[clang] [CMake][Fuchsia] Compile runtimes with -ffuchsia-api-level (PR #211753)

[Roland McGrath via cfe-commits]

https://github.com/frobtech created https://github.com/llvm/llvm-project/pull/211753

Use the supplied Fuchsia SDK's version_history.json to deduce the
oldest API level still supported, and target that for toolchain
runtimes so they'll be runtime-compatible when linked into user
programs that target any SDK-supported API level.


>From 110d2087f27e22c8db4beadfbe70e8e0a907f705 Mon Sep 17 00:00:00 2001
From: Roland McGrath <mcgrathr at google.com>
Date: Fri, 24 Jul 2026 01:41:07 -0700
Subject: [PATCH] [CMake][Fuchsia] Compile runtimes with -ffuchsia-api-level

Use the supplied Fuchsia SDK's version_history.json to deduce the
oldest API level still supported, and target that for toolchain
runtimes so they'll be runtime-compatible when linked into user
programs that target any SDK-supported API level.
---
 clang/cmake/caches/Fuchsia-stage2.cmake | 20 +++++++++++++++++++-
 1 file changed, 19 insertions(+), 1 deletion(-)

diff --git a/clang/cmake/caches/Fuchsia-stage2.cmake b/clang/cmake/caches/Fuchsia-stage2.cmake
index a2c2a83be221b..be31d68efc5bc 100644
--- a/clang/cmake/caches/Fuchsia-stage2.cmake
+++ b/clang/cmake/caches/Fuchsia-stage2.cmake
@@ -199,13 +199,31 @@ foreach(target aarch64-unknown-linux-gnu;armv7-unknown-linux-gnueabihf;i386-unkn
 endforeach()
 
 if(FUCHSIA_SDK)
+  # Since there is only one build of the toolchain runtimes for users doing any
+  # supported -ffuchsia-api-level=... build, target the oldest unretired level.
+  file(READ "${FUCHSIA_SDK}/version_history.json" FUCHSIA_SDK_VERSION_HISTORY)
+  string(JSON FUCHSIA_SDK_API_LEVEL_COUNT LENGTH "${FUCHSIA_SDK_VERSION_HISTORY}" "data" "api_levels")
+  math(EXPR FUCHSIA_SDK_API_LEVEL_MAXIDX "${FUCHSIA_SDK_API_LEVEL_COUNT} - 1")
+  set(FUCHSIA_API_LEVEL 0)
+  foreach(idx RANGE ${FUCHSIA_SDK_API_LEVEL_MAXIDX})
+    string(JSON FUCHSIA_API_LEVEL_NAME MEMBER "${FUCHSIA_SDK_VERSION_HISTORY}" "data" "api_levels" ${idx})
+    string(JSON FUCHSIA_API_LEVEL_PHASE GET "${FUCHSIA_SDK_VERSION_HISTORY}" "data" "api_levels" "${FUCHSIA_API_LEVEL_NAME}" "phase")
+    if(NOT "${FUCHSIA_API_LEVEL_PHASE}" STREQUAL "retired")
+      set(FUCHSIA_API_LEVEL_INT ${FUCHSIA_API_LEVEL_NAME})
+      if(FUCHSIA_API_LEVEL EQUAL 0 OR FUCHSIA_API_LEVEL_INT LESS FUCHSIA_API_LEVEL)
+        set(FUCHSIA_API_LEVEL ${FUCHSIA_API_LEVEL_INT})
+      endif()
+    endif()
+  endforeach()
+  message(STATUS "Runtimes target Fuchsia API level ${FUCHSIA_API_LEVEL}")
+
   set(FUCHSIA_aarch64-unknown-fuchsia_NAME arm64)
   set(FUCHSIA_arm-unknown-fuchsia_NAME arm)
   set(FUCHSIA_i386-unknown-fuchsia_NAME x64)
   set(FUCHSIA_x86_64-unknown-fuchsia_NAME x64)
   set(FUCHSIA_riscv64-unknown-fuchsia_NAME riscv64)
   foreach(target i386-unknown-fuchsia;x86_64-unknown-fuchsia;aarch64-unknown-fuchsia;arm-unknown-fuchsia;riscv64-unknown-fuchsia)
-    set(FUCHSIA_${target}_COMPILER_FLAGS "--target=${target} -I${FUCHSIA_SDK}/pkg/sync/include -I${FUCHSIA_SDK}/pkg/fdio/include -fstack-size-section -fexperimental-call-graph-section")
+    set(FUCHSIA_${target}_COMPILER_FLAGS "--target=${target} -ffuchsia-api-level=${FUCHSIA_API_LEVEL} -I${FUCHSIA_SDK}/pkg/sync/include -I${FUCHSIA_SDK}/pkg/fdio/include -fstack-size-section -fexperimental-call-graph-section")
     set(FUCHSIA_${target}_LINKER_FLAGS "-L${FUCHSIA_SDK}/arch/${FUCHSIA_${target}_NAME}/lib")
     set(FUCHSIA_${target}_SYSROOT "${FUCHSIA_SDK}/arch/${FUCHSIA_${target}_NAME}/sysroot")
   endforeach()