compiler: Fix "nowrite" miscompilation for sret functions

This affected e.g. rtio_input_timestamped_data().

artiq/compiler/transforms/llvm_ir_generator.py

@@ -1409,7 +1409,10 @@ class LLVMIRGenerator:
                 llfun.args[idx].add_attribute(attr)
             if 'nounwind' in insn.target_function().type.flags:
                 llfun.attributes.add('nounwind')
-            if 'nowrite' in insn.target_function().type.flags:
+            if 'nowrite' in insn.target_function().type.flags and not is_sret:
+                # Even if "nowrite" is correct from the user's perspective (doesn't
+                # access any other memory observable to ARTIQ Python), this isn't
+                # true on the LLVM IR level for sret return values.
                 llfun.attributes.add('inaccessiblememonly')
 
         return llfun, list(llargs), llarg_attrs, llcallstackptr

artiq/test/lit/embedding/syscall_flags.py

@@ -13,6 +13,14 @@ from artiq.language.types import *
 def foo() -> TNone:
     pass
 
+# sret nowrite functions shouldn't be marked inaccessiblememonly.
+# CHECK-L: ; Function Attrs: nounwind
+# CHECK-NEXT-L: declare void @bar({ i32, i64 }* sret)
+@syscall(flags={"nounwind", "nowrite"})
+def bar() -> TTuple([TInt32, TInt64]):
+    pass
+
 @kernel
 def entrypoint():
     foo()
+    bar()