diff --git a/llvm-runtimes/compiler-rt/Manifest b/llvm-runtimes/compiler-rt/Manifest
index 83ba13a279677..40848786e7bce 100644
--- a/llvm-runtimes/compiler-rt/Manifest
+++ b/llvm-runtimes/compiler-rt/Manifest
@@ -17,3 +17,4 @@ DIST llvm-project-1c3a9a853ce68c3c539e7cab21380b9d8b18f426.tar.gz 229448743 BLAK
 DIST llvm-project-1d133420dc512487e2be0e9d86755f2a7c3497a7.tar.gz 229268288 BLAKE2B aca79bcca631deeab429a03cd2732d78d8ecf0c9615bce181445ed875436806609dc85a57d4f97109492833d620d060ea93fed450a4912c2d94597aeabf84974 SHA512 24ecadfccdf785bfbf27d5007afd2a4d5868493b2c5f48d6cf07c335bfbfef5cba4f12a639971e5f95877ae3b8c7b7fc0c210e404d21036af553de82a5059037
 DIST llvm-project-20.1.1.src.tar.xz 147215020 BLAKE2B d1774d060036b5340f765d68cc23cbd1e00730874cb3e65b7236303238d7922a7fa191b631c20c3bf17e54c933672a355d4424f2beea17cd25fcf9f5c4c625dd SHA512 b851d3e24a2775f6e789720614a8192d72956f447a39d75e5160a8acf6bd3a3386fab2cca5590b7e020b65e408ccf32432f8dc03519fd4cf60b4b5674fe0547e
 DIST llvm-project-20.1.1.src.tar.xz.sig 566 BLAKE2B 76c49401d9b8e283fc29586c32fc1b5205d76b3b83d0f99aa2ac09a63b058478e08588dcfbfc728110172b639eeaf04a884f4e6d707b94cad0dbf2d0dcb30a4f SHA512 469b03f3ddbe56b074724375b09a45ee2730bee0ba4b25f64f93d93ef609c33d39136f051c216a74a35d98a70ffcd3d7507e38d19a20d5006c29a954ca32813b
+DIST llvm-project-21eeca3db0341fef4ab4a6464ffe38b2eba5810c.tar.gz 229546432 BLAKE2B e783f08b93498b252741a07fd3aa3383ca93f66688f9bbdb2e11185c660dfff1020ab4a1ca803ab69ab28dc2fd8b5c3855f45c0b77851b8e6edeb0dc5bf6cbd4 SHA512 54420956b36f3bccabea2171a4103ca20d6b136e1d1f7f217e0ec735be4542bbd840ef228c55f3e9103ad2f9fb41393e611d5aaa1521d01fdd163b770b80ba0b
diff --git a/llvm-runtimes/compiler-rt/compiler-rt-21.0.0_pre20250329.ebuild b/llvm-runtimes/compiler-rt/compiler-rt-21.0.0_pre20250329.ebuild
new file mode 100644
index 0000000000000..812a99f9d2a6d
--- /dev/null
+++ b/llvm-runtimes/compiler-rt/compiler-rt-21.0.0_pre20250329.ebuild
@@ -0,0 +1,175 @@
+# Copyright 1999-2025 Gentoo Authors
+# Distributed under the terms of the GNU General Public License v2
+
+EAPI=8
+
+PYTHON_COMPAT=( python3_{10..13} )
+inherit cmake crossdev flag-o-matic llvm.org llvm-utils python-any-r1
+inherit toolchain-funcs
+
+DESCRIPTION="Compiler runtime library for clang (built-in part)"
+HOMEPAGE="https://llvm.org/"
+
+LICENSE="Apache-2.0-with-LLVM-exceptions || ( UoI-NCSA MIT )"
+SLOT="${LLVM_MAJOR}"
+IUSE="+abi_x86_32 abi_x86_64 +atomic-builtins +clang +debug test"
+REQUIRED_USE="atomic-builtins? ( clang )"
+RESTRICT="!test? ( test ) !clang? ( test )"
+
+DEPEND="
+	llvm-core/llvm:${LLVM_MAJOR}
+"
+BDEPEND="
+	clang? ( llvm-core/clang:${LLVM_MAJOR} )
+	test? (
+		$(python_gen_any_dep ">=dev-python/lit-15[\${PYTHON_USEDEP}]")
+		=llvm-core/clang-${LLVM_VERSION}*:${LLVM_MAJOR}
+	)
+	!test? (
+		${PYTHON_DEPS}
+	)
+"
+
+LLVM_COMPONENTS=( compiler-rt cmake llvm/cmake )
+LLVM_TEST_COMPONENTS=( llvm/include/llvm/TargetParser )
+llvm.org_set_globals
+
+python_check_deps() {
+	use test || return 0
+	python_has_version ">=dev-python/lit-15[${PYTHON_USEDEP}]"
+}
+
+pkg_pretend() {
+	if ! use clang && ! tc-is-clang; then
+		ewarn "Building using a compiler other than clang may result in broken atomics"
+		ewarn "library. Enable USE=clang unless you have a very good reason not to."
+	fi
+}
+
+pkg_setup() {
+	if target_is_not_host || tc-is-cross-compiler ; then
+		# strips vars like CFLAGS="-march=x86_64-v3" for non-x86 architectures
+		CHOST=${CTARGET} strip-unsupported-flags
+		# overrides host docs otherwise
+		DOCS=()
+	fi
+	python-any-r1_pkg_setup
+}
+
+test_compiler() {
+	target_is_not_host && return
+	$(tc-getCC) ${CFLAGS} ${LDFLAGS} "${@}" -o /dev/null -x c - \
+		<<<'int main() { return 0; }' &>/dev/null
+}
+
+src_configure() {
+	llvm_prepend_path "${LLVM_MAJOR}"
+
+	# LLVM_ENABLE_ASSERTIONS=NO does not guarantee this for us, #614844
+	use debug || local -x CPPFLAGS="${CPPFLAGS} -DNDEBUG"
+
+	# pre-set since we need to pass it to cmake
+	BUILD_DIR=${WORKDIR}/${P}_build
+
+	if use clang && ! is_crosspkg; then
+		# Only do this conditionally to allow overriding with
+		# e.g. CC=clang-13 in case of breakage
+		if ! tc-is-clang ; then
+			local -x CC=${CHOST}-clang
+			local -x CXX=${CHOST}-clang++
+		fi
+
+		strip-unsupported-flags
+	fi
+
+	if ! is_crosspkg && ! test_compiler ; then
+		local nolib_flags=( -nodefaultlibs -lc )
+
+		if test_compiler "${nolib_flags[@]}"; then
+			local -x LDFLAGS="${LDFLAGS} ${nolib_flags[*]}"
+			ewarn "${CC} seems to lack runtime, trying with ${nolib_flags[*]}"
+		elif test_compiler "${nolib_flags[@]}" -nostartfiles; then
+			# Avoiding -nostartfiles earlier on for bug #862540,
+			# and set available entry symbol for bug #862798.
+			nolib_flags+=( -nostartfiles -e main )
+
+			local -x LDFLAGS="${LDFLAGS} ${nolib_flags[*]}"
+			ewarn "${CC} seems to lack runtime, trying with ${nolib_flags[*]}"
+		fi
+	fi
+
+	local mycmakeargs=(
+		-DCOMPILER_RT_INSTALL_PATH="${EPREFIX}/usr/lib/clang/${LLVM_MAJOR}"
+
+		-DCOMPILER_RT_EXCLUDE_ATOMIC_BUILTIN=$(usex !atomic-builtins)
+		-DCOMPILER_RT_INCLUDE_TESTS=$(usex test)
+		-DCOMPILER_RT_BUILD_CTX_PROFILE=OFF
+		-DCOMPILER_RT_BUILD_LIBFUZZER=OFF
+		-DCOMPILER_RT_BUILD_MEMPROF=OFF
+		-DCOMPILER_RT_BUILD_ORC=OFF
+		-DCOMPILER_RT_BUILD_PROFILE=OFF
+		-DCOMPILER_RT_BUILD_SANITIZERS=OFF
+		-DCOMPILER_RT_BUILD_XRAY=OFF
+
+		-DPython3_EXECUTABLE="${PYTHON}"
+	)
+
+	if use amd64 && ! target_is_not_host; then
+		mycmakeargs+=(
+			-DCAN_TARGET_i386=$(usex abi_x86_32)
+			-DCAN_TARGET_x86_64=$(usex abi_x86_64)
+		)
+	fi
+
+	if is_crosspkg; then
+		# Needed to target built libc headers
+		export CFLAGS="${CFLAGS} -isystem /usr/${CTARGET}/usr/include"
+		mycmakeargs+=(
+			# Without this, the compiler will compile a test program
+			# and fail due to no builtins.
+			-DCMAKE_C_COMPILER_WORKS=1
+			-DCMAKE_CXX_COMPILER_WORKS=1
+
+			# Without this, compiler-rt install location is not unique
+			# to target triples, only to architecture.
+			# Needed if you want to target multiple libcs for one arch.
+			-DLLVM_ENABLE_PER_TARGET_RUNTIME_DIR=ON
+
+			-DCMAKE_ASM_COMPILER_TARGET="${CTARGET}"
+			-DCMAKE_C_COMPILER_TARGET="${CTARGET}"
+			-DCOMPILER_RT_DEFAULT_TARGET_ONLY=ON
+		)
+	fi
+
+	if use prefix && [[ "${CHOST}" == *-darwin* ]] ; then
+		mycmakeargs+=(
+			# setting -isysroot is disabled with compiler-rt-prefix-paths.patch
+			# this allows adding arm64 support using SDK in EPREFIX
+			-DDARWIN_macosx_CACHED_SYSROOT="${EPREFIX}/MacOSX.sdk"
+			# Set version based on the SDK in EPREFIX.
+			# This disables i386 for SDK >= 10.15
+			-DDARWIN_macosx_OVERRIDE_SDK_VERSION="$(realpath "${EPREFIX}/MacOSX.sdk" | sed -e 's/.*MacOSX\(.*\)\.sdk/\1/')"
+			# Use our libtool instead of looking it up with xcrun
+			-DCMAKE_LIBTOOL="${EPREFIX}/usr/bin/${CHOST}-libtool"
+		)
+	fi
+
+	if use test; then
+		mycmakeargs+=(
+			-DLLVM_EXTERNAL_LIT="${EPREFIX}/usr/bin/lit"
+			-DLLVM_LIT_ARGS="$(get_lit_flags)"
+
+			-DCOMPILER_RT_TEST_COMPILER="${EPREFIX}/usr/lib/llvm/${LLVM_MAJOR}/bin/clang"
+			-DCOMPILER_RT_TEST_CXX_COMPILER="${EPREFIX}/usr/lib/llvm/${LLVM_MAJOR}/bin/clang++"
+		)
+	fi
+
+	cmake_src_configure
+}
+
+src_test() {
+	# respect TMPDIR!
+	local -x LIT_PRESERVES_TMP=1
+
+	cmake_build check-builtins
+}