bootstrap: work around GCC bug

GCC bug 119085, which affects versions 13.0--15.1, is being triggered by
the C backend, resulting in a (small but critical) miscompilation in
zig2. Unfortunately, we can't really work around that bug without a
command-line flag, because the bug is sensitive to things like the order
in which `struct` types are defined.

Therefore, I have added an option to `bootstrap.c` which causes it to
pass the appropriate flag to `gcc` to disable the optimization pass in
question. This flag can likely be removed in future, once the affected
GCC versions become less common.

At least one of our x86_64-linux CI machines is using an affected GCC
version, so I also updated the relevant CI script to pass this flag when
testing no-LLVM bootstrap.

CMakeLists also has a workaround, but no user intervention is required
there: the GCC version range is automatically detected by CMakeLists.

CMakeLists.txt

@@ -330,7 +330,6 @@ set(ZIG_STAGE2_SOURCES
     src/Air/Liveness.zig
     src/Air/Liveness/Verify.zig
     src/Air/print.zig
-    src/Air/types_resolved.zig
     src/Builtin.zig
     src/Compilation.zig
     src/Compilation/Config.zig
@@ -344,6 +343,7 @@ set(ZIG_STAGE2_SOURCES
     src/Sema.zig
     src/Sema/bitcast.zig
     src/Sema/comptime_ptr_access.zig
+    src/Sema/type_resolution.zig
     src/Type.zig
     src/Value.zig
     src/Zcu.zig
@@ -360,7 +360,8 @@ set(ZIG_STAGE2_SOURCES
     src/codegen/aarch64/Mir.zig
     src/codegen/aarch64/Select.zig
     src/codegen/c.zig
-    src/codegen/c/Type.zig
+    src/codegen/c/type.zig
+    src/codegen/c/type/render_defs.zig
     src/codegen/llvm.zig
     src/codegen/llvm/bindings.zig
     src/crash_report.zig
@@ -375,6 +376,7 @@ set(ZIG_STAGE2_SOURCES
     src/libs/libunwind.zig
     src/link.zig
     src/link/C.zig
+    src/link/ConstPool.zig
     src/link/Coff.zig
     src/link/Dwarf.zig
     src/link/Elf.zig
@@ -623,6 +625,12 @@ else()
   else()
     set(ZIG2_LINK_FLAGS "-Wl,-z,stack-size=0x10000000")
   endif()
+  # Prevent GCC from miscompiling 'zig2.c'. See also 'workaround_gcc_sra_miscomp' in 'bootstrap.c'.
+  if (CMAKE_C_COMPILER_ID STREQUAL "GNU" AND
+      CMAKE_C_COMPILER_VERSION VERSION_GREATER_EQUAL "13.0" AND
+      CMAKE_C_COMPILER_VERSION VERSION_LESS_EQUAL "15.2")
+    set(ZIG2_COMPILE_FLAGS "${ZIG2_COMPILE_FLAGS} -fno-tree-sra")
+  endif()
 endif()
 
 set(ZIG1_WASM_MODULE "${PROJECT_SOURCE_DIR}/stage1/zig1.wasm")

bootstrap.c

@@ -102,6 +102,26 @@ int main(int argc, char **argv) {
     const char *cc = get_c_compiler();
     const char *host_triple = get_host_triple();
 
+    // GCC versions 13.0--14.1 have a miscompilation where some bytes of a union may get clobbered
+    // depending on the union layout and the order in which types are defined. This miscompilation
+    // affects the output of the C backend, and thus can affect the bootstrap process. Specifically,
+    // we observe that using the self-hosted x86_64 backend in 'zig2' will cause all function calls
+    // to be relocated incorrectly, causing immediate crashes on any binary produced by it.
+    //
+    // The only reliable workaround for this bug is to disable the optimization pass containing it,
+    // so here we check for a CLI flag requesting that workaround.
+    //
+    // The upstream bug is fixed in GCC version 15.2 onwards (and was also backported to the 13 and
+    // 14 branches). Once this bug is no longer widespread, we can remove this CLI flag.
+    //
+    // Upstream bug report: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119085
+    bool workaround_gcc_sra_miscomp = false;
+    for (int i = 1; i < argc; ++i) {
+        if (!strcmp(argv[i], "--workaround-gcc-sra-miscomp")) {
+            workaround_gcc_sra_miscomp = true;
+        }
+    }
+
     {
         const char *child_argv[] = {
             cc, "-o", "zig-wasm2c", "stage1/wasm2c.c", "-O2", "-std=c99", NULL,
@@ -193,6 +213,7 @@ int main(int argc, char **argv) {
 #if defined(__GNUC__)
             "-pthread",
 #endif
+            workaround_gcc_sra_miscomp ? "-fno-tree-sra" : NULL,
             NULL,
         };
         print_and_run(child_argv);

ci/x86_64-linux-release.sh

@@ -21,7 +21,8 @@ export ZIG_LOCAL_CACHE_DIR="$PWD/zig-local-cache"
 
 # Test building from source without LLVM.
 cc -o bootstrap bootstrap.c
-./bootstrap
+# See comments in bootstrap.c for an explanation of the flag given here.
+./bootstrap --workaround-gcc-sra-miscomp
 ./zig2 build -Dno-lib
 ./zig-out/bin/zig test test/behavior.zig