Unfortunately Apple decided to drop support for PPC, SDKs 10.4/10.5 and GCC 4.0 in Xcode4. This hasn't been done for technical reasons, though, but only because Apple wanted to do that. If you have a parallel installation of Xcode3 on your Mac, you can easily restore all that functionality to Xcode4. I posted a detailed step by step guide at Stack Overflow how this can be done. For convenience and to reach a wider audience I'm posting it here again.

Before I get to the how it is done part, here are some notes about my patch/hack/fix. Right from the start the major goals have been:

• Keep all modifications to an absolute minimum.
  We want to keep the Xcode setups as original as possible.
• By all means, try to avoid patching or modifying any files.
  We want all files to stay untouched and keep their original content.
• Try to avoid moving or copying files around, unless absolutely necessary.

I was able to keep all those goals. Almost everything is done by creating symlinks. Only a single existing symlink had to be replaced and we'll back it up before replacement, just in case.

If you are no expert on terminal operations, I strongly advise you to copy/paste all terminal commands from my reply to your terminal, to avoid typos. Bear in mind that even spacing, quoting and especially capitalization can be important. Copy/paste them line by line, never more than one line at once and hit return after each pasted line to execute the command. Should any operation ever prompt you for a password, this will be the password of the currently logged in administrator user (your keystrokes are not displayed while typing, this is normal, don't worry, just keep typing the password and hit return; re-try if you had a typo and get prompted again).

Prerequisites

Before we can start, make sure the following conditions are true:

• You are logged in as an administrator user.
• You have started Terminal.app (Applications/Utilities) and a terminal window is open.
• You have a copy of the Xcode3 (e.g. 3.2.5) and Xcode4 disk image (DMG) or installer available.
• If you already have either Xcode version installed, consider uninstalling it first, so you can start with a fresh/clean setup. Uninstalling Xcode will not remove your preferences, color scheme or key binding customizations. Ideally you'd start with a system that has no Xcode version (neither 3 nor 4) currently installed.

Step 1: Installing Xcode3

Important: Do not install either the System Tools or Unix Development packages of Xcode3.
Whether you want to install Mac OS X 10.4 SDK and/or Documentation is up to you. If that is a Xcode3 with iOS SDKs, whether you install those or not is also up to you.

You are free to choose any destination folder for your installation. For this guide I have chosen /Xcode3, but feel free to pick a different one. Just make sure to alter all terminal commands accordingly.

The order of the steps given here is usually not really important, but I strongly advise you to not swap step 1 and step 2. Xcode always installs a couple of files outside of the chosen destination folder and trust me, in the end you want the Xcode4 versions of those files on your disk. By installing Xcode3 before Xcode4, you can be sure that Xcode4 will overwrite those files if necessary. I once swapped steps 1 and 2 and in the end I had some rather strange issues that might have been related to the incorrect order (I cannot say for sure, but after re-installing in the correct order the issues were gone).

Step 2: Installing Xcode4

Chose any packets you like. Installing System Tools is advisable, but not strictly necessary (though most people will sooner or later miss that functionality).

Again, feel free to pick any target folder you like. For this guide I chose the normal target folder /Developer; if you take a different one, alter all terminal commands accordingly.

Step 3: Restoring 10.4/10.5 SDK Support

Switch to your terminal window and run the following commands: Of course only run the command for 10.4u if you also installed SDK 10.4 in step 1.

This is enough to bring the SDKs 10.5 (and possibly 10.4) back to the selection list in Xcode4. Give it a try if you like. Fire up Xcode4, open a project, try changing the selected SDK. That was easy, huh? Be sure to close Xcode4 again (the application, not just the window) before proceeding with the next step.

Step 4: Restoring GCC 4.0 Support

If you have not installed MacOS 10.4 SDK or if you don't plan to ever use it, you can safely skip this step and proceed with step 5.

To use SDK 10.4, you'll have to use GCC 4.0, GCC 4.2 won't work. Apple claims that GCC 4.2 is not compatible with SDK 10.4, well, if you ask me, this is a hoax. I have already overwritten this limitations more than once and there was never the tiniest issue because of it. It would be easy to modify SDK 10.4 so that Xcode will allow you to use GCC 4.2 for it, but my goal was to avoid all file modifications, so we just add GCC 4.0 support back to Xcode, which is also a good thing, because some projects really depend on GCC 4.0 (e.g. there are some bugs in GCC 4.2 that prevent valid inline assembly code to compile without errors, while the same code compiles flawlessly on GCC 4.0 and GCC 4.4).

Back to terminal: Right now we have restored full GCC 4.0 support except for the fact that GCC 4.0 is still not selectable in Xcode4. That is because Xcode4 has no GCC 4.0 compiler plug-in any longer. Fortunately the Xcode3 plug-in also works in Xcode4, only the position has radically changed. Apple now hides those plug-ins deep within a bundle and only plug-ins there seem to work, placing them to their old position seems to have no effect. Now fire up Xcode4 again, open a project and try selecting the compiler. You should have GCC 4.0 back on the list. Now you can actually already select SDK 10.4 or 10.5, GCC 4.0 and you should have no issue to build a PPC binary. Just select 'Other...' for the 'Architecture' build setting and manually enter 'ppc', then alter 'Valid Architectures' to also include 'ppc'. We are almost done, except that trying to build a PPC binary using GCC 4.2 and SDK 10.5 will still fail.

Step 5: Restoring PPC Support for GCC 4.2

Since Apple is only supporting Intel platforms in Xcode4, their fat binaries only have i386 and x86_64 code, but no ppc or ppc64 code. For most binaries this is no issue, as you can only run Xcode4 on an Intel platform in the first place, ppc/ppc64 fat binaries are not necessary, even if the resulting binary is compiled/linked for those platforms. However, there is one tool where this is important. The tool is named 'as' and it is the GNU Assembler. To compile ppc/ppc64 binaries with GCC 4.2 we need a fat binary 'as' version with ppc/ppc64 code. This is the one and only file (actually it also a symlink) we have to first move aside (making a backup copy) before we can replace it by a symlink: Step 6: There is No Step 6

That's all folks. Considering how easy that was, you can imagine that Apple has certainly not dropped SDK 10.4/10.5 or ppc/ppc64 or GCC 4.0 support because this was a necessity, they dropped all that because they wanted to drop it.

I hope this setup works as well for you as it does for me. I have been able to compile all my old projects in Xcode4 without any major changes, except for having to alter a search path here and there.

[crarko adds: I haven't tested this one. But considering that Xcode4 is final now it's likely support for Xcode3 will fade pretty quickly. It's very handy to have the ability to both update the environment but still be able to build the earlier projects. I currently have machines with Xcode3 and one with Xcode4, and would like to update the others. Here's the link to the original article, and there is other related information available there as well.]