Count the sret register as well when keeping track of how many integer registers are available for extern(C) functions on x86-64. Interestingly, llvm-g++ seems to have a very similar bug: http://llvm.org/pr4242 (So this breaks ABI-compatibility with llvm-gcc for this corner case, but gains it with gcc...) To clarify, this is about code like this: {{{ struct S { void*[3] data; } struct T { void*[2] data; } // The T should be passed in memory, and p in the last int register. extern(C) S fail(int, int, int, int, T t, void* p) { S s; s.data[0] = t.data[0]; s.data[1] = t.data[1]; s.data[2] = p; return s; } }}} which should generate code functionally equivalent to this: {{{ extern(C) S* succeed(S* s, int, int, int, int, T t, void* p) { s.data[0] = t.data[0]; s.data[1] = t.data[1]; s.data[2] = p; return s; } }}} (with the same definitions for S and T)

--- a/gen/abi-x86-64.cpp	Fri May 22 10:18:04 2009 +0200
+++ b/gen/abi-x86-64.cpp	Fri May 22 13:17:06 2009 +0200
@@ -562,7 +562,14 @@
             return false;
         
         Classification cl = classify(rt);
-        return cl.isMemory;
+        if (cl.isMemory) {
+            assert(state().int_regs > 0
+                && "No int registers available when determining sret-ness?");
+            // An sret parameter takes an integer register.
+            state().int_regs--;
+            return true;
+        }
+        return false;
     }
 }