compiler: Fix crash with try/finally and stack-return function calls

The previous code could have never worked as-is, as the result slot
went unused, and it tried to append the load instruction to the
block just terminated with the invoke.

GitHub: Fixes #1506, #1531.

artiq/compiler/transforms/llvm_ir_generator.py

@@ -1474,19 +1474,22 @@ class LLVMIRGenerator:
             llstackptr = self.llbuilder.call(self.llbuiltin("llvm.stacksave"), [])
 
             llresultslot = self.llbuilder.alloca(llfun.type.pointee.args[0].pointee)
-            llcall = self.llbuilder.invoke(llfun, llargs, llnormalblock, llunwindblock,
+            llcall = self.llbuilder.invoke(llfun, [llresultslot] + llargs,
-                                           name=insn.name)
+                                           llnormalblock, llunwindblock, name=insn.name)
+
+            self.llbuilder.position_at_start(llnormalblock)
             llresult = self.llbuilder.load(llresultslot)
 
             self.llbuilder.call(self.llbuiltin("llvm.stackrestore"), [llstackptr])
         else:
             llcall = self.llbuilder.invoke(llfun, llargs, llnormalblock, llunwindblock,
                                            name=insn.name)
+            llresult = llcall
 
             # The !tbaa metadata is not legal to use with the invoke instruction,
             # so unlike process_Call, we do not set it here.
 
-        return llcall
+        return llresult
 
     def _quote_listish_to_llglobal(self, value, elt_type, path, kind_name):
         llelts    = [self._quote(value[i], elt_type, lambda: path() + [str(i)])

artiq/test/lit/regression/issue_1506.py

@@ -0,0 +1,44 @@
+# RUN: %python -m artiq.compiler.testbench.jit %s
+
+#
+# Check various sret-ized return types integrate properly with try/finally, which lowers
+# to `invoke` on the LLVM level (code adapted from GitHub #1506).
+#
+
+LIST = [1, 2]
+
+
+def get_tuple():
+    return (1, 2)
+
+
+def get_list():
+    return LIST
+
+
+def get_range():
+    return range(10)
+
+
+def main():
+    try:
+        a, b = get_tuple()
+        assert a == 1
+        assert b == 2
+    finally:
+        pass
+
+    try:
+        for _ in get_list():
+            pass
+    finally:
+        pass
+
+    try:
+        for _ in get_range():
+            pass
+    finally:
+        pass
+
+
+main()