[experiment] opt-in clang-cl support for Windows

[EgorBo]

Experiment: building CoreCLR's JIT (and ~most of native) on Windows with clang-cl

Opt-in path to build CoreCLR's native components on Windows using LLVM clang-cl instead of MSVC cl.exe. Not intended to merge as-is — opening as draft so we can discuss whether the approach is interesting and where it could go.

Two commits:

1. JIT-only (verified end-to-end): Clr.Jit builds + JIT smoke test passes with the clang-cl-built clrjit.dll (Vector SIMD, exception throw/catch, loop arithmetic — exit code 100, output identical to MSVC).
2. Root-level switch + broader native (~109/2551 native objects + libs + host before blocker): -clangcl promoted to a top-level build.cmd switch and many additional source/cmake fixes for CoreCLR runtime, libs native, and host. Hits one upstream LLVM crash that we couldn't sidestep (see "Known limitation").

Usage

Requires clang-cl.exe on %PATH% or under %ProgramFiles%\LLVM\bin (winget install LLVM.LLVM). Tested with clang 22.1.5.

# JIT only (works end-to-end):
build.cmd Clr.Jit -c Release -clangcl /p:NoPgoOptimize=true

# Full native (currently blocked by upstream LLVM bug, see below):
build.cmd clr+libs+host -c Release -clangcl /p:NoPgoOptimize=true

Build infrastructure

• eng/build.ps1 — new -clangcl switch. Sets =1 so it propagates through Arcade → MSBuild Exec → all three native build scripts (src/coreclr/build-runtime.cmd, src/native/libs/build-native.cmd, src/native/corehost/build.cmd). All three call eng/native/gen-buildsys.cmd which already reads __UseClangCl, so no further plumbing was needed.
• src/coreclr/build-runtime.cmd — -clangcl flag (also honors a pre-set __UseClangCl=1 env var).
• eng/native/gen-buildsys.cmd — discovers clang-cl.exe, injects -DCMAKE_C_COMPILER / -DCMAKE_CXX_COMPILER, forces Ninja generator.
• eng/native/configurecompiler.cmake — clang-cl branch inside if (MSVC). Each suppression empirically verified to actually fire (audit method: replace -Wno-foo with -Wno-error=foo and rebuild). IPO/LTO is also disabled for clang-cl since ThinLTO triggers additional LLVM crashes.

Source fixes (all MSVC-safe; verified MSVC builds still pass)

File	Why
utilcode/pedecoder.cpp	Stray line-continuation \ silently merged two STATIC_CONTRACT_* statements (real bug; was masking a clang frontend crash).
jit/gentree.cpp (line 31645)	Compare-greater-than scalar branch was missing id =, computing a ?: value and discarding it. Real JIT bug found by -Wunused-value.
jit/importercalls.cpp	Replace two impPopStack().val; discards with one impPopStack(2);.
jit/utils.cpp	(int) cast on size_t template arg compared against int.
inc/sigparser.h	operator= = default to satisfy -Wdeprecated-copy-with-user-provided-copy.
utilcode/debug.cpp	__assume(0) after RaiseFailFastException so clang treats FailFastOnAssert as truly noreturn.
utilcode/fstring.cpp + gc/gcpriv.h	Gate #pragma optimize("t", on) on _MSC_VER && !__clang__.
utilcode/pedecoder.cpp + utilcode/stacktrace.cpp	Explicit (const char *) and reinterpret_cast<LPVOID> casts.
minipal/{guid.c,xoshiro128pp.c}	(unsigned int)/size_t casts for format/sign-compare.
gc/vxsort/{vxsort,vxsort_targets_*,smallsort/*}.h + codegen/*.py	#ifdef __GNUC__ excluded clang-cl; widen to #if defined(__GNUC__) || defined(__clang__) so the AVX2/AVX512 __attribute__((target(...))) push gets emitted. The python codegen is updated too.
vm/threads.{h,cpp}	Split AreShadowStacksEnabled into an out-of-line definition decorated with [[gnu::target("shstk")]] for clang-cl, since cl.exe always exposes _rdsspq but clang-cl gates it behind the shstk target feature.
vm/dispatchinfo.h	Forward declaration enum BinderMethodID widened to enum BinderMethodID : int to match the actual definition's fixed underlying type.
native/eventpipe/ds-ipc-pal-namedpipe.c	Restructure case-fallthrough-with-goto pattern in ds_ipc_poll to use a flag — cleaner code, helps clang's CFG analyzer (but does NOT fully sidestep the upstream LLVM crash on this file).
native/libs/System.Globalization.Native/pal_icushim.c	(unsigned int) cast for %u of GetLastError().

Known limitation: upstream LLVM bug

clang-cl 22.1.5 crashes (clang frontend SIGSEGV) when compiling src/native/eventpipe/ds-ipc-pal-namedpipe.c. We tried:

• __attribute__((optnone)) on ds_ipc_poll
• __attribute__((noinline)) combined with optnone
• File-wide #pragma clang attribute push(__attribute__((optnone, noinline)), apply_to = function)
• Per-file /Od
• Per-file -mllvm -opt-bisect-limit=0 (which disables every LLVM pass)
• Disabling IPO/LTO globally for clang-cl
• Restructuring the goto-from-switch pattern that initially looked suspicious

None sidestep the crash, which appears to happen in clang's IR emission rather than in a pass we can disable. cl.exe builds the same source cleanly. This one file currently gates the rest of CoreCLR from building under clang-cl. Filing upstream is the next step — the file is small enough to make a usable repro from.

Verification

• ✅ MSVC build.cmd Clr.Jit -c Release /p:NoPgoOptimize=true still passes.
• ✅ clang-cl build.cmd Clr.Jit -c Release -clangcl /p:NoPgoOptimize=true passes; JIT smoke test succeeds (Vector, exception, loop) with output identical to MSVC.
• ⚠️ clang-cl build.cmd clr+libs+host -c Release -clangcl /p:NoPgoOptimize=true reaches CMake configure cleanly and compiles ~109/2551 native objects before being blocked by the LLVM bug above. Libs and host targets are unaffected by that one file and would build to completion.

Discussion points

• Worth landing the JIT-only path as an experimental opt-in knob even if the broader CoreCLR build is blocked upstream?
• Are the source fixes individually acceptable as standalone cleanups (regardless of the clang-cl story)? Notably gentree.cpp:31645 is a real JIT bug, and the \ removal in pedecoder.cpp fixes a typo cl.exe also miscompiles silently.
• Should -clangcl be promoted to a top-level build.cmd flag (as done here) or kept under -cmakeargs?

Note

This PR was authored with assistance from GitHub Copilot CLI (Claude Opus 4.7).

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[EgorBo]

seems like a bug that clang's warning highlighted

[Copilot]

Pull request overview

Experimental, opt-in path to build CoreCLR's JIT on Windows using LLVM clang-cl instead of MSVC cl.exe. Adds a -clangcl flag (and __UseClangCl env-var bridge) to the native build, wires CMake to locate clang-cl.exe and force the Ninja generator, adds a clang-cl-specific warning-suppression block in configurecompiler.cmake, and applies a handful of small source fixes (one of which is a real JIT bug — a missing assignment in GetHWIntrinsicIdForCmpOp's scalar greater-than branch).

Changes:

• New -clangcl opt-in build flag plumbed through build-runtime.cmd and gen-buildsys.cmd, with a clang-cl branch in configurecompiler.cmake that suppresses MSVC-only command-line warnings and clang-only diagnostics that fire widely in current sources.
• Source fixes to compile cleanly under clang-cl: stray line continuation in pedecoder.cpp (also a real bug), missing id = assignment in gentree.cpp (real JIT bug), impPopStack(2) instead of two discarded impPopStack().val, sign/cast tweaks (utils.cpp, stacktrace.cpp, pedecoder.cpp, guid.c, xoshiro128pp.c), defaulted operator= for SigParser, __assume(0) after RaiseFailFastException, and gating an MSVC #pragma optimize("t", on) on !__clang__.

Reviewed changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/coreclr/build-runtime.cmd	Adds -clangcl flag, env-var default, validation against -msbuild, and help text.
eng/native/gen-buildsys.cmd	When __UseClangCl=1, locates clang-cl.exe and injects -DCMAKE_C_COMPILER/-DCMAKE_CXX_COMPILER; requires Ninja.
eng/native/configurecompiler.cmake	Adds a clang-cl branch inside the existing if (MSVC) block to suppress unused-cli-arg and several clang-only diagnostics.
src/coreclr/jit/gentree.cpp	Adds missing id = assignment in scalar greater-than branch (real JIT bug).
src/coreclr/jit/importercalls.cpp	Replaces two discarded impPopStack().val statements with a single impPopStack(2).
src/coreclr/jit/utils.cpp	Casts bufferSize to int for sign-compare cleanliness.
src/coreclr/inc/sigparser.h	Adds defaulted operator= to satisfy -Wdeprecated-copy-with-user-provided-copy.
src/coreclr/utilcode/debug.cpp	__assume(0) after RaiseFailFastException so clang treats it as truly noreturn.
src/coreclr/utilcode/fstring.cpp	Gates #pragma optimize("t", on) on _MSC_VER && !__clang__.
src/coreclr/utilcode/pedecoder.cpp	Removes stray line-continuation that merged two STATIC_CONTRACT_* statements; explicit (const char *) cast for __unaligned.
src/coreclr/utilcode/stacktrace.cpp	Cast ARRAY_SIZE to int and reinterpret_cast<LPVOID> around GetProcAddress.
src/native/minipal/guid.c	(unsigned int) cast on guid.Data1 for %08x.
src/native/minipal/xoshiro128pp.c	Loop index changed from int to size_t to match sizeof-based bound.

[EgorBo]

Ran into a bunch of LLVM crashes, seems like it needs more efforts I was willing to put in.

Will upstream the bug-fix and some other source changes.