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)

// Compiler implementation of the D programming language
// Copyright (c) 1999-2006 by Digital Mars
// All Rights Reserved
// initial header generation implementation by Dave Fladebo
// http://www.digitalmars.com
// License for redistribution is by either the Artistic License
// in artistic.txt, or the GNU General Public License in gnu.txt.
// See the included readme.txt for details.


struct HdrGenState
{
    int hdrgen;		// 1 if generating header file
    int ddoc;		// 1 if generating Ddoc file
    int console;	// 1 if writing to console
    int tpltMember;
    int inCallExp;
    int inPtrExp;
    int inSlcExp;
    int inDotExp;
    int inBinExp;
    int inArrExp;
    int emitInst;
    struct
    {
        int init;
        int decl;
    } FLinit;

    HdrGenState() { memset(this, 0, sizeof(HdrGenState)); }
};