[clang] [llvm] [SystemZ] Add support for __builtin_setjmp and __builtin_longjmp (PR #116642)

[Eli Friedman via cfe-commits]

================
@@ -4619,6 +4619,31 @@ RValue CodeGenFunction::EmitBuiltinExpr(const GlobalDecl GD, unsigned BuiltinID,
     // Buffer is a void**.
     Address Buf = EmitPointerWithAlignment(E->getArg(0));
 
+    if (getTarget().getTriple().getArch() == llvm::Triple::systemz) {
+      // Call LLVM's EH setjmp, which is lightweight.
+      // We're not filling any fields of the jmp_buf here and leave
+      // it all to the back-end.
+      // Current LLVM implementation and documentation of the builtin is
+      // somewhat inconsistent with itself. The documentation starts out with:
+      // The buffer format and the overall functioning of this intrinsic is
+      // compatible with the GCC __builtin_setjmp implementation allowing code
+      // built with the clang and GCC to interoperate.
+      // But in GCC the format of the buffer is completely target-dependent,
+      // while LLVM attempts to impose some constraints on certain fields.
+      // 1. LLVM documentation continues with - The front end places the frame
+      // pointer in the first word - What clang puts into the first word,
+      // however, is the result of the frameaddress intrinsic - this value
+      // matches the frame pointer register contents on some but not all
+      // targets. On SystemZ these values differ, which would cause
+      // incompatibilties with GCC.
+      // 2. Clang front-end also writes the stack pointer into the third
+      // slot. Not only is this not even mentioned in the docs, it also does
+      // not match GCC behavior on all targets. On SystemZ we use the fourth
+      // slot.
----------------
efriedma-quic wrote:

I think we should specifically call out which targets are which, for reference.  No point in being overly generic.  Otherwise I guess that's fine.

It's worth pointing out that if the backend completely fills the buffer, it doesn't really matter if the frontend stores to the buffer before calling setjmp.

https://github.com/llvm/llvm-project/pull/116642