[lli] Make sure the exported __chkstk functions are included when exporting them

The trick we use (since cbc2a06)
for exporting the __chkstk function (with various per-arch names)
that is defined in a different object file, relies on the function
already being linked in (by some function referencing it).

This function does end up referenced if there's a function that
allocates more than 4 KB on the stack. In most cases, it's referenced
somewhere, but in the case of builds with LLVM_LINK_LLVM_DYLIB
enabled (so most of the code resides in a separate libLLVM-<ver>.dll)
the only code in lli.exe is the lli tool specific code and the
mingw-w64 crt startup code. In the case of GCC based MinGW i386
builds with LLVM_LINK_LLVM_DYLIB, nothing else references it though.

Manually add a reference to the function to make sure it is linked
in (from libgcc or compiler-rt builtins) so that it can be exported.

This fixes one build issue encountered in
msys2/MINGW-packages#18002.

Differential Revision: https://reviews.llvm.org/D159085

Author: mstorsjo

llvm/tools/lli/lli.cpp

@@ -1206,18 +1206,32 @@ Expected<std::unique_ptr<orc::ExecutorProcessControl>> launchRemote() {
 // For real JIT uses, the real compiler support libraries should be linked
 // in, somehow; this is a workaround to let tests pass.
 //
+// We need to make sure that this symbol actually is linked in when we
+// try to export it; if no functions allocate a large enough stack area,
+// nothing would reference it. Therefore, manually declare it and add a
+// reference to it. (Note, the declarations of _alloca/___chkstk_ms/__chkstk
+// are somewhat bogus, these functions use a different custom calling
+// convention.)
+//
 // TODO: Move this into libORC at some point, see
 // https://github.com/llvm/llvm-project/issues/56603.
 #ifdef __MINGW32__
 // This is a MinGW version of #pragma comment(linker, "...") that doesn't
 // require compiling with -fms-extensions.
 #if defined(__i386__)
+#undef _alloca
+extern "C" void _alloca(void);
+static __attribute__((used)) void (*const ref_func)(void) = _alloca;
 static __attribute__((section(".drectve"), used)) const char export_chkstk[] =
     "-export:_alloca";
 #elif defined(__x86_64__)
+extern "C" void ___chkstk_ms(void);
+static __attribute__((used)) void (*const ref_func)(void) = ___chkstk_ms;
 static __attribute__((section(".drectve"), used)) const char export_chkstk[] =
     "-export:___chkstk_ms";
 #else
+extern "C" void __chkstk(void);
+static __attribute__((used)) void (*const ref_func)(void) = __chkstk;
 static __attribute__((section(".drectve"), used)) const char export_chkstk[] =
     "-export:__chkstk";
 #endif