstAdd and LLVMConstSub are still there and carry no deprecation, but ponyc calls none of the six.

Deprecated to newer API variants

On the clang and gcc builds ponyc's C files compile with -Werror, so a deprecated LLVM C function fails the build if its declaration carries LLVM_ATTRIBUTE_C_DEPRECATED, which produces a compiler warning. The MSVC build passes /wd4996 alongside /WX, so the same call compiles clean on Windows.

A function marked deprecated only in a doc comment produces no warning anywhere. The headers use two spellings for that, @deprecated and a plain Deprecated: use X instead, so grep for both when you want to know what ponyc calls that upstream has marked. The list below is a starting point, not a complete one.

LLVMGetMDKindID carried the attribute rather than a doc comment, so it did fail the build. That migration is done; src/ now calls LLVMGetMDKindIDInContext. LLVMBuildNUWNeg also carries the attribute — its replacement is LLVMBuildNeg followed by LLVMSetNUW, which ponyc wraps as LLVMSetNoUnsignedWrap in host.cc. LLVMBuildNSWNeg is not deprecated.

A new link-time dependency

An LLVM upgrade can make ponyc need a system library it did not need before, and the need can be Windows-only, so a local Linux build will not show it. The LLVM 21 upgrade (22e84c4a1) added ntdll.lib to the Windows link line in src/libponyc/platform/vcvars.c and to three CMakeLists.txt files.

Intrinsic signature changes

LLVM can change an intrinsic's signature in a major version. These do not show up as compile errors; ponyc builds clean. They surface as a module verification failure when ponyc compiles a Pony program (Incorrect number of arguments passed to called function!), which is why step 7 runs the tests.

• LLVM 22: llvm.lifetime.start and llvm.lifetime.end dropped their leading i64 size argument and now take a single pointer. ponyc emits these in gencall_lifetime_start and gencall_lifetime_end (gencall.c). Drop the size operand, drop the type parameter the size was computed from, and call with one argument:

// Before (2 args: size, ptr):
LLVMValueRef args[2];
args[0] = LLVMConstInt(c->i64, size, false);
args[1] = ptr;
LLVMBuildCall2(c->builder, func_type, func, args, 2, "");
// After (1 arg: ptr):
LLVMBuildCall2(c->builder, func_type, func, &ptr, 1, "");

CodegenTest.LifetimeIntrinsicsHaveSinglePointerArgument in test/libponyc/codegen.cc asserts the emitted argument count. When a later LLVM changes another intrinsic's signature, assert its argument count the same way.

C++ API changes

ponyc also embeds LLD (genexe.cc) and clang's frontend (gencshim.cc). Those C++ APIs carry no stability guarantee and break more often than LLVM's C API.

• LLVM 19: DIBuilder::insertDeclare returns DbgInstPtr (PointerUnion) instead of Instruction*. If callers don't use the return value, change the wrapper's return type to void.
• LLVM 20: Intrinsic::getDeclaration to Intrinsic::getOrInsertDeclaration.
• LLVM 20: Optimizer extension point callbacks (registerOptimizerEarlyEPCallback, registerOptimizerLastEPCallback) now take a ThinOrFullLTOPhase parameter in the lambda.
• LLVM 21: Attribute::NoCapture to Attribute::getWithCaptureInfo(ctx, CaptureInfo::none()).
• LLVM 21: LintPass() requires a bool AbortOnError parameter: LintPass(false).
• LLVM 22: createTargetMachine takes a const Triple& instead of a StringRef; wrap with Triple(opt->triple). ponyc made this change under LLVM 21, in 3eae0a119, while the StringRef overload was still there and deprecated.

APIs to check on every upgrade

APIs ponyc calls that a future LLVM may break:

The first three carry an @deprecated doc comment; the other four are not deprecated. The build fails once one of them gains the deprecation attribute or is removed. Re-check this table against the new headers as part of the upgrade.

Files typically modified

• lib/CMakeLists.txt — LLVM_DESIRED_HASH, PATCHES_DESIRED_HASH
• lib/llvm/patches/ — patch files, added or removed
• .release-notes/ — the release note
• src/libponyc/codegen/genoperator.c — constant expression replacements
• src/libponyc/codegen/genreference.c — constant expression replacements
• src/libponyc/codegen/gendebug.cc, gendebug.h — debug info API changes
• src/libponyc/codegen/host.cc — target machine, intrinsics
• src/libponyc/codegen/genopt.cc — optimizer pass pipeline
• src/libponyc/codegen/gendesc.c — array and descriptor construction
• src/libponyc/codegen/genident.c — array construction
• src/libponyc/codegen/codegen.c — core codegen types and helpers
• src/libponyc/codegen/gencontrol.c — metadata APIs
• src/libponyc/codegen/gencall.c, gencall.h — intrinsic emission (lifetime markers, memcpy and memmove)
• src/libponyc/codegen/genexe.cc — embedded LLD
• src/libponyc/codegen/gencshim.cc — embedded clang frontend
• src/libponyc/platform/vcvars.c, src/ponyc/CMakeLists.txt, test/libponyc/CMakeLists.txt, benchmark/libponyc/CMakeLists.txt — link dependencies
• test/libponyc/codegen.cc — codegen assertions

A bigger LLVM can also push a CI job past its disk limit. During the LLVM 22 upgrade we moved the OpenBSD tier-3 job onto a dedicated 50 GB data disk; the script that creates it is in .ci-scripts/bsd/.